Famicom Network System

System Overview

The Famicom Network System is a complicated device with its own memory mapping system and internal CPU. The RF5C66 chip provides the main mapper functionality, delegating its own registers at $40A0, RF5A18 Famicom registers at $40D0, an internal Kanji ROM at $5000, an internal 8kByte W-RAM at $6000. It also controls the bank of a built-in 16kByte CHR-RAM.

The RF5A18 contains CPU2, which is a 65C02 processor with its own independent CPU clock. It has a built-in 4kByte ROM. This chip is responsible for controlling the modem communications. It communicates with the Famicom CPU through four bidirectional registers at $40Dx.

The Famicom Network System plugs into the Famicom through its cartridge connector and provides the user a ZIF style slot to insert a "card". The card is similar to a normal cartridge but does not have access to any PPU signals. Commercial cards are observed to have their own MMC1 memory mapper, which does not interfere with any of the registers of the Famicom Network System. The CPU data bus is routed through the RF5C66 chip before making it to the card, possibly acting just as a bidirectional buffer / signal conditioner. It is unknown what other reason the data bus would be passed through the RF5C66 like that. Older revisions of Famicom Network System buffered the Famicom address bus with 74HC541 chips, so it is plausible that this function is literally just a bidirectional buffer integrated into the RF5C66.

LH5323M1 Kanji Graphic ROM

The LH5323M1 is a 256kByte graphics ROM containing primarily Kanji data that is mapped at $5000-5FFF. Each index in this range is a 32-byte space containing 16x16 1bpp graphics, usually for a single character, and each read automatically advances to the next byte in the sequence. There are 2 128kByte banks, and the low bank is default at power-on. Writing 1 to $40B0.0 selects the high bank. Reading from $40B0 resets to the beginning of the 32-byte sequence. Writing $40B0 does not reset the sequence however. No values written to $40B0 have been observed to arbitrarily change or reset the position in the sequence.

Expansion Audio

The Famicom Network System does have expansion audio capabilities. The Famicom audio is routed through the modem module, but nowhere directly to either of the large ASICs. Dial tones have been observed through the television speakers. It is unlikely but unknown if there are other possible sources of sound.

Disk Drive Support

According to a block diagram with potentially dubious origins, the RF5C66 chip contains a disk drive controller. Similar design in several ways to the Famicom Disk System, it is suspected that a disk drive can be connected to the expansion port and controlled by the RF5C66. Since this feature was never used, it is unknown how to use or activate it, or even if that feature is fully implemented. The original FDS has a large DRAM that is not present as a discrete chip in the Famicom Network System. It is unknown if such a DRAM could be already integrated into the RF5C66, or could be attached externally and simply not populated, or if a special card was to be constructed containing this RAM. All original FDS registers are notably absent and all discovered registers start immediately after where the FDS registers would normally be. There is no obvious path to produce FDS expansion audio. This remains a mystery presently.

One possibility if the RF5C66 follows a similar pinout to the FDS RP2C33:

        79 / -> Exp P3-2  (reg unknown) Serial Out (observed 95.95kHz)
       78 / <- Exp P3-3  (reg unknown) Serial In
      77 / -> Exp P3-4  (!$40A4.2)    Read / Write
     76 / -> Exp P3-5  (!$40A4.1)    Reset Transfer Timing
    75 / -> Exp P3-6  ($40A3.0)     Turn on Motor
   74 / <- Exp P3-7  ($40A5.2)     Write Protect
  73 / <- Exp P3-8  ($40A5.1)     Disk Not Ready
 72 / <- Exp P3-9  ($40A5.0)     Disk Missing
71 / <- Exp P3-11 ($40A5.7)     Battery Health

Memory Map

+================+ $0000 - NES internal RAM
| NES internal   |
| RAM            |
+----------------+ $0800
|   (Mirrors of  |
|   $0000-$07FF) |
+================+ $2000 - NES PPU Registers
| NES PPU        |
| Registers      |
+----------------+ $2008
|   (Mirrors of  |
|   $2000-$2007) |
+================+ $4000 - NES APU, IO, and Test Registers
| NES APU and IO |
| Registers      |
+----------------+ $4018
| NES Test Mode  |
| Registers      |
+----------------+ $4020
|   (Open Bus)   |
+================+ $40A0 - Famicom Modem Registers
| Famicom Modem  |
| RF5C66         |
| Registers      |
+----------------+ $40D0
| Famicom Modem  |
| RF5A18         |
| Registers      |
+----------------+ $40D8
|   (Mirror of   |
|   $40D0-$40D7) |
+----------------+ $40E0
|   (Open Bus)   |
+----------------+ $4100
|   (Open Bus)   |
+----------------+ $41A0
|   (Mirror of   |
|   $40A0-$40FF) |
+----------------+ $4200
|   (Mirrors of  |
|   $4100-$41FF) |
+================+ $5000 - Famicom Modem Kanji ROM
| Famicom Modem  |
| LH5323M1       |
| Kanji ROM      |
+================+ $6000 - Famicom Modem Internal RAM
| Famicom Modem  |
| Internal RAM   |
+================+ $8000 - Famicom Modem Card Space
|                |
| Card Space     |
|                |
+================+ $10000

Known Registers

Note: All registers available to the Famicom ignore address bits 8-11 because those bits are not physically connected to the RF5C66. Therefore, register $4xA0 has mirrors that exist at $40A0, $41A0 ... $4FA0. For simplicity, this page shows all registers as the $40xx mirror.

Address	Read Has Effect	Read Has Data	Write	Owner	Function
$40A1	Unknown	Yes	Unknown	RF5C66	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 ++++++++-- Bits exist but function is unknown Reference Data  Bench test observed value $FF with pull-downs. Test code writing values $01,02,04,08,10,20,40,80 always read back $FF after each write.
$40A2	Yes	Yes	Unknown	RF5C66	IRQ Acknowledge, similar to FDS register $4030 Write 76543210 ++++++++-- (unknown) Read 76543210 |||||||+-- Timer Interrupt (1: an IRQ occurred) ||||||+--- Bit exists but function is unknown ||||++---- (unlikely to exist) ++++------ Bits exist but function is unknown Reading this register acknowledges /IRQ. Observed inconsistent behavior acknowledging, possibly suggesting multiple IRQ sources. Reference Data  Bench test observed value $20 with pull-downs, $2C with pull-ups. Test code writing values $01,02,04,08,10,20,40,80 always read back $20 after each write. JRA-PAT: Reads this register with a BIT op-code right before CLI op-code. Reads this register and makes a decision based on D7. Super Mario Club reads this register with BIT op-code.
$40A3	Unknown	No	Yes	RF5C66	Unknown Function. Write 76543210 |||||||+-- EXP 6 = $40A3.0 (POR value = 0) +++++++--- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Various values written to this register affect the behavior of pulses on pin 79. Reference Data  Bench test found open bus when reading this register. JRA-PAT writes $2F to this register and appears to keep a RAM copy at $15. Super Mario Club writes $2F to this register and appears to keep a RAM copy at $15.
$40A4	Unknown	No	Yes	RF5C66	Expansion Port Control Write 76543210 |||||||+-- (unknown) ||||||+--- EXP 5 = !($40A4.1) (POR value = 0) |||||+---- EXP 4 = !($40A4.2) (POR value = 0) +++++----- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Note: Reading test should be repeated with pull-ups and pull-downs on expansion pins. This register has not been observed read or written to by any commercial software. Reference Data  Bench test found open bus when reading this register.
$40A5	Unknown	Yes	Unknown	RF5C66	Expansion Port Input Data Write 76543210 ++++++++-- (unknown) Read 76543210 |||||||+-- Input value of EXP 9 ||||||+--- Input value of EXP 8 |||||+---- Input value of EXP 7 |++++----- (unlikely to exist) +--------- Input value of EXP 11 Reference Data  Bench test observed value $00 with pull-downs, $78 with pull-ups.
$40A6	Unknown	Yes	Yes	RF5C66	M2 Cycle Counter LSB, similar to FDS register $4020 Write 76543210 ++++++++-- Cycle counter reload value (LSB) Read 76543210 ++++++++-- Cycle counter present value (LSB) Writing to this register writes to the cycle counter reload value. Reading this register gives the present value of the counter. Writing any value to $40A8 resets the counter to the reload value. This value counts down. When the value reaches $0000, the next count rolls over to $FFFF or auto-reloads depending on $40A8.0.
$40A7	Unknown	Yes	Yes	RF5C66	M2 Cycle Counter MSB, similar to FDS register $4021 Write 76543210 ++++++++-- Cycle counter reload value (MSB) Read 76543210 ++++++++-- Cycle counter present value (MSB) Refer to description in $40A6, this register being the MSB portion of the counter.
$40A8	Unknown	No	Yes	RF5C66	IRQ Control, similar to FDS register $4022 Write 76543210 |||||||+-- IRQ Repeat Flag ||||||+--- IRQ Enable ++++++---- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Writing anything to this register resets the cycle counter to the reload value. Observed writing $02 makes /IRQ go low, $00 makes /IRQ go high. Reading $40A2 with /IRQ low acknowledges it back high. Acknowledging with $40A2 first before writing $02 here prevents IRQ immediately going low. Observed rollover of cycle counter to $FFFF with repeat flag = 0 and auto-reload when flag = 1. Reference Data  Bench test found open bus when reading this register. JRA-PAT: Writes $00 to this register. Writes $00 again later, potentially connected to RAM $4F. Later, right after having written $25 to $40A7 and $20 to $40A6, writes $02 to this register. Has these various sequences hard-coded: $25->$40A7, $20->$40A6, $02->$40A8 $1C->$40A7, $10->$40A6, $02->$40A8 $03->$40A7, $19->$40A6, $02->$40A8 $06->$40A7, $F1->$40A6, $02->$40A8 Super Mario Club: Writes $00 to this register. Later, right after having written $24 to $40A7 and $F8 to $40A6, writes $02 to this register.
$40A9	Yes	Yes	Unknown	RF5C66	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 ++++++++-- Bits exist but function is unknown Bench test observed value $00 with pull-ups. When driving RF5C66 pin 45 high, this 8-bit value changes. Pin 45 high causes the value to change continuously, as if counting cycles. The value does not appear to match $40A7 or $40A6, though further testing is required to say that for sure. Reading this register causes its contents to be loaded into register $40AA.
$40AA	No	Yes	Unknown	RF5C66	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 ++++++++-- Bits exist but function is unknown This register maintains the most recent value that was read from $40A9. Reference Data  Bench test observed value $00 with pull-ups.
$40AB	Yes	No	Yes	RF5C66	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Reading this register resets the value of $40A9 back to $00. Reference Data  Bench test found open bus when reading this register. JRA-PAT writes $00 to this register. Super Mario Club writes $00 to this register.
$40AC	Yes	No	Unknown	RF5C66	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Reading this register prevents timed toggle on RF5C66-69 (see notes in pinout). Reference Data  Bench test: Observed $00 with pull-downs and $FF with pull-ups (totally open bus). JRA-PAT reads this register with a BIT op-code and throws away the result. Super Mario Club reads this register with a BIT op-code.
$40AD	Unknown	Yes	Yes	RF5C66	Mirroring Control Write 76543210 |+++++++-- (unknown) +--------- Mirroring (POR value = 0) 0 = Vertical Mirroring (CIRAM A10 = PPU A10) 1 = Horizontal Mirroring (CIRAM A10 = PPU A11) Read 76543210 |+++++++-- (unlikely to exist) +--------- Present value of CIRAM A10 Reference Data  Bench test observed $00 with pull-downs, $7F with pull-ups. JRA-PAT writes $00 to this register. Super Mario Club writes $80 to this register.
$40AE	Unknown	No	Yes	RF5C66	Unknown Function. Write 76543210 |||||||+-- Built-in RAM /CE control (POR value = 1) ||||||| 1 = Built-in RAM /CE enabled to go low for reads and writes in the range $6000-7FFF. ||||||| Pin 5C66.28 = 1 at all address ranges. (This pin normally n/c.) ||||||| 0 = Built-in RAM /CE is always high, preventing all reads and writes of the built-in RAM. ||||||| Pin 5C66.28 = 0 at all address ranges. (This pin normally n/c.) +++++++--- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Refer also to $40C0.0 for built-in RAM enabling. Reference Data  Bench test found open bus when reading this register. JRA-PAT: Writes $00 to this register. Later writes $01 to this register. Super Mario Club: Writes $00 to this register. Later writes $01 to this register.
$40B0	Yes	No	Yes	RF5C66	Kanji Graphic ROM Control Write 76543210 |||||||+-- Kanji ROM Bank Select (POR value = 0) +++++++--- (unknown) Read 76543210 ++++++++-- (unlikely to exist) All reads reset the Kanji auto-increment counter. D0 is written with 0 or 1 in Kanji graphics-loading code depending on the Kanji character index, changing the Kanji bank. For an unknown reason, RF5C66 registers can't be written on test setups. They only work in a real Famicom so far. Reference Data  Bench test found open bus when reading this register. Is read with BIT and results discarded before reading Kanji data out of $5000-5FFF. Super Mario Club: Stores X to this register after incrementing X (observed value $20). Reads this register and throws away the value read.
$40B1	Unknown	Yes	Yes	RF5C66	Modem Control Write 76543210 |||||||+-- Modem Module pin 29 = $40B1.0, rises slowly, goes low fast (POR value = 1) ||||||+--- Modem Module pin 32 = $40B1.1, rises slowly, goes low fast (POR value = 1) |||||+---- Modem Module pin 31 = $40B1.2, rises slowly, goes low fast (POR value = 1) ||||+----- Pin 5C66.68: CPU2 Reset on new revision Famicom Network Systems (POR value = 1) |||| CPU2 /Reset = !($40B1.3) |||| CPU2 runs when $40B1.3 = 0. |||| See also $40C0.2. |||+------ Exp 15 = $40B1.4, rises slowly, goes low fast (POR value = 1) ||+------- Exp 14 = $40B1.5, rises slowly, goes low fast (POR value = 1) |+-------- Exp 13 = $40B1.6, rises slowly, goes low fast (POR value = 1) +--------- Exp 12 = $40B1.7, rises slowly, goes low fast (POR value = 1) Read 76543210 |||||||+-- Input value of Modem Module pin 29 ||||||+--- Input value of Modem Module pin 32 |||||+---- Input value of Modem Module pin 31 ||||+----- Input value of 5C66 pin 63 (normally n/c) |||+------ Input value of EXP 15 ||+------- Input value of EXP 14 |+-------- Input value of EXP 13 +--------- Input value of EXP 12 Reference Data  Bench test observed value $FF with pull-downs. JRA-PAT: Writes $F7 to this register and appears to keep a RAM copy at $17. Later writes the value from $17, ORed with #$08. (Bit 3 being set to 1.) Also writes the value from $17, ANDed with #$F7. (Bit 3 being set to 0.) Super Mario Club: Writes $F7 to this register and appears to keep a RAM copy at $17. Later writes the value from $17, ORed with #$08. (Bit 3 being set to 1.)
$40C0	Unknown	Yes	Yes	RF5C66	CIC Status, CHR Bank, and RAM Control Write 76543210 |||||||+-- Pin 5C66.35 = $40C0.0 (POR value = 0) ||||||| RAM +CE Enable (1 = enabled, 0 = disabled) ||||||+--- Pin 5C66.36 = $40C0.1 (POR value = 0) |||||| (This pin normally n/c) |||||+---- Pin 5C66.37 = $40C0.2 (POR value = 0) ||||| Old Revision Famicom Network System: CPU2 /Reset (This pin n/c on newer revisions) ||||| CPU2 runs on old revisions when $40C0.2 = 1. ||||| See also $40B1.3. ||||+----- Pin 5C66.38 = $40C0.3 (POR value = 0) |||| CHR-RAM Bank Select ++++------ (unknown) Read 76543210 |||||||+-- Input value of pin 5C66.31: ||||||| Host CIC /Reset ||||||+--- Input value of pin 5C66.32: |||||| Host CIC /Fail |||||+---- Input value of pin 5C66.33: ||||| CPU2 /Reset ||||+----- Input value of pin 5C66.34: |||| Selected CHR RAM Bank |+++------ (unlikely to exist) +--------- Input value of pin 5C66.29: Filtered Host CIC +Start RAM +CE always reflects bit 0 of this register regardless of address space. Refer also to $40AE.0 for built-in RAM enabling. All examined software waits for D7 = 1 at initialization. D7 is normally 1, but becomes and stays 0 if the cartridge is removed or is not present on power-on. Reference Data  Bench test observed value $00 with pull-downs, $70 with pull-ups. JRA-PAT: Writes to this register from what appears to be a RAM copy at $18 ORed with #$01. Also writes from $18 ANDed with #$FE. Super Mario Club: Writes $00 to this register and appears to keep a RAM copy at $18. Later writes the value from $18, ANDed with #$FB. (Bit 2 being set to 0.) Reads this register and makes a decision using D7.
$40D0	Unknown	Yes	Yes	RF5A18	Famicom CPU <-> CPU2 Interface, Data Byte 0 Write 76543210 ++++++++-- 8-bit value written here can be read by CPU2 from register $4123. Read 76543210 ++++++++-- 8-bit value read here was written by CPU2 to register $4123. Reference Data  Super Mario Club: Does multiple writes of various values to this register when opening a connection. Reads this register when closing a modem connection, which reports value $80 and stores it at $701. Writing a value to this register does not cause the read value to change. This shows that Famicom -> CPU2 and CPU2 -> Famicom directions each use separate physical registers.
$40D1	Unknown	Yes	Yes	RF5A18	Famicom CPU <-> CPU2 Interface, Data Byte 1 Write 76543210 ++++++++-- 8-bit value written here can be read by CPU2 from register $4124. Read 76543210 ++++++++-- 8-bit value read here was written by CPU2 to register $4124. Reference Data  Super Mario Club: Does multiple writes of various values to this register when opening a connection. Reads this register when closing a modem connection, which reports value $01 and stores it at $702. Writing a value to this register does not cause the read value to change. This shows that Famicom -> CPU2 and CPU2 -> Famicom directions each use separate physical registers.
$40D2	Unknown	Yes	Yes	RF5A18	Famicom CPU <-> CPU2 Interface, Data Byte 2 Write 76543210 ++++++++-- 8-bit value written here can be read by CPU2 from register $4125. Read 76543210 ++++++++-- 8-bit value read here was written by CPU2 to register $4125. Reference Data  Super Mario Club: Does multiple writes of various values to this register when opening a connection. Reads this register when closing a modem connection, which reports value $00 and stores it at $703. Writing a value to this register does not cause the read value to change. This shows that Famicom -> CPU2 and CPU2 -> Famicom directions each use separate physical registers.
$40D3	Unknown	Yes	Yes	RF5A18	Famicom CPU <-> CPU2 Interface, Data Acknowledge Write 76543210 |||+++++-- (unknown) +++------- 3-bit value written here can be read by CPU2 from register $4122. Read 76543210 |||+++++-- (unlikely to exist) +++------- 3-bit value read here was written by CPU2 to register $4122. Reference Data  JRA-PAT writes $FF to this register and appears to keep a RAM copy at $19. Super Mario Club: Writes $FF to this register and appears to keep a RAM copy at $19. Does a BIT operation on this register and makes decisions based on D7 and D6. Probably the same: Reads this register once per frame, which reports value $E0. Also writes value $BF to this register, not in connection with $19. Does multiple writes of various values to this register when opening and closing a connection. Bits 4,3,2,1,0 follow pull-ups and downs on the data bus. Writing a value to this register does not cause the read value to change. This shows that Famicom -> CPU2 and CPU2 -> Famicom directions each use separate physical registers.
$40D4	Unknown	Yes	Yes	RF5A18	Tone Rx and Expansion I/O Control Write 76543210 |||||||+-- RF5A18 Pin 65 (Exp 17) = $40D4.0 (open-drain) ||||||+--- RF5A18 Pin 67 (Exp 19) = $40D4.1 (push-pull) |||||+---- RF5A18 Pin 66 (Exp 18) = $40D4.2 (open-drain) +++++----- (unknown) Read 76543210 |||||||+-- $40D4.0 = I/O value of RF5A18 Pin 65 (Exp 17) ||||||+--- $40D4.1 = output value of RF5A18 Pin 67 (Exp 19) |||||+---- $40D4.2 = I/O value of RF5A18 Pin 66 (Exp 18) ||||+----- $40D4.3 = input value of RF5A18 Pin 69 (Tone Rx D1) |||+------ $40D4.4 = input value of RF5A18 Pin 70 (Tone Rx D2) ||+------- $40D4.5 = input value of RF5A18 Pin 71 (Tone Rx D4) |+-------- $40D4.6 = input value of RF5A18 Pin 72 (Tone Rx D8) +--------- $40D4.7 = input value of RF5A18 Pin 73 (Tone Rx DV) Reference Data  JRA-PAT writes $FF to this register and appears to keep a RAM copy at $1A. Super Mario Club writes $FF to this register and appears to keep a RAM copy at $1A. Bits 7,6,5,4,3 read back as always 0 when writing inverting values to these bits. Bits 2,1,0 read back with the same value that was written.
$40D5	Unknown	Yes	Unknown	RF5A18	Clocks Write 76543210 ++++++++-- (unknown) Read 76543210 ||++++++-- Bits exist but function is unknown ++-------- (unlikely to exist) The purpose is unknown but it could potentially serve to generate random numbers. The Famicom CPU and CPU2 have separate clocks and are asynchronous, making the value read especially random. Bit 5 is a 0.05% duty square wave, measured 1.200kHz (19.6608MHz CPU2 crystal / 16384) Negative pulse width measures 406 nsec, the same as period of bit 2. Bit 4 is a 50% duty square wave, depending on $4114.1 and $4114.0: (0,0) measured 19.20kHz (19.6608MHz CPU2 crystal / 1024) (0,1) measured 38.40kHz (19.6608MHz CPU2 crystal / 512) (1,0) measured 76.80kHz (19.6608MHz CPU2 crystal / 256) (1,1) measured 153.6kHz (19.6608MHz CPU2 crystal / 128) Bit 3 is a 50% duty square wave, measured 307.3kHz (19.6608MHz CPU2 crystal / 64) Bit 2 is a 50% duty square wave, measured 2.451MHz (19.6608MHz CPU2 crystal / 8) Bit 1 is a 50% duty square wave, measured 4.902MHz (19.6608MHz CPU2 crystal / 4) This bit toggles at the same rate as RF5A18 pin 42, though there is a 129.9 degree phase difference. Bit 0 is a 50% duty square wave, measured 9.804MHz (19.6608MHz CPU2 crystal / 2) The phasing of these signals do not line up nicely but they do stay in perfect sync with CPU2 M2. Reference Data  $40Dx registers can be monitored in real-time by statically giving the RF5A18 the bus signals for a read from this register. M2 = 1, Card R/W (i.e. buffered CPU R/W) = 1, Famicom /CE = 0, CPU A0,A1,A2 = x,x,x Unimplemented bits will latch the Famicom data bus's last value when toggling M2. Bits 7,6 follow pull-ups and downs on the data bus.
$40D6	Unknown	Yes	Unknown	RF5A18	UART Status Write 76543210 ++++++++-- (unknown) Read 76543210 |||||||+-- Bit exists but function is unknown ||||||+--- $40D6.1 = !($4112.0) read by CPU2. (0 = $4110 buffer has a byte ready to be read.) |||||+---- $40D6.2 = !($4112.1) read by CPU2. (0 = $4110 buffer can be written.) ||||+----- $40D6.3 = !($4113.1 OR $4113.2) written by CPU2. |||+------ $40D6.4 = !($4113.7) written by CPU2. (1 = Transmit repeat.) ||+------- $40D6.5 = !($4113.6) written by CPU2. ++-------- (unlikely to exist) $40D6.2 previously observed to equal !($4113.7 AND $4112.1) written by CPU2. Writing these bits likely has direct effect on the read value of $4112.1. Reference Data  Bits 7,6 follow pull-ups and downs on the data bus. Bits 1 and 0 only ever observed with value 1 regardless of various techniques writing to lots of CPU2 registers. Writing a value to this register does not seem to cause its own read value to change.

RF5A18 Internal 65C02 CPU

The RF5A18 contains its own CPU, termed "CPU2" on this page. It is a 65C02 supporting bitwise set/clear/branch instructions. Note that CPU2 has its own parallel execution with its own address and data busses that are not available to the Famicom's CPU. CPU2 also has its own clock source, so it does not execute synchronously with the Famicom CPU. CPU2 clock speed is 2.4576 MHz (19.6608 MHz crystal / 8). This section describes CPU2's own memory mapping and its own internal registers.

CPU2 /Reset is controlled 2 different ways depending on the revision of Famicom Network System. To support all revisions when enabling CPU2, both of these bits should be written:

• $40B1.3 = 0 (new revisions Famicom Network System, and old revisions with J1 closed)
• $40C0.2 = 1 (old revisions Famicom Network System with J2 closed)

CPU2 Memory Map

+================+ $0000
| CPU2 RAM       |
| (U6)           |
+================+ $2000
|   (Returns     |
|   last fetch)  |
+================+ $4100
| CPU2 Control   |
| Registers      |
+================+ $4140
|   (Returns     |
|   last fetch)  |
+================+ $C000
|   (Open Bus)   |
|                |
+================+ $E000
| RF5A18         |
| Internal ROM   |
+================+ $10000

CPU2 Known Registers

Address	Read Has Effect	Read Has Data	Write	Function
$4100	No	No	Yes	NMI Timer 1 Period, low byte. Write 76543210 ++++++++-- NMI timer 1 period low byte. Read 76543210 ++++++++-- (does not exist) Time (seconds) * 1200 = 16-bit timer 1 register value. 1 count = 2048 CPU cycles. This is the LSB of the 16-bit period. Refer to $4101 for the MSB. Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected. When tone dialing, the digit length is $0078 (~100ms) and the inter-digit gap is $0046 (~60ms). Slow pulse dialing uses on-hook pulses of $004f (~67ms) and off-hook gaps of $0027 (~33ms) and the inter-digit gap is $031e (~666ms). Fast pulse dialing uses on-hook pulses of $0027 (~33ms) and off-hook gaps of $0013 (~17ms) and the inter-digit gap is $027e (~533ms). The times are approximate and assume a 1200Hz clock source, but the actual clock source is unknown. Theory: 19.6608MHz RF5A18 crystal / 2^14 = 1200Hz.
$4101	No	No	Yes	NMI Timer 1 Period, high byte. Write 76543210 ++++++++-- NMI timer 1 period high byte. Read 76543210 ++++++++-- (does not exist) Time (seconds) * 1200 = 16-bit timer 1 register value. 1 count = 2048 CPU cycles. This is the MSB of the 16-bit period. Refer to $4100 for the LSB. Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected.
$4102	No	No	Yes	NMI Timer 1 Restart. Write 76543210 |||||||+-- $4102.0 = timer 1 loop. 1 = Loop, 0 = One-Shot ||||||+--- $4102.1 = timer 1 restart. 1 = Restart. ++++++---- (unknown) Read 76543210 ++++++++-- (does not exist) When setting bit 1 = 1, timer 1 restarts with the period specified in $4100, $4101. The values of $4100, $4101 are not affected by running timer 1 and need not be rewritten each time. NMI will be generated when timer 1 expires if $412F.0 = 1. The behavior is untested when setting bits 0 or 1 = 1 while timer 1 is already running. Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected. ROM $E136 writes value #$02 to this address. ROM $F3D6 writes value #$00 to this address (initialization). ROM $F840 writes value #$00 to this address (Famicom command $61).
$4103	Yes	Yes	Unknown	NMI Timer 1 Acknowledge. Write 76543210 ++++++++-- (unknown) Read 76543210 |||||||+-- $4103.0 = Timer 1 NMI flag. 1 = Timer 1 NMI triggered. +++++++--- (does not exist) Reading from this register likely acknowledges timer 1 NMI. The read value of $4103.0 reflects the timer 1 NMI flag, but does not reflect external NMI via pin 29. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $40. ($41 XOR $40 = $01 existence mask) Value $40 from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $BE. ($BF XOR $BE = $01 existence mask) $01 OR $01 = $01 accumulated existence mask. ROM $E825 reads from this register at the beginning of the NMI handler and throws away the value. ROM $F3D9 reads from this register during initialization and throws away the value.
$4104	No	No	Yes	IRQ Timer 2 Period, low byte. Write 76543210 ++++++++-- IRQ timer 2 period low byte. Read 76543210 ++++++++-- (does not exist) Time (seconds) * 2,457,600 = 16-bit timer 2 register value. 1 count = 1 CPU cycle. This is the LSB of the 16-bit period. Refer to $4105 for the MSB Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected. ROM $F415 writes value #$FD to this address.
$4105	No	No	Yes	IRQ Timer 2 Period, high byte. Write 76543210 ++++++++-- IRQ Timer 2 Period, high byte. Read 76543210 ++++++++-- (does not exist) Time (seconds) * 2,457,600 = 16-bit timer 2 register value. 1 count = 1 CPU cycle. This is the MSB of the 16-bit period. Refer to $4104 for the LSB Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected. ROM $F410 writes value #$2F to this address.
$4106	No	No	Yes	IRQ Timer 2 Restart. Write 76543210 |||||||+-- $4106.0 = timer 2 loop. 1 = Loop, 0 = One-Shot ||||||+--- $4106.1 = timer 2 restart. 1 = Restart. ++++++---- (unknown) Read 76543210 ++++++++-- (does not exist) Theory: When setting bit 1 = 1, timer 2 restarts with the period specified in $4100, $4101. Theory: The values of $4104, $4105 are not affected by running timer 2 and need not be rewritten each time. IRQ will be generated when timer 2 expires if $412F.6 = 1. The behavior is untested when setting bits 0 or 1 = 1 while timer 2 is already running. Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected. ROM $F3DE writes value #$00 to this address. ROM $F41A writes value #$03 to this address.
$4107	Yes	Yes	Unknown	IRQ Timer 2 Acknowledge. Write 76543210 ++++++++-- (unknown) Read 76543210 |||||||+-- (unlikely to exist) ||||||+--- Likely to be the Timer 2 IRQ flag ++++++---- (unlikely to exist) Reading from this register likely acknowledges timer 2 IRQ. $4107.1 is probably the timer 2 IRQ flag. Reference Data  Read Bit Existence: Value $41 and $40 from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $BD. ($BF XOR $BD = $02 existence mask) ROM $E78A reads from this register directly after seeing IRQ flag $412F.6 and throws away the value. ROM $F3E1 reads from this register during initialization and throws away the value.
$4110	Yes	Yes	Yes	UART Rx/Tx Data Buffer Write 76543210 ++++++++-- UART Tx Data Byte Bits 7:0 in 8-bit mode, bits 6:0 in 7-bit mode Read 76543210 ++++++++-- UART Rx Data Byte Bits 7:0 in 8-bit mode, bits 6:0 in 7-bit mode Writing a byte to this register automatically clears $4112.1 read value and triggers the byte to be sent on UART Tx. Reading a byte from this register automatically clears $4112.0 read value, to be triggered high again next byte received on UART Rx. This UART is connected to the MSM6827L TXD and RXD pins. The byte is ready to read here when the IRQ flag $4112.0 is high. The byte is ready to be written here when flag $4112.1 is high. Writes to $4110 are specifically prevented by built-in ROM in modes 0 and 5. Bench testing confirmed reads and writes of this register correspond to data read by UART Rx and written to UART Tx. In 7-bit mode, Rx bit 7 (unused in this case) observed to be 0 but not confirmed always to be 0. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $FF. ($41 XOR $FF = $BE existence mask) Value $41 from previous fetch also observed changed to $00. ($41 XOR $00 = $41 existence mask) Value $40 from previous fetch observed changed to $00. ($40 XOR $00 = $40 existence mask) Value $BF from previous fetch observed changed to $00. ($BF XOR $00 = $BF existence mask) $BE OR $41 OR $40 OR $BF = $FF accumulated existence mask. ROM $E69B reads from this address inside of an IRQ handler, seemingly caused by its own IRQ, with flag at $4112.0. ROM $E72F writes to this address inside of an IRQ handler, which may have been caused by timer 2. ROM $E754 writes to this address inside of an IRQ handler, which may have been caused by timer 2.
$4111	Unknown	Yes	Yes	UART Configuration Write 76543210 |||||||+-- $4111.0 = UART Rx Enable (1 = enabled) ||||||+--- $4111.1 = UART Tx Enable (1 = enabled) |||||+---- $4111.2 = Baudrate Scaler (0 = multiply baudrate x4) ||||+----- $4111.3 = Data bits (0 = 7 bits, 1 = 8 bits) |||+------ $4111.4 = Tx Stop bits (0 = 1 stop bit, 1 = 2 stop bits) ||+------- $4111.5 = Parity bit (0 = don't use, 1 = append parity bit) |+-------- $4111.6 = Parity type (0 = odd parity, 1 = even parity) +--------- $4111.7 = Send Tx Break (1 = force UART Tx low (break), 0 = normal) Read 76543210 ++++++++-- Reads back the value written. See register $4114 for baudrate table. Confirmed write bits 2,3,5,6 apply to both Rx and Tx. Write bit $4111.4 confirmed not to enforce 2 stop bits for UART Rx. Consecutive Rx bytes with 1 stop bit separation can still be received when $4111.4 = 1. Write bit $4111.7 requires manual operation to send a break character and has no effect on Rx operation. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $00. ($41 XOR $00 = $41 existence mask) Value $40 from previous fetch observed changed to $00. ($40 XOR $00 = $40 existence mask) Value $BF from previous fetch observed changed to $00. ($BF XOR $00 = $BF existence mask) $41 OR $40 OR $BF = $FF accumulated existence mask. The built-in ROM function at $E1BC determines the UART settings to be written to this register. Clears bits 0,1,7. Sets bit 2 (slow baudrate scaler). Figures out bits 3,4,5,6 (UART bits/parity settings). Returns that $4111 value in accumulator A. Also sets baudrate to 1200 bits/sec by writing to $4114 before returning. ROM $E151 runs $E1BC, sets bits [1,0] = 0,0, and writes it to this register. ROM $ECE3 runs $E1BC, sets bits [1,0] = 1,1, and writes it to this register. ROM $ED20 runs $E1BC, sets bits [1,0] = 0,1, and writes it to this register. ROM $ED4E reads from this address, OR's it with #$03 (i.e. sets bits [1,0] = 1,1), and writes it back to this register. ROM $EE83 runs $E1BC, sets bits [1,0] = 0,1, and writes it to this register. ROM $F0E8 reads from this address, AND's it with #$FE (i.e. clears bit 7), and writes it back to this register. ROM $F2DC runs $E1BC, sets bits [1,0] = 0,1, and writes it to this register.
$4112	Unknown	Yes	Yes	UART Status Write 76543210 |||||||+-- Bit exists but function is unknown ||||||+--- $4112.1: Tx Silence |||||| 0 = Set UART Tx directly high (idle state), 1 = Allow sending data. |||||| This bit figures into $40D6.2 read by the Famicom. (See register $40D6.) |+++++---- (unknown) +--------- $4112.7 = $4110 UART Rx Data IRQ Acknowledge, 1 = Acknowledge IRQ. Read 76543210 |||||||+-- $4112.0 = $4110 UART Rx Buffer Ready Flag (1 = $4110 buffer has a byte ready to be read.) ||||||+--- $4112.1 = $4110 UART Tx Buffer Ready Flag (1 = $4110 buffer can be written.) |||||+---- $4112.2 = UART Tx Idle, 1 = Idle, 0 = Active. ||||+----- Bit exists but function is unknown |||+------ $4112.4 = Rx Parity Error. (1 = error detected) ||+------- $4112.5 = Rx Framing Error. (1 = error detected) |+-------- $4112.6 = Rx Break Received. (1 = break detected) +--------- Bit exists but function is unknown Error and Break bits persist through $4110 operations, but can be cleared by writing 1 to $4112.7. Found $4112.4 read became 1 when setting in parity mode ($4111.5 = 1) and injecting a wrong parity bit. Confirmed this flag not getting set when changing parity even/odd (i.e. inverted $4111.6). $4112.6 = 1 (break) comes accompanied with $4112.5 = 1 (framing error). Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $04. ($41 XOR $04 = $45 existence mask) Value $40 from previous fetch observed changed to $04. ($40 XOR $04 = $44 existence mask) Value $BF from previous fetch observed changed to $04. ($BF XOR $04 = $BB existence mask) $45 OR $44 OR $BB = $FF accumulated existence mask. ROM implements a shadow read register at $0011. ROM implements a shadow write register at $0012. ROM $E159 writes value #$80 to this address when disconnecting. ROM $E4D8 reads from this address in the IRQ handler and uses bit 0. ROM $E4F9 writes to this address in the IRQ handler after having set bit 7 in its shadow register. If $4112.0 read = 1, it writes a 1 to $4112.7. ROM $ECDB writes value #$81 to this address. ROM $ED18 writes value #$81 to this address. ROM $ED4B writes value #$81 to this address. ROM $EE7B writes value #$81 to this address. ROM $F2D4 writes value #$81 to this address. ROM $E4F2 uses read bit 0. ROM $E67C uses read bit 5. ROM $E68E uses read bit 4. ROM $E703 uses read bit 1.
$4113	Unknown	Yes	Yes	UART Configuration Write 76543210 |||||||+-- (unknown) ||||||+--- $4113.1 figures into $40D6.3 read by the Famicom. (See register $40D6.) |||||+---- $4113.2 figures into $40D6.3 read by the Famicom. (See register $40D6.) ||+++----- (unknown) |+-------- $4113.6 figures into $40D6.5 read by the Famicom. (See register $40D6.) +--------- $4113.7: Tx Repeat 1 = Send once on pin 90 (UART Tx), 0 = Send repeatedly This figures into $40D6.2 and $40D6.4 read by the Famicom. (See register $40D6.) Read 76543210 ++++++++-- Bits exist but function is unknown Reference Data  Writing to Bit 4 appeared to have an effect in some tests, apparently sending only 1 bit on the UART Tx, but this was not able to be reproduced in later tests. Read Bit Existence: Value $41 from previous fetch observed changed to $10. ($41 XOR $10 = $51 existence mask) Value $40 from previous fetch observed changed to $10. ($40 XOR $10 = $50 existence mask) Value $04 from previous fetch observed changed to $10. ($04 XOR $10 = $14 existence mask) Value $BF from previous fetch observed changed to $10. ($BF XOR $10 = $AF existence mask) $51 OR $50 OR $14 OR $AF = $FF accumulated existence mask. ROM implements a shadow write register at $0013. ROM $E161 writes value #$00 to this address. ROM $ECD3 writes value #$C0 to this address. ROM $ED11 writes value #$40 to this address. ROM $ED44 writes value #$C0 to this address. ROM $EE73 writes value #$40 to this address. ROM $F2CD writes value #$40 to this address. Bits 7 and 6 differ in these writes, suggesting that they exist.
$4114	Unknown	No	Yes	UART Baudrate Write 76543210 ||||||++-- UART Baud Rate (Rx and Tx) |||||| This is a clock divider that also affects frequency seen on $40D5.4. ++++++---- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Baud Rate Selection: $4111.2 $4114.1 $4114.0 Baudrate 0 0 0 1200 0 0 1 2400 0 1 0 4800 0 1 1 9600 1 0 0 300 1 0 1 600 1 1 0 1200 1 1 1 2400 Reference Data  $40D5.4 shows these frequencies depending on $4114 bits 1 and 0: (0,0) measured 19.20kHz (19.6608MHz CPU2 crystal / 1024) (0,1) measured 38.40kHz (19.6608MHz CPU2 crystal / 512) (1,0) measured 76.80kHz (19.6608MHz CPU2 crystal / 256) (1,1) measured 153.6kHz (19.6608MHz CPU2 crystal / 128) Note: $40D5.4 is not affected by $4111.2. Read Bit Existence: Values $41, $40, $FF, and $BF from previous fetch observed unaffected. ROM $E1DE writes value #$02 to this address.
$4120	Unknown	Yes	Yes	Unknown Function. Write 76543210 |||||||+-- Pin 39 (MSM6827L AD0) = $4120.0 ||||||+--- Pin 38 (MSM6827L AD1) = $4120.1 |||||+---- Pin 37 (n/c) = $4120.2 ||||+----- Pin 36 (MSM6827L /RD) = $4120.3 |||+------ Pin 35 (MSM6827L /WR) = $4120.4 ||+------- Pin 34 (MSM6827L EXCLK) = $4120.5 |+-------- Pin 32 (MSM6827L Data): Direction = $4120.6: 1 = input, 0 = output (refer to $4121.0) +--------- (unknown) Read 76543210 |+++++++-- Bits exist but function is unknown +--------- $4120.7 = Pin 33 (MSM6827L /INT) When pin 32 is set as input, it is unknown if or how the value can be read. Pin 32 floats in input mode, regardless of the value written to $4121.0. When pin 32 is set as output, it is a push-pull output. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $FF. ($41 XOR $FF = $BE existence mask) Value $40 from previous fetch observed changed to $FF. ($40 XOR $FF = $BF existence mask) Value $BF from previous fetch observed changed to $FF. ($BF XOR $FF = $40 existence mask) $BE OR $BF OR $40 = $FF accumulated existence mask. Sequence: ROM $E3BC writes value #$38 to this address. ROM $E3C7 writes value #$38 & #$DF = #$18 to this address. ROM $E3CC writes value #$18 | #$20 = #$38 to this address. ROM $E3D4 writes value #$38 & #$DF = #$18 to this address. ROM $E3D9 writes value #$18 & #$EF = #$08 to this address. ROM $E3DE writes value #$08 | #$10 = #$18 to this address. ROM $E3E3 writes value #$58 to this address. ROM $E3E9 writes value #$5A to this address. ROM $E3EE writes value #$52 to this address. ROM $E3F5 writes value #$72 to this address. ROM $E3FA writes value #$52 to this address. ROM $E402 writes value #$72 to this address.
$4121	Unknown	Yes	Yes	Unknown Function. Write 76543210 |||||||+-- Pin 32 (MSM6827L Data) = $4121.0 when set as output (refer to $4120.6) +++++++--- (unknown) Read 76543210 ++++++++-- Bits exist but function is unknown Reading $4121.0 reflects the value written to $4121.0 when $4120.6 is set as output. Surprisingly, reading $4121.0 is always 1 when $4120.6 is set as input regardless of: Driving pin 32 high or low. Writing 1 or 0 to $4121.0. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $FF. ($41 XOR $FF = $BE existence mask) Value $40 from previous fetch observed changed to $FF. ($40 XOR $FF = $BF existence mask) Value $FF from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $FF. ($BF XOR $FF = $40 existence mask) $BE OR $BF OR $40 = $FF accumulated existence mask. ROM $E3BF writes to this address. ROM $E405 reads this address.
$4122	Unknown	Yes	Yes	Famicom CPU <-> CPU2 Interface, Data Acknowledge Write 76543210 |||+++++-- (unlikely to exist) +++------- 3-bit value written here can be read by Famicom CPU from register $40D3. Read 76543210 |||+++++-- (unlikely to exist) +++------- 3-bit value read here was written by Famicom CPU to register $40D3. Read and write directions are represented by separate physical registers. The read value can only be affected by the Famicom CPU. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $E1. ($41 XOR $E1 = $A0 existence mask) Value $40 from previous fetch observed changed to $E0. ($40 XOR $E0 = $A0 existence mask) Value $FF from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $FF. ($BF XOR $FF = $40 existence mask) $A0 OR $A0 OR $40 = $E0 accumulated existence mask. ROM $F3E6 writes value #$FF to this address. ROM $FA70 writes value #$E0 to this address. ROM $FA78 writes value #$E0 to this address. ROM $FA89 reads this address using BIT operation and uses BMI, indicating bit 7 used. ROM $FAA7 reads this address using BIT operation and uses BPL, indicating bit 7 used. ROM $FAB4 reads this address and checks if bits 7 and 6 both = 1. ROM $FABF writes #$40 to this address, reads it right back, and checks if bits 7 and 6 both = 1. ROM $FACF writes value #$20 to this address. ROM $FAF3 writes value #$A0 to this address. ROM $FB00 writes value #$20 to this address. ROM $FB0D writes value #$A0 to this address. ROM $FB15 writes value #$E0 to this address. ROM $FB1D writes value #$20 to this address. ROM $FB25 writes value #$E0 to this address. ROM $FB8F writes value #$E0 to this address. ROM $FC02 writes value #$E0 to this address. ROM $FC13 reads this address using BIT operation and uses BMI, indicating bit 7 used. ROM $FC2D reads this address using BIT operation and uses BPL, indicating bit 7 used. ROM $FC3A reads this address and checks if bits 7 and 6 both = 1. ROM $FC43 writes value #$60 to this address. ROM $FC69 writes value #$E0 to this address. ROM $FC76 writes value #$60 to this address. ROM $FC83 writes value #$E0 to this address. ROM $FC8D reads this address using BIT operation and uses BPL and BVC, indicating bits 7 and 6 are used. ROM $FC96 writes value #$E0 to this address. ROM $FCA4 writes value #$60 to this address.
$4123	Unknown	Yes	Yes	Famicom CPU <-> CPU2 Interface, Data Byte 0 Write 76543210 ++++++++-- 8-bit value written here can be read by Famicom CPU from register $40D0. Read 76543210 ++++++++-- 8-bit value read here was written by Famicom CPU to register $40D0. Read and write directions are represented by separate physical registers. The read value can only be affected by the Famicom CPU. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $FF. ($41 XOR $FF = $BE existence mask) Value $40 from previous fetch observed changed to $FF. ($40 XOR $FF = $BF existence mask) Value $FF from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $FF. ($BF XOR $FF = $40 existence mask) $BE OR $BF OR $40 = $FF accumulated existence mask. ROM $F3E9 writes value #$FF to this address. ROM $FAD9 writes to this address. ROM $FB32 writes to this address. ROM $FC49 reads from this address. ROM $FCAE reads from this address.
$4124	Unknown	Yes	Yes	Famicom CPU <-> CPU2 Interface, Data Byte 1 Write 76543210 ++++++++-- 8-bit value written here can be read by Famicom CPU from register $40D1. Read 76543210 ++++++++-- 8-bit value read here was written by Famicom CPU to register $40D1. Read and write directions are represented by separate physical registers. The read value can only be affected by the Famicom CPU. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $FF. ($41 XOR $FF = $BE existence mask) Value $40 from previous fetch observed changed to $FF. ($40 XOR $FF = $BF existence mask) Value $FF from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $FF. ($BF XOR $FF = $40 existence mask) $BE OR $BF OR $40 = $FF accumulated existence mask. ROM $F3EC writes value #$FF to this address. ROM $FADF writes to this address. ROM $FB38 writes to this address. ROM $FC55 reads from this address and checks if it equals $00, indicating all bits exist. ROM $FCB5 reads from this address.
$4125	Unknown	Yes	Yes	Famicom CPU <-> CPU2 Interface, Data Byte 2 Write 76543210 ++++++++-- 8-bit value written here can be read by Famicom CPU from register $40D2. Read 76543210 ++++++++-- 8-bit value read here was written by Famicom CPU to register $40D2. Read and write directions are represented by separate physical registers. The read value can only be affected by the Famicom CPU. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $FF. ($41 XOR $FF = $BE existence mask) Value $40 from previous fetch observed changed to $FF. ($40 XOR $FF = $BF existence mask) Value $FF from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $FF. ($BF XOR $FF = $40 existence mask) $BE OR $BF OR $40 = $FF accumulated existence mask. ROM $F3EF writes value #$FF to this address. ROM $FAE7 writes to this address. ROM $FB3E writes to this address. ROM $FC4F reads from this address. ROM $FCBC reads from this address.
$4126	Unknown	Yes	Unknown	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 |||||||+-- $4126.0 = Pin 47 (+5V) ||||||+--- $4126.1 = !(Pin 48) (+5V) |||||+---- $4126.2 = Pin 49 (from 5C66-69) ||||+----- $4126.3 = Pin 51 (Switch SW1-2) |||+------ $4126.4 = Pin 52 (Switch SW1-4) ||+------- $4126.5 = Pin 53 (Modem P4-25) |+-------- $4126.6 = Pin 54 (Modem P4-28) +--------- $4126.7 = Pin 55 (Modem P4-23) SW1 selects between pulse and DTMF dialing. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $FB. ($41 XOR $FB = $BA existence mask) Value $41 from previous fetch also observed changed to $FF. ($41 XOR $FF = $BE existence mask) Value $40 from previous fetch observed changed to $FF. ($40 XOR $FF = $BF existence mask) Value $FF from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $FF. ($BF XOR $FF = $40 existence mask) $BA OR $BE OR $BF OR $40 = $FF accumulated existence mask. ROM $E482 reads this address and checks Bit 2. ROM $E7BC reads this address and checks Bits 7 and 6. ROM $E886 reads this address and checks Bits 7 and 6. ROM $E8DC reads this address. ROM $E8FC reads this address and checks Bits 7 and 6. ROM $E9C0 reads this address. ROM $EBAB reads this address and checks Bits 7 and 6. ROM $F1B6 reads this address. ROM $F4F2 reads this address.
$4127	Unknown	No	Yes	Unknown Function. Write 76543210 |||||||+-- Pin 56 MSM6827L +Reset = $4127.0 ||||||+--- Pin 57 /Red LED = $4127.1 |||||+---- Pin 58 /Green LED = $4127.2 ||||+----- Pin 59 (n/c) = $4127.3 |||+------ Pin 60 /Phone Off Hook = $4127.4 ||+------- Pin 61 /DTMF Output Enable = $4127.5 |+-------- Pin 62 /Phone Audio Enable = $4127.6 +--------- Pin 63 (Modem P4-19) = $4127.7 Read 76543210 ++++++++-- (unlikely to exist) Pin 60 observed low corresponding to off hook and pulse dialing pulses. Pin 61 observed low for the duration of DTMF numbers being dialed, but not in pulse mode. Pin 62 observed low corresponding to modem sounds coming through the TV speakers. Pin 63 observed always high. Reference Data  Read Bit Existence: Values $41, $40, $FF, and $BF from previous fetch observed unaffected. ROM implements a shadow write register at $0014. ROM $E237 writes the shadow register $0014 to this register, modified bit 6 (/Phone Audio Enable). ROM $E24D writes the shadow register $0014 to this register, modified bit 7 (Modem P4-19). ROM $E263 writes the shadow register $0014 to this register, modified bit 4 (/Phone Off Hook). ROM $E27D writes the shadow register $0014 to this register, modified bit 1 (/Red LED). ROM $E29A writes the shadow register $0014 to this register, modified bit 2 (/Green LED). ROM $E3B0 writes the shadow register $0014 to this register, modified bit 5 (/DTMF Output Enable). ROM $E987 writes the shadow register $0014 to this register, modified bit 4 (/Phone Off Hook). ROM $EA83 writes the shadow register $0014 to this register, modified bit 4 (/Phone Off Hook). ROM $F3BF writes value #$FF directly to this register, then clears bit 0 with value #$FE via the shadow register $0014.
$4128	Unknown	Yes	Yes	Unknown Function. Write 76543210 |||||||+-- Pin 68 (Tone Rx GT) = $4128.0 +++++++--- (unknown) Read 76543210 |||||+++-- (unlikely to exist) ||||+----- $4128.3 = Pin 69 (Tone Rx D1) |||+------ $4128.4 = Pin 70 (Tone Rx D2) ||+------- $4128.5 = Pin 71 (Tone Rx D4) |+-------- $4128.6 = Pin 72 (Tone Rx D8) +--------- $4128.7 = Pin 73 (Tone Rx DV) Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $01. ($41 XOR $01 = $40 existence mask) Value $41 from previous fetch also observed changed to $F9. ($41 XOR $F9 = $B8 existence mask) Value $40 from previous fetch observed changed to $F8. ($40 XOR $F8 = $B8 existence mask) Value $FF from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $FF. ($BF XOR $FF = $40 existence mask) $40 OR $B8 OR $B8 OR $40 = $F8 accumulated existence mask. ROM $E599 reads from this address. ROM $E5BB reads from this address and uses bits 3,4,5,6. ROM $F3F7 writes value #$00 to this address.
$4129	Unknown	Unknown	Yes	P5 Expansion Port Write 76543210 ||||++++-- Data nybble written to device attached to P5 connector. ||++------ Used for sequencing writes to the device. ++-------- (unknown) Read 76543210 ||++++++-- Controlled by device attached to P5 connector, though the ROM code never reads it. ++-------- Not used The data bus floats when reading this register, presumably to be driven by a device connected to P5. Pin 23 is a /CE low when writing to this register (untested for reads). Data bits 6 and 7 are not available in the P5 connector. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $FF with pull-ups on data bus. Value $41 from previous fetch observed changed to $00 with pull-downs on data bus. Value $41 from previous fetch observed changed to $BF, when data bus configured with pull-ups and -downs to make $BF. Value $40 from previous fetch observed changed to $BF, when data bus configured with pull-ups and -downs to make $BF. Value $FF from previous fetch observed changed to $BF, when data bus configured with pull-ups and -downs to make $BF. Sequence: ROM $E4B6 writes a value to this address. ROM $E4BB writes previous value again, OR'd with #$10. ROM $E4C0 writes previous value again, AND'd with #$2F.
$412F	Unknown	Yes	Yes	Unknown Function. Write 76543210 |||||||+-- $412F.0 = Timer 1 NMI enable. 1 = Enabled. ||||+++--- (unknown) |||+------ $412F.4 = Pin 33 (MSM6827L /INT) IRQ enable. 1 = Enabled. ||+------- $412F.5 = Pin 73 (Tone Rx DV) IRQ enable. 1 = Enabled. |+-------- $412F.6 = Timer 2 IRQ enable. 1 = Enabled. +--------- $412F.7 = UART Rx IRQ enable. 1 = Enabled. Read 76543210 |||||||+-- Bit exists but function is unknown |||||++--- (unlikely to exist) ||||+----- Bit exists but function is unknown |||+------ $412F.4 = !(Pin 33) (MSM6827L /INT) ||+------- $412F.5 is set when IRQ is triggered by writing 1 to $412F.5. || This is probably the Tone Rx DV interrupt flag. |+-------- $412F.6 is an IRQ flag. 1 = IRQ pending. +--------- Bit exists but function is unknown ROM tends to clear $412F.0 immediately after disabling interrupts with SEI, and sets it right before enabling with CLI. The ROM is apparently disabling/enabling both IRQs and the timer NMI this way. None of the write bits in this register prevent external NMI generated from pin 29 low. None of the write bits prevent exteral IRQ generated from pin 28 low either. Pin 33 (MSM6827L /INT) low triggers interrupt. Pin 73 (Tone Rx DV) high triggers interrupt. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $08. ($41 XOR $08 = $49 existence mask) Value $41 from previous fetch also observed changed to $20. ($41 XOR $20 = $61 existence mask) Value $40 from previous fetch observed changed to $20. ($40 XOR $20 = $60 existence mask) Value $BF from previous fetch observed changed to $26. ($BF XOR $26 = $99 existence mask) $49 OR $61 OR $60 OR $99 = $F9 accumulated existence mask. ROM implements a shadow read register at $0015. ROM implements a shadow write register at $0016. ROM $E31C clears bit 0 of this register. ROM $E33B writes to this address, setting bit 0 to 1. ROM $E4D3 reads from this address in the IRQ handler and uses bit 6. ROM $F377 writes value #$00 to this address. ROM $F40A writes value #$C1 to this address. ROM $F4E1 writes value #$00 to this address. ROM $F4E9 writes the shadow register to this address. ROM $F99C writes value #$00 to this address. Bits 7, 6, and 0 differ in values written, suggesting that they exist.
$4130	Unknown	No	Yes	Unknown Function. Write 76543210 |||||||+-- Serial bit to be written +++++++--- (unknown, unlikely to exist) Read 76543210 ++++++++-- (unlikely to exist) The buffer at RAM $0330 is written to this register 1 bit at a time, seemingly using only bit 0 of the register. The most significant bit of each byte is sent first. 40 bytes are sent from $0330, then only the 6 most significant bits of the 41st byte. A total of 326 bits are sent to this register. Note: CPU2 commands $10 and $11 fill this buffer. Reference Data  Read Bit Existence: Values $41, $40, $26, and $BF from previous fetch observed unaffected. ROM $FF2D writes to this address. ROM $FF40 writes to this address.
$4131	Unknown	No	Yes	Unknown Function. Write 76543210 |||||||+-- Serial bit to be written +++++++--- (unknown, unlikely to exist) Read 76543210 ++++++++-- (unlikely to exist) The buffer at RAM $0360 is written to this register 1 bit at a time, seemingly using only bit 0 of the register. The most significant bit of each byte is sent first. 40 bytes are sent from $0360, then only the 6 most significant bits of the 41st byte. A total of 326 bits are sent to this register. Note: CPU2 commands $10, $11, and $6A fill this buffer. Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected. ROM $FF51 writes to this address. ROM $FF64 writes to this address.
$4132	Unknown	No	Yes	Unknown Function. Write 76543210 |||||||+-- Serial bit to be written +++++++--- (unknown, unlikely to exist) Read 76543210 ++++++++-- (unlikely to exist) The buffer at RAM $0390 is written to this register 1 bit at a time, seemingly using only bit 0 of the register. The most significant bit of each byte is sent first. 40 bytes are sent from $0390, then only the 6 most significant bits of the 41st byte. A total of 326 bits are sent to this register. Note: CPU2 commands $10, $11, and $6A fill this buffer. Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected. ROM $FF75 writes to this address. ROM $FF88 writes to this address.
$4133	Unknown	No	Yes	Unknown Function. Write 76543210 |||||||+-- Serial bit to be written +++++++--- (unknown, unlikely to exist) Read 76543210 ++++++++-- (unlikely to exist) The buffer at RAM $0320 is written to this register 1 bit at a time, seemingly using only bit 0 of the register. The most significant bit of each byte is sent first. 4 bytes are sent from $0320, then only the 4 most significant bits of the 5th byte. A total of 36 bits are sent to this register. Note: CPU2 commands $10 and $11 fill this buffer. Reference Data  Read Bit Existence: Values $41, $40, and $BF from previous fetch observed unaffected. ROM $FF99 writes to this address. ROM $FFAC writes to this address.
$4134	Yes	Yes	Unknown	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 |||||||+-- Bit exists but function is unknown +++++++--- (unlikely to exist) This register has serial data that can be read 1 bit at a time from $4134.0. Without any other register reads/writes, the next bit is provided the next time this register is read. In each byte read, the most significant bit comes first. CPU2 built-in ROM reads 40 bytes this way and stores them starting at RAM $03C0. Then 6 additional bits are read and stored as the 41st byte bits 7:2, and 0 for bits 1:0. Reference Data  Read Bit Existence: Value $41 from previous fetch observed changed to $40. ($41 XOR $40 = $01 existence mask) Value $40 from previous fetch observed unaffected. Value $BF from previous fetch observed changed to $BE. ($BF XOR $BE = $01 existence mask) $01 OR $01 = $01 accumulated existence mask. ROM $FFB8 reads this address and uses bit 0 ROM $FFCD reads this address and uses bit 0
$4135	Unknown	Yes	Unknown	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 |||||||+-- $4135.0 = $4134 Serial Data Ready (1 = ready) +++++++--- (unlikely to exist) If $4135.0 = 1, Built-in ROM proceeds to read the serial data from $4134. Note: Built-in ROM only treats this register as a flag and does not treat it as serial data. Reference Data  Read Bit Existence: Values $41, $40, and $BE from previous fetch observed unaffected. Value $41 from previous fetch also observed changed to $40. ($41 XOR $40 = $01 existence mask) Value $BF from previous fetch observed changed to $BE. ($BF XOR $BE = $01 existence mask) $01 OR $01 = $01 accumulated existence mask. ROM $FD35 reads this address and uses bit 0.
$4136	Unknown	No	Yes	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Built-in ROM writes #$00 to $4137, then writes serial data to $4130, $4131, $4132, $4133, then writes #$00 to this register. Theory: This is a "start" flag that starts using new data from $4130-4133 when the flag is cleared. Reference Data  Read Bit Existence: Values $41, $40, and $BE from previous fetch observed unaffected. ROM $F74A writes value #$00 to this address.
$4137	Unknown	No	Yes	Unknown Function. Write 76543210 ++++++++-- (unknown) Read 76543210 ++++++++-- (unlikely to exist) Built-in ROM writes #$00 to this register, then writes serial data to $4130, $4131, $4132, $4133, then writes #$00 to $4136. Theory: This is a "clear" flag that resets write bit counters and/or clears old data from $4130-4133 when the flag is cleared. Reference Data  Read Bit Existence: Values $41, $40, and $BE from previous fetch observed unaffected. ROM $F3FA writes value #$00 to this address. ROM $F73B writes value #$00 to this address.

Communication Between Famicom and CPU2

Registers $40D0, $40D1, $40D2, and $40D3 are used for communication between Famicom and CPU2. One would tend to expect the Famicom to receive the same value from any of these four registers if read back right after writing. However, each register is actually a separate register in each direction. The Famicom only controls the value read by CPU2, and CPU2 only controls the value read by the Famicom. It may be that CPU2 echoes the value back in some cases, but don't be fooled.

Data packets are sent in both directions between the Famicom and CPU2 using these registers. The data flow is controlled by status and acknowledge flags in $40D3, and data is sent 3 bytes at a time using registers $40D0, $40D1, and $40D2. When CPU2 receives each 3-byte chunk, it buffers it in its RAM starting at address $0401 until the full message has been received. The maximum message length is at most 255 bytes, possibly less.

Each message starts with a command byte, followed by a byte count. The byte after the byte count is not used and not counted towards the byte count in most commands. There are a total of 25 command bytes, which are stored in a lookup table at CPU2 ROM address $FB52. The index of this lookup table corresponds to the index of a function pointer table at address $FBB2, and a command mode support bitfield table at address $FB6B. This is how CPU2 efficiently directs to a unique message handler function for each command byte. The mode bitfield checks against the mode byte at $0051. It is probably used for enforcing the correct sequence of commands, as the command handlers themselves seem to be a major contributor changing the mode. It seems there are 6 possible modes: 0, 1, 2, 3, 4, and 5, though command $03 is set to support modes 6 and 7 as well. The mode may actually represent a global state machine, such as 0 = disconnected, 1 = dialing, etc.

CPU2 Commands

Commands Read by the Famicom from CPU2

Command Byte	Description
$80	This command is received by the Famicom in response to writing to command $00. (See next section for details.)
$81	Unknown function. [$81] [count=$01] [$00] [connection status] It is unknown what events trigger this command to be sent to the Famicom. [connection status] comes from RAM $0052, see section below for enumerated values. This response can only happen in mode 3 via NMI. Theory: This command is a response to writing to command $01.
$82	Unknown function. [$82] [count=$01] [$00] [connection status] It is unknown what events trigger this command to be sent to the Famicom. [connection status] comes from RAM $0052, see section below for enumerated values. Response: [parameter] $00 means success, all other values indicate an error. When successful, the mode changes to $02. When unsuccessful, the mode changes to $00. [parameter] comes from RAM $0052 This response can only happen in mode 4 via NMI. Theory: This command is a response to writing to command $02.
$83	This command is received by the Famicom in response to writing to command $03. (See next section for details.)
$90	This command is received by the Famicom in response to writing to command $10. (See next section for details.)
$91	This command is received by the Famicom in response to writing to command $11. (See next section for details.)
$92	This command is received by the Famicom in response to writing to command $12. (See next section for details.)
$C0	UART Rx Packet There are 2 different parts of the CPU2 code that can send this message: [$C0] [count=N] [parameter] [...N bytes...] This command can only be sent to the Famicom in mode 2. [parameter] comes from RAM $0024. [...N bytes...] are read from register $4110, most recent reads first, 1 byte read from $4110 per IRQ. Count N does not include the parameter byte. This is serial data received from the Oki MSM6827L integrated modem chip. This response can only happen in mode 2 via IRQ. [$C0] [count=$01] [$00] [parameter] It is unknown what events trigger this command to be sent to the Famicom. The modem abruptly disconnects when sending this command. [parameter] can be #$00 or #$01. This response can only happen in mode 2 via IRQ.
$C1	Tone Rx Data Packet. When the packet only contains 1 nybble: [$C1] [count=$01] [$80] [tone Rx nybble] [tone Rx nybble] is register ($4128 read value >> 3) & 0x0F. When there is more than 1 nybble in the packet: [$C1] [count=N] [parameter] [tone Rx bytes] [parameter] is either $00 or $01 based on the value of RAM $007A. Theory: [parameter] may indicate an even or odd number of nybbles in the packet. Each Tone Rx byte is packed with 2 nybbles. Count N does not include the parameter byte.
$E1	This command indicates that CPU2 had a failure receiving a command from the Famicom. Command from Famicom that failed due to invalid command byte, or a command not supported in the present mode: Message from Famicom: [invalid command] [count] [byte 0] [byte 1] ... Response back to Famicom: [$E1] [count=$03] [?] [$00] [invalid command] [byte 1] [?] byte appears to be open to sending an artifact from the previous response. Observed JRA-PAT rev 05 read this response after attempting to write to command $01 after a failed connection. E1 03 (00) 00 01 01 Command from Famicom that failed due to invalid byte count: Message from Famicom: [command] [invalid count] [byte 0] [byte 1] ... Response back to Famicom: [$E1] [count=$03] [$00] [$01] [command] [byte 1]
$F0	This command is received by the Famicom in response to writing to command $7C. (See next section for details.)

Commands Written by the Famicom to CPU2

Note that most descriptions are incomplete.

Command Byte	CPU2 Handler Address	Byte Count Expected	Modes Supported	Description
$00	$F43A	>= 0 <= 60	0	Hook And Dialing Sequence Configuration Message Bytes: [$00] [count=N] [xx] [...N bytes command string...] Response: [$80] [count=$01] [$00] [status byte] [xx] byte is ignored. All observed software uses this command when opening a connection. Changes to Command Mode 1 when the full message has been received from the Famicom. [command string] is a series of one-byte commands executed in Command Mode 1. $00-$1F: ignored $20: wait up to 3 seconds for call progress (dial) tone to stop Fails with status $03 if the tone never stops $21: switch to DTMF dialing $22: switch to normal (10pps) pulse dialing $23: switch to fast (20pps) pulse dialing $24: turn on audio output $25: turn off audio output $26: disconnect phone line (on hook) $27: connect phone line (off hook) $28-$2F: ignored $30: dial 0 (10 pulses) $31-$39: dial 1-9 (1-9 pulses) $3A: dial D (no pulses) $3B: dial * (11 pulses) $3C: dial # (12 pulses) $3D: dial A (13 pulses) $3E: dial B (14 pulses) $3F: dial C (15 pulses) $40-7F: currently unknown, but not ignored $80-FF: delay 0.2-25.6 seconds (in 200ms increments) Response: [connection status] comes from RAM $0052, see section below for enumerated values. When successful, the mode changes to $02. When unsuccessful, the mode changes to $00. This response can only happen in mode 1 via NMI. Reference Data  Starts NMI timer 1 with period $0006 (5 msec), one-shot: Writes #$06 to $4100 Writes #$00 to $4101 Writes #$02 to $4102 Observed Messages (unused bytes in parentheses): Super Mario Club rev 09: 00 13 (00) 42 31 20 36 33 57 30 36 30 37 38 31 34 34 31 33 25 67 25 (A2 01) "...B1 63W0607814413%g%¢." Note: The first '%' is definitely not a '$' in this one. Heart no Benrikun Mini rev 01 A: 00 13 (00) 42 31 20 36 33 57 30 36 30 33 31 33 35 38 35 35 24 67 25 (37 31) "...B1 63W0603135855$g%71" Okasan no Famicom Trade rev 01: 00 13 (00) 42 31 20 36 33 57 30 36 30 33 31 33 33 33 39 30 24 67 25 (00 00) "...B1 63W0603133390$g%.." JRA-PAT rev 05: 00 0F (00) 88 30 42 30 20 36 34 32 35 36 34 36 36 36 24 "...ˆ0B0 642564666$" JRA-PAT rev 05, with phone number manually set to 6666666666 00 0F (00) 42 36 20 36 36 36 36 36 36 36 36 36 24 67 25 "...B6 666666666$g%" PiT Motorboat Race rev 02, with phone number manually set to 1234567890: 00 0E (00) 24 42 31 20 32 33 34 35 36 37 38 39 30 00 (00) "...$B1 234567890.." Okasan no Famicom Trade rev 01 responses observed: 80 01 00 01 after failure due to no phone line connected, reported Error 4001. 80 01 00 03 after failure due to dial tone not going away, reported Error 4003. 80 01 00 06 after failure due to server never answering, reported Error 4006. PiT Motorboat Race rev 02 responses observed: 80 01 00 01 after failure due to no phone line connected. 80 01 00 0D after failure due to server never answering. JRA-PAT rev 05 responses observed: 80 01 00 01 after failure due to no phone line connected, reported Error 390010019999. 80 01 00 03 after failure due to dial tone not going away, reported Error 390030019999. 80 01 00 04 after failure due to server never answering, reported Error 390040019999. The response [parameter] byte comes from RAM $0052.
$01	$F46C	1	2	Unknown Function. Message Bytes: [$01] [count=$01] [xx] [parameter] [xx] byte is ignored. [parameter] Values: $00 = Write #$00 to $0031 $01-FF = Write #$02 to $0031 JRA-PAT and PiT Motorboat Race use this command when closing a connection. Observed Messages (unused bytes in parentheses): JRA-PAT rev 05: 01 01 (00) 01 (30 42) PiT Motorboat Race rev 02: 01 01 (00) 01 (02 31) Checks if the byte received is 00 and if so, acts differently. Starts NMI timer 1 with period $0006 (5 msec), one-shot: Writes #$06 to $4100 Writes #$00 to $4101 Writes #$02 to $4102 Changes to Command Mode 3 when complete. Theory: This command generates response command $81 (see previous section).
$02	$F48A	2	0	Unknown Function. Message Bytes: [$02] [count=$02] [xx] [parameter 0] [parameter 1] [xx] byte is ignored. [parameter 0] gets written to $002E. [parameter 1] gets written to $002B and $002C. 9 zero-page addresses get zeroed out; seems to be initializing something. Starts NMI timer 1 with period $0006 (5 msec), one-shot: Writes #$06 to $4100 Writes #$00 to $4101 Writes #$02 to $4102 Changes to Command Mode 4 when complete. Theory: This command generates response command $82 (see previous section).
$03	$F4BB	0	0,1,2,3, 4,5,6,7	System Revision and Status Information Message Bytes: [$03] [count=$00] Response: [$83] [count=$0A] [$00] [ROM revision] [ROM checksum MSB] [ROM checksum LSB] [byte 4] ... [byte 10] Writes to $4120. Writes $412F shadow register to $412F (update only, no apparent modification). Response: [byte 1] = Comes from ROM address $FFF9, which seems to be a ROM revision byte. Observed value $03. [byte 2], [byte 3] = 16-bit ROM checksum. Computed at bootup with the function located at $FD7D. It adds each byte of ROM. [byte 4] = Value read from register $4121 & #$3F. If RAM $0023 is not equal to #$04, it also sets bit 7. [byte 5] = Value read from register $4126. [byte 6] = Value most recently written to $4127. [byte 7] = Present mode. [byte 8] = Number of 256-byte data blocks presently queued up in RAM range $1300-$1FFF. [byte 9] = Value of RAM $0076. [byte 10] = Value of RAM $0077. Observed Messages (unused bytes in parentheses): Super Mario Club rev 09 at power-on: Write: 03 00 (00) Read: 83 0A 00 03 0C AF A1 FB FE 00 0D 00 00 JRA-PAT rev 05 and 06 at power-on: Write: 03 00 (00) Read: 83 0A 00 03 0C AF 81 FB FE 00 0D 00 00
$10	$F71A	40	0,2,5	Unknown Function. Message Bytes: [$10] [count=$28] [parameter] [...40 bytes...] Response: [$90] [count=$28] [response parameter] [...40 bytes...] This command is similar to command $11, except using pre-defined data from ROM instead of the additional chunks in command $11. [parameter] is used but does not count towards the byte count. [parameter] value #$00 - #$0F: Pre-defined ROM data gets copied from tables. 40 bytes from ROM are copied starting at RAM address $0360. ROM table index using parameter bits 2,1,0. $0388 (i.e. 41st byte) set to #$00. See Register $4131. 40 bytes from ROM are copied starting at RAM address $0390. ROM table index using parameter bits 1,0. $03B8 (i.e. 41st byte) set to #$00. See Register $4132. 40 bytes from the command are copied starting at RAM address $0330. $0358 (i.e. 41st byte) set to #$00. $0330 (i.e. 1st byte) set to #$00. See Register $4130. 5 static bytes from ROM are copied starting at RAM address $0320: 01 40 50 00 60 See Register $4133. [parameter] value #$10: Skips the $0360 and $0390 ROM copy routines if RAM $0055 == #$00. If $0055 is not #$00, it aborts the command. Writes #$00 to $4137 Writes the 4 data chunks shifting 1 bit at a time to $4130, $4131, $4132, $4133 Writes #$00 to $4136 For the response, the count has not been confirmed to include [response parameter] or not. The response comes from RAM starting at $03C0. (See register $4134.) ROM Data  Data Loaded to $0330 (which later gets written to register $4130): (These are the 40 bytes supplied by the Famicom to this command, with 1st byte and 41st byte set to $00.) Data Loaded to $0360 (which later gets written to register $4131): Parameter Bits Data xxxxx000 00000c6bbc41351b1e0fa372963f7af704415090ee9a139ff87c60ca610dec66f74bd5b992c8868300 xxxxx001 0000027473468ebd1f161331ad0e5512ba56487f7b30037dac78ed9c51b308b6d8c597ca8b59cc1300 xxxxx010 000001c89b4630b518cbc342a45282fa0908773df56830150ec1955c9ec1abbd1100d4d878604afd00 xxxxx011 00000939c25f1859304000e9fdab927959a49d1acdd136da4ddb2b551b2bc942a36077d1d09dbc3500 xxxxx100 3cfed7ad4cd7435c3544144572d1d0e83d1859864bf8c7412cac015ced49a1cabce13fbd6262af0b00 xxxxx101 8aac02df739c8c0a2572493bd95755450b06ea0d77f1f08bc665636ed3cf317cb89f6f301ef03a2f00 xxxxx110 39b07c4f12614dadb4eb214af3a6a0dc046b47d10c184b04d16c91d9eeb5e0c21a052529738c6f0100 xxxxx111 36a6a31fa79ba49a9f1d1985c640ad9a6c79a4195d06ece8726fc006e1d347efa5d129cdb79017b900 Data Loaded to $0390 (which later gets written to register $4132): Parameter Bits Data xxxxxx00 882a14099f3f32b2419d0b09237a252ece6534274700628043a6df86657a84c0c4463fa72b49ad4900 xxxxxx01 bbb57552f19d8a71d6ed68fd037188b700eab270071acedc624ff4c64b53ed1c4e1046ed89e775fd00 xxxxxx10 cfb8e78ec54b81a1100f02f524acf0e05a45cc36a98f161c115a41082abb587ecabad5453511877100 xxxxxx11 aa189432166db82500fffbb1107ced2469b12f0959b263c63a0332bb4a8e6ea378a0a47150cb296100 Data Loaded to $0320 (which later gets written to register $4133): Parameter Bits Data xxxxxxxx 0140500060
$11	$F75C	127	0,2,5	Unknown Function. Message Bytes: [$11] [count=$7F] [...5 byte chunk...] [...41 byte chunk 0...] [...41 byte chunk 1...] [...41 byte chunk 2...] Response: [$91] [count=$29] [response parameter] [...41 bytes...] Note that the first byte of the 5 byte chunk is not ignored in this command like it is in most others, though it still does not count towards the byte count. The 5 byte chunk gets written to RAM starting at $0320 with lots of bit shifting and byte swapping. (See Register $4133.) The 41 byte chunk 0 gets copied to RAM starting at $0330. (See Register $4130.) The 41 byte chunk 1 gets copied to RAM starting at $0360. (See Register $4131.) The 41 byte chunk 2 gets copied to RAM starting at $0390. (See Register $4132.) Shares the remaining code in common with command $10. For the response, the count has not been confirmed to include [response parameter] or not. The response comes from RAM starting at $03C0. (See register $4134.)
$12	$F773	>= 3 <= 252	0,2,5	CRC Calculator Message Bytes: [$12] [count=N] [initial] [polynomial 1] [polynomial 2] [...N-2 bytes...] Response: [$92] [count=$02] [$00] [CRC 2] [CRC 1] The [initial] byte does not count towards the byte count. See next section for information about CRC keys. Doesn't seem to write to any registers.
$40	$F7AF	>= 1 <= 252	2	Unknown Function. Message Bytes: [$40] [count=N] [xx] [...N-1 bytes...] [xx] byte is ignored, but does count towards the byte count. Copies the N-1 bytes data directly into memory starting at the location +2 pointed to by $0000:0001. Copies the byte count to the location +1 pointed to by $0000:0001. Writes value $FF directly to the location pointed to by $0000:0001. Not known what sets up that pointer. This command is tricky to disassemble because it actually does copy the [xx] byte, then writes over it with the byte count, ultimately not using it. Theory: This command sends a UART Tx Packet to the Oki MSM6827L, and connected with response command $C0.
$41	$F7D0	>= 1 <= 100	5	Unknown Function. Message Bytes: [$41] [count=N] [xx] [...N bytes...] [xx] byte is ignored and does not count towards the byte count. Writes ((Byte count + 2) * 2) to the location +1 pointed to by $0000:0001. Writes value $00 to the location +2 pointed to by $0000:0001. Writes value $0F to the location +3 pointed to by $0000:0001. For each of N bytes: Writes upper nybble of the byte (>> 4) to the next byte pointed to by $0000:0001. Writes the lower nybble of the byte (& $0F) to the next byte pointed to by $0000:0001. Appends the value of $007C similarly as 2 nybbles at the end. Writes value $FF directly to the location pointed to by $0000:0001. Not known what sets up that pointer. Theory: This command is something to do with the Tone Rx chip, and connected with response command $C1.
$60	$F829	6	0	Unknown Function. Message Bytes: [$60] [count=$06] [...6 data bytes...] Note that the first data byte is not ignored in this command, and it does count towards the byte count. The 6 data bytes written by this command get copied to CPU2 RAM starting at address $84. PiT Motorboat Race uses this command when opening a connection. Observed Messages (unused bytes in parentheses): PiT Motorboat Race rev 02: 60 06 00 01 80 00 00 00 (00)
$61	$F839	0	1,3,4	Disconnect Message Bytes: [$61] [count=$00] This command abruptly disconnects the modem. Writes #$00 to register $4102. (Stops NMI timer 1.) Changes mode to 0 when complete.
$62	$F849	1	0,2,5	Unknown Function. Message Bytes: [$62] [count=$01] [xx] [parameter] [xx] byte is ignored. Disables interrupts while executing this command. Valid values of the [parameter] byte are $00 and $01. Other values abort the command. Appears to write to $4120
$63	$F887	0	0,1,2,3, 4,5	NOP Command Message Bytes: [$63] [count=$00] This command only validates the byte count = #$00 and is then complete.
$64	$F88D	1	0,2,5	Modem Off Hook Control Message Bytes: [$64] [count=$01] [xx] [Modem Off Hook mode] [xx] byte is ignored. [Modem Off Hook mode] Values: $00 = Don't update but save mode as $00. $01 = Set $4127.4 (/Modem Off Hook) = 0 (off hook). (Also stores #$00 to $0049) $02-FF = Set $4127.4 (/Modem Off Hook) = 1 (hang up). (Also stores #$00 to $0049) Value gets saved to $004E. PiT Motorboat Race uses this command when closing a connection. Observed Messages (unused bytes in parentheses): PiT Motorboat Race rev 02: 64 01 (00) 02 (A9 31) Note: This description is fairly complete.
$65	$F89D	2	0,2,5	Modem Audio Enable and P4-19 Control Message Bytes: [$65] [count=$02] [xx] [Modem Audio Enable mode] [P4-19 mode] [xx] byte is ignored. [Modem Audio Enable mode] Values: $00 = Don't update but save mode as $00. $01 = Set $4127.6 (/Modem Audio Enable) = 0 (enabled to TV speakers). $02-FF = Set $4127.6 (/Modem Audio Enable) = 1 (disabled). Value gets saved to $004F. [P4-19 mode] Values: $00 = Don't update but save mode as $00. $01 = Set $4127.7 (Modem P4-19) = 0. $02-FF = Set $4127.7 (Modem P4-19) = 1. Value gets saved to $0050. Heart no Benrikun Mini rev 01 A uses this command when closing a connection. Observed Messages (unused bytes in parentheses): Heart no Benrikun Mini rev 01 A: 65 02 (00) 02 00 (20) Note: This description is fairly complete.
$66	$F8B7	2	0,2,5	LED Control Message Bytes: [$66] [count=$02] [xx] [red LED mode] [green LED mode] [xx] byte is ignored. [red LED mode] Values: $00 = Don't update but save mode as $00. $01 = Set Red LED on. $02-7F = Set Red LED off. $80-FF = Set Red LED to be controlled automatically by network status. Value gets saved to $004C. [green LED mode] Values: $00 = Don't update but save mode as $00. $01 = Set Green LED on. $02-7F = Set Green LED off. $80-FF = Set Green LED to be controlled automatically by network status. Value gets saved to $004D. PiT Motorboat Race uses this command when closing a connection. Observed Messages (unused bytes in parentheses): PiT Motorboat Race rev 02: 66 02 (00) 02 02 (31) Note: This description is fairly complete.
$67	$F8D1	2	0,5	Unknown Function. Message Bytes: [$67] [count=$02] [xx] [parameter 0] [parameter 1] [xx] byte is ignored. If [parameter 0] is $00: Abruptly disconnects the modem. Zeroes out RAM $0076 and $0077. Changes to mode 0 when complete. Else if [parameter 0] is not $00: Writes [parameter 0] to $74. Writes [parameter 1] to $75. Refreshes Green LED based on its mode. Refreshes P4-27 (Phone Off Hook) based on its mode. Zeroes out RAM $76,77,37,39,38. Refreshes P4-21 (Phone Audio Enable) based on its mode. Zeroes out all queued up outgoing modem Tx messages at RAM $1300-1FFF. Each message has space $100 and number of queued messages is at RAM $0100. Zeroes out Tone Rx incoming data buffer at RAM $0500-05FF. Changes to mode 5 when complete.
$68	$F912	1	0,5	Unknown Function. Message Bytes: [$68] [count=$01] [xx] [parameter] [xx] byte is ignored. The code has 3 different paths depending on parameter. [parameter] < #$10 [parameter] == #$FF All other values of [parameter]
$69	$F951	10	0	Unknown Function. Message Bytes: [$69] [count=$0A] [xx] [...10 data bytes...] [xx] byte is ignored. Observed Messages (unused bytes in parentheses): PiT Motorboat Race rev 02 when opening a connection: 69 0A (00) 00 04 14 1D 28 80 00 30 26 21 (00 00) JRA-PAT rev 05 and 06 at power-on: 69 0A (00) 00 04 14 1D 28 30 00 30 26 21 (00 00) The 10 data bytes are copied to CPU2 RAM starting at $8A. If the 6th data byte (location $8F) is > #$FC, it gets overwritten by #$01.
$6A	$F96F	80	0,2,5	Unknown Function. Message Bytes: [$6A] [count=$50] [xx] [...40 byte chunk 0...] [...40 byte chunk 1...] [xx] byte is ignored. Copies the 40 byte chunk 0 to RAM starting at $0360. (See Register $4131.) Copies the negative (2's complement) of each byte in 40 byte chunk 1 to RAM starting at $0390. Writes #$00 to $0388 and $03B8. (i.e. 41st byte of each.) Writes #$10 to $0055. Note: This description is fairly complete.
$7B	$F994	No Check	0,1,2,3, 4,5	Software Reset Message Bytes: [$7B] [count=xx] [xx] Does not care about any bytes beyond the command byte. Refreshes the green LED. Disables interrupts. Writes #$00 to $412F. Sorts through some memory comparing values then generates a software reset. Stores #$09 as the reset type code into $00FF.
$7C	$F9AC	5	0,1,2,3, 4,5	Arbitrary Memory Read Message Bytes: [$7C] [count=$05] [xx] [CRC 1] [CRC 2] [address MSB] [address LSB] [read count N] Response: [$F0] [read count N] [$00] ...N bytes from memory... [xx] byte does not count towards the byte count. See next section for information about CRC bytes. If [address MSB] >= #$80, exits early with no response. (Prevent ROM dump?? :) ) If [read count N] >= #$39, exits early with no response.
$7D	$F9D9	>= 5	0,1,2,3, 4,5	Arbitrary Memory Write Message Bytes: [$7D] [count=N] [xx] [CRC 1] [CRC 2] [address MSB] [address LSB] [...N-4 bytes...] [xx] byte does not count towards the byte count. See next section for information about CRC bytes. count - 4 is the number of bytes to write to memory. Writes $00 to $006E. Super Mario Club rev 09 uses this command at the welcome screen. 7D 1A (00) 07 60 01 04 09 01 06 E0 16 A2 90 A5 29 F0 06 C9 FF D0 04 A2 01 86 29 4C 20 F4 (30) CRC bytes: 07 60 Starting address: $0104 (farthest reaches of the stack) Disassembly  .org $0104 .dd2 T0109 ;Wrote over Main Super-loop Function Pointer, now executes code below. .dd2 TE006 ;Wrote over IRQ Handler Function Pointer, now immediately exits IRQ. .dd1 $16 ;Satisfy periodic checksum of bytes $102-109. T0109 ldx #$90 ;Arbitrary code inserted. lda $29 beq L0115 cmp #$ff bne L0117 ldx #$01 L0115 stx $29 L0117 jmp $f420 ;Continue to original main super-loop.  ; Code Below Existing In CPU2 ROM: .org $e006 TE006 jmp TE502 .org $e502 TE502 ply ;Return from IRQ. plx pla rti JRA-PAT rev 05 and 06 use this command at power-on. 7D 29 (00) EC AD 01 02 1A 01 09 01 06 E0 DB A2 90 A5 29 F0 06 C9 FF D0 04 A2 01 86 29 4C 20 F4 AD 32 00 C9 05 D0 03 EE 32 00 4C 39 E8 (00) CRC bytes: EC AD Starting address: $0102 (farthest reaches of the stack) Disassembly  .org $0102 .dd2 T011A ;Wrote over NMI function pointer, now executes code below. .dd2 T0109 ;Wrote over Main Super-loop Function Pointer, now executes code below. .dd2 TE006 ;Wrote over IRQ Handler Function Pointer, now immediately exits IRQ. .dd1 $db ;Satisfy periodic checksum of bytes $102-109. T0109 ldx #$90 ;Arbitrary code inserted. lda $29 beq L0115 cmp #$ff bne L0117 ldx #$01 L0115 stx $29 L0117 jmp $f420 ;Continue to original main super-loop. T011A lda $0032 ;Arbitrary NMI handler code inserted. cmp #$05 bne L0124 inc $0032 L0124 jmp TE839  ; Code Below Existing In CPU2 ROM: .org $e006 TE006 jmp TE502 .org $e502 TE502 ply ;Return from IRQ. plx pla rti .org $e839 TE839 ply ;Return from NMI. plx pla rti
$7E	$F9FE	5	0,1,2,3, 4,5	Unknown Function. Message Bytes: [$7E] [count=$05] [xx] [CRC 1] [CRC 2] [parameter 0] [parameter 1] [parameter 2] [xx] byte does not count towards the byte count. See next section for information about CRC bytes. Writes a lot of stuff to $4120 and $4121 Writes to $4127 bit 5 (/DTMF Output Enable)
$7F	$FA16	5	0,1,2,3, 4,5	Unknown Function. Message Bytes: [$7F] [count=$05] [parameter 0] [CRC 1] [CRC 2] [parameter 1] [parameter 2] [parameter 3] [parameter 0] byte does not count towards the byte count. See next section for information about CRC bytes. If [parameter 0] is $00-7F, write value $0C out the P5 Expansion Port. If [parameter 0] is $80-FF, dump RAM $0240-$0280 out the P5 Expansion Port. The P5 Expansion port sequences out 1 nybble at a time. [parameter 1] gets written to $00B7. [parameter 2] gets written to $00B6. [parameter 3] gets written to $00B8. [parameter 0] gets written to $00B5.

CPU2 Commands with CRC key bytes

CPU2 command $12 calculates the CRC-16 of its message using [polynomial 1] and [polynomial 2] as the CRC polynomial, with [initial] placed in both bytes of the initial value, and performs the calculation LSB-first or "reflected". This command requires the polynomial to be specified in reciprocal form. For example, to calculate the standard CRC-16 with polynomial $8005, [polynomial 1] would be set to $40, [polynomial 2] would be set to $03, and [initial] would be set to $00.

CPU2 Commands $7C, $7D, $7E, and $7F use CRC key bytes [CRC 1] and [CRC 2] to validate the message. These commands calculate a CRC-16 across the message backwards, starting from the last byte and ending at [CRC 1]. The result must be 0 or the command is ignored. The CRC-16 polynomial is $8385 and the initial value is $35AC. The calculation is done LSB-first, or "reflected".

Connection Status Byte

Response commands $80, $81, and $82 send an enumerated value indicating the result when attempting to make a telephone connection. These enumerations are used clearly in the CPU2 ROM: 00, 01, 02, 03, 04, 05, 06, 07, 08, 0B, 0C, 0D, 0E. CPU2 ROM keeps track of this value at RAM location $0052. $00 indicates a successful connection, and all other values indicate a failure. Super Mario Club shows these failures with error code 40xx, xx directly reflecting this byte. Super Mario Club's manual gives troubleshooting information for most of these values. That information was used to create the table below.

Status Byte	Error Code	Description
$00	(n/a)	Connection was successful.
$01	4001	No dial tone was detected.
$02	4002	An incoming phone call is being received.
$03	4003	Pulse vs. Tone setting is incorrect.
$04	4004	"0" Outgoing call setting is incorrect.
$05	4005	DDX-TP connection completed earlier than expected.
$06	4006	Unknown.
$07	4007	Unknown.
$08	4008	Server was busy, disconnected, or rejected the password.
$09	4009	Server was busy, disconnected, or rejected the password.
$0A	400A	DDX-TP registration incorrect or server disconnected.
$0B	400B	DDX-TP registration incorrect or server disconnected.
$0C	400C	DDX-TP registration incorrect or server disconnected.
$0D	400D	Problem with telephone line connection.
$0E	400E	Problem with telephone line connection.

Pinouts

RF5C66 Mapper and Disk Drive Controller

                                                       _____
                                                      /     \
                                           CPU A0 -> / 1 100 \ -- +5Vcc
                                          CPU A1 -> / 2    99 \ -- n/c
                                         CPU A2 -> / 3      98 \ <> CPU D0
                                        CPU A3 -> / 4        97 \ <> CPU D1
                                       CPU A4 -> / 5          96 \ <> CPU D2
                                      CPU A5 -> / 6            95 \ <> CPU D3
                                     CPU A6 -> / 7              94 \ <> CPU D4
                                    CPU A7 -> / 8                93 \ <> CPU D5
                                  CPU A12 -> / 9                  92 \ <> CPU D6
                                 CPU A13 -> / 10                   91 \ <> CPU D7
                                CPU A14 -> / 11                     90 \ -- GND
                               /ROMSEL -> / 12                       89 \ <> Card D0
                              CPU R/W -> / 13                         88 \ <> Card D1
                                  M2 -> / 14                           87 \ <> Card D2
          P6-1 Lid Switch, Card R/W <- / 15                             86 \ <> Card D3
          (20k resistor to 5Vcc) ? -> / 16                               85 \ <> Card D4
                             /IRQ <- / 17                                 84 \ <> Card D5
                           +5Vcc -- / 18                                   83 \ <> Card D6
                            n/c -- / 19                                     82 \ <> Card D7
              21.47727MHz Xtal -- / 20                                       81 \ -- +5Vcc
                         Xtal -- / 21                                            \
                         n/c -- / 22                                     O       /
                        GND -- / 23                                          80 / -- n/c
        (n/c) Xtal Osc Out <- / 24                                          79 / -> Exp P3-2
                      n/c -- / 25                                          78 / <- Exp P3-3
   ToneRx Xin, CIC Clock <- / 26              Nintendo RF5C66             77 / -> Exp P3-4
                    n/c -- / 27       Package QFP-100, 0.65mm pitch      76 / -> Exp P3-5
         (n/c) $40AE.0 <- / 28                                          75 / -> Exp P3-6
Filt'd HostCIC +Start -> / 29                Mapper and                74 / <- Exp P3-7
         Host CIC-12 <- / 30           Disk Drive Controller          73 / <- Exp P3-8
                       /       O                                     72 / <- Exp P3-9
                       \                                            71 / <- Exp P3-11
     Host CIC /Reset -> \ 31                                       70 / -- GND
       Host CIC /Fail -> \ 32                                     69 / -> 5A18-49
           CPU2 /Reset -> \ 33                                   68 / -> CPU2 /Reset (new rev)
    CHR RAM /CE (input) -> \ 34                                 67 / <> Exp P3-12           Orientation:
                 RAM +CE <- \ 35                               66 / <> Exp P3-13            --------------------
            (n/c) $40C0.1 <- \ 36                             65 / <> Exp P3-14                 80         51
 CPU2 /Reset (old rev: J2) <- \ 37                           64 / <> Exp P3-15                   |         |
                CHR RAM /CE <- \ 38                         63 / <- $40B1.3 (n/c)               .-----------.
                         GND -- \ 39                       62 / <> Modem P4-31               81-|O Nintendo |-50
Built-in RAM /CE ($6000-7FFF) <- \ 40                     61 / <> Modem P4-32                   |  RF5C66   |
      (n/c) ? /CE ($4xE0-4xEF) <- \ 41                   60 / <> Modem P4-29                100-|  GCD 4R  O|-31
       5A18-85 /CE ($4xD0-4xDF) <- \ 42                 59 / -- +5Vcc                           \-----------'
                         (GND) ? -> \ 43               58 / -- n/c                               |         |
                          (GND) ? -> \ 44             57 / -> Kanji ROM A17                     01         30
                           (GND) ? -> \ 45           56 / -> Kanji ROM A4
                            (GND) ? -> \ 46         55 / -> Kanji ROM A3          Legend:
                           CIRAM A10 <- \ 47       54 / -> Kanji ROM A2           ------------------------------
                              PPU A11 -> \ 48     53 / -> Kanji ROM A1            --[RF5C66]-- Power
                               PPU A10 -> \ 49   52 / -> Kanji ROM A0             ->[RF5C66]<- RF5C66 input
             Kanji ROM /CE ($5000-5FFF) <- \ 50 51 / -- n/c                       <-[RF5C66]-> RF5C66 output
                                            \     /                               <>[RF5C66]<> Bidirectional
                                             \   /                                    f      Famicom connection
                                              \ /                                     r      ROM chip connection
                                               V                                      R      RAM chip connection
Notes:
- +5Vcc pins 18, 59, 81, 100 are all connected together internally.
- GND pins 23, 39, 70, 90 are all connected together internally.
- 43, 44, 45, 46 are GND on the PCB, but have internal protection diodes from GND, suggesting they are logic pins.
- 24, 28, 36, 37, 41, 63 are n/c on the PCB, but have protection diodes from GND, suggesting they may have a function.
- Pins 41 and 42 ranges shown are duplicated at $Cxxx (i.e. ignores /ROMSEL).
  - It is unknown how the 5A18 prevents bus conflict at $Cxxx range when it has no known access to /ROMSEL.
- Pins 45-46, when pulled high, causes oscillation on pin 56.
- Pin 29 (CIC-11) observed only high (with or without card inserted).
  - Seems to be a /reset because it sets pins 52-57 low when this pin is low, and possibly lots of other things.
- Pin 31 (CIC-10) observed only high (with or without card inserted).
- Pin 32 (CIC-15) observed high with card inserted and low with no card inserted or when hot-removed.
  - Does not go high when hot-inserted or Famicom reset in low state, only high when card is present at power-on.
- Pin 16 Pull-up of 20k to 5V is also required in order to avoid triggering reset.
- Pin 16 seems to be related to pin 29.  With pin 29 floating and pin 16 pulled high at power on, the chip runs for 5 seconds, then enters reset.
- Tested 10k instead of 20k (per original PCB) on pin 16, found no difference in time or function.
- Pin 69 has a high pulse of 11.9085 usec at any time that register $4xAC has not been read for 12.4892 seconds.
  - Each additional 12.4892 seconds generates another pulse.
  - It has very repeatable precision, at least 6 figures on each.
  - It is not synchronized to M2 or any other inputs.
  - Note that 12.4892 sec * 21.47727 MHz = 2^28, with an error of 0.075%. (Nominal would be 12.4986 sec.)
  - Note that 11.9085 usec * 21.47727 MHz = 2^8, with an error of 0.093%. (Nominal would be 11.9196 usec.)
- Pins 52-56 drive the address pins of the Kanji ROM.  (See notes below the LH5323M1 pinout.)
- Pin 15 (Card R/W) is a non-inverted buffer of CPU R/W.  This signal connects through the lid switch.
- Pin 26 puts out a 3.58 MHz square wave, ~50% duty.  This corresponds to 21.47727 MHz / 6.
- Pin 79 (Exp 2) puts out a 95.95 kHz square wave, 93.7% duty.  Clock source unknown.
  - Note that this seems similar to FDS serial bitrate.
  - Standalone chip can get into a 341.2 kHz mode when touching pin 80, though pulling 80 high or low doesn't correlate.
  - Either frequency, the negative pulse width is 650 nsec.
- Pins 71-79 appear strikingly similar to an FDS interface.
- CIRAM A10 follows PPU A10 by default.

RF5A18 CPU2 / Modem Controller

                                                     _____
                                                    /     \
                                        CPU2 A0 <- / 1 100 \ -- GND
                                       CPU2 A1 <- / 2    99 \ <> CPU2 D0         
                                      CPU2 A2 <- / 3      98 \ <> CPU2 D1         
                                     CPU2 A3 <- / 4        97 \ <> CPU2 D2         
                                    CPU2 A4 <- / 5          96 \ <> CPU2 D3         
                                   CPU2 A5 <- / 6            95 \ <> CPU2 D4         
                                  CPU2 A6 <- / 7              94 \ <> CPU2 D5         
                                 CPU2 A7 <- / 8                93 \ <> CPU2 D6         
                                  +5Vcc -- / 9                  92 \ <> CPU2 D7         
                               CPU2 A8 <- / 10                   91 \ -- +5Vcc
                              CPU2 A9 <- / 11                     90 \ -> UART Tx (MSM6827L TXD)
                            CPU2 A10 <- / 12                       89 \ <- UART Rx (MSM6827L RXD)
                           CPU2 A11 <- / 13                         88 \ <- CPU A2
                          CPU2 A12 <- / 14                           87 \ <- CPU A1
                   (n/c) CPU2 A13 <- / 15                             86 \ <- CPU A0
                  (n/c) CPU2 A14 <- / 16                               85 \ <- /CE (5C66-42)
                 (n/c) CPU2 A15 <- / 17                                 84 \ <- P6-1 Lid Switch, Card R/W
                           GND -- / 18                                   83 \ <- M2
   (2.4576 MHz) (n/c) CPU2 M2 <- / 19                                     82 \ <> Card D7
                    CPU2 R/W <- / 20                                       81 \ <> Card D6
       RAM /CE ($0000-1FFF) <- / 21                                            \
(n/c) ROM /CE ($C000-xFFF) <- / 22                                     O       /
      P5 /CE ($4129 Only) <- / 23                                          80 / <> Card D5
       (GND) CPU2 +Reset -> / 24                                          79 / <> Card D4
                (GND) ? -> / 25                                          78 / -- GND
  /Internal ROM Enable -> / 26             Nintendo RF5A18              77 / <> Card D3
(5C66-68) CPU2 /Reset -> / 27      Package QFP-100, 0.65mm pitch       76 / <> Card D2
  (10k up) CPU2 /IRQ -> / 28                                          75 / <> Card D1
 (10k up) CPU2 /NMI -> / 29             Modem Controller             74 / <> Card D0
               n/c -- / 30                   CPU2                   73 / <- Tone Rx DV
                     /       O                                     72 / <- Tone Rx D8
                     \                                            71 / <- Tone Rx D4
             +5Vcc -- \ 31                                       70 / <- Tone Rx D2
      MSM6827L DATA <> \ 32                                     69 / <- Tone Rx D1
       MSM6827L /INT -> \ 33                                   68 / -> Tone Rx GT
         MSM6827L /RD <- \ 34                                 67 / -> Exp P3-19
          MSM6827L /WR <- \ 35                               66 / <> Exp P3-18
         MSM6827L EXCLK <- \ 36                             65 / <> Exp P3-17
           (n/c) $4120.2 <- \ 37                           64 / -- +5Vcc                  Orientation:
             MSM6827L AD1 <- \ 38                         63 / -> Modem P4-19             --------------------
              MSM6827L AD0 <- \ 39                       62 / -> /Phone Audio Enable          80         51
              (n/c) CPU2 D0 <- \ 40                     61 / -> /DTMF Output Enable            |         |
                     (n/c) ? <- \ 41                   60 / -> /Phone Off Hook                .-----------.
       (4.9152 MHz) Exp P3-16 <- \ 42                 59 / -> $4127.3 (n/c)                81-|O  RF5A18  |-50
                           GND -- \ 43               58 / -> /Green LED                       |  Nintendo |
                19.6608MHz Xtal -- \ 44             57 / -> /Red LED                      100-|  GCD 8C  O|-31
                      1k to Xtal -- \ 45           56 / -> MSM6827L +Reset                    \-----------'
                              GND -- \ 46         55 / <- Audio from phone line                |         |
                   (+5Vcc) $4126.0 -> \ 47       54 / <- Modem P4-28                          01         30
                 (+5Vcc) !($4126.1) -> \ 48     53 / <- Modem P4-25             
                             5C66-69 -> \ 49   52 / <- Switch SW1-4             Legend:
                                  n/c -- \ 50 51 / <- Switch SW1-2              ------------------------------
                                          \     /                               --[RF5A18]-- Power, n/a
                                           \   /                                ->[RF5A18]<- RF5A18 input
                                            \ /                                 <-[RF5A18]-> RF5A18 output
                                             V                                  <>[RF5A18]<> Bidirectional
Notes:
- This chip contains its very own 65C02 CPU, with built-in ROM.
- +5Vcc pins 9, 31, 64, 91 are all connected together internally.
- GND pins 18, 43, 46, 78, 100 are all connected together internally.
- 24, 25, 26 are GND on the PCB, but have internal protection diodes from GND, suggesting they are logic pins.
- 47, 48 are +5Vcc on the PCB, but have internal protection diodes to +5Vcc, suggesting they are logic pins.
- 15, 16, 17, 19, 22, 37, 40, 41, 59 are n/c on the PCB, but have protection diodes from GND, suggesting they may have a function.
- Pin 42 (Exp 16) puts out a 4.92 MHz square wave, ~50% duty.  This is 19.6608 MHz / 4.
- CPU2 /Reset comes from RF5C66 pin 68 on new revisions and selectable with J1, J2 on old revisions:
  - J2 closed = RF5C66 pin 37 (default)
  - J1 closed = RF5C66 pin 68
- Pin 24 prevents CPU2 functioning when held high at power-on.  If the pin is then driven low, the reset vector is then fetched after that.
  - Pin 24 can be freely used as a +reset at any time this way.
- Pin 25 low at any time causes address bus to go to $FFFF and data bus shows a toggle on bits 2,5,6,7: period 208.7 usec, low for 7.93 usec.
  - Other data bits always low.
  - Shortly after applying power, the toggle has a lot of variations for a period of about 1.5 seconds, including a 225 msec gap where the bits are low.
  - The mentioned data bits all appear to have the same data.
  - Held low at power-on will fetch the reset vector later when driven high.
  - Held high at power-on, driven low later, enters the data bus toggle mode but:
    - Does not appear to fetch the reset vector when driven high after that.
    - Does execute code, possibly resuming from where it left off.
- When pin 26 is set low (default case: This pin is tied directly to GND on the PCB):
  - Internal ROM is enabled in range $E000-FFFF.
  - Pin 22 (ROM /CE) is enabled low in range $C000-DFFF.
- When pin 26 is set high:
  - Internal ROM is disabled, leaving open bus in range $E000-FFFF.
  - Pin 22 (ROM /CE) is enabled low in the entire range $C000-FFFF, which allows a custom interrupt vector table.

LH5323M1 Kanji Graphic ROM

                              _____  Note: Flat spot does not correspond to pin 1.
                             /     \
                     n/c -- / 12 11 \ -- n/c
           (5C66-52) A0 -> / 13   10 \ -- n/c
                CPU D0 <> / 14      9 \ <- A1 (5C66-53)
               CPU D1 <> / 15        8 \ <- A2 (5C66-54)
              CPU D2 <> / 16          7 \ <- A3 (5C66-55)
                GND -- / 17            6 \ -- GND
            CPU D3 <> / 18              5 \ <- A5 (CPU A0)
           CPU D4 <> / 19                4 \ -- n/c
          CPU D5 <> / 20                  3 \ <- A6 (CPU A1)
         CPU D6 <> / 21                    2 \ <- A7 (CPU A2)
        CPU D7 <> / 22  Nintendo LH5323M1   1 \ -- n/c
                 /        Package QFP-44       \
                 \         0.8mm pitch         /
           n/c -- \ 23                     44 / <- A8 (CPU A3)
            n/c -- \ 24   Kanji Graphic   43 / <- A13 (CPU A8)
       (GND) /OE -- \ 25       ROM       42 / <- A16 (CPU A11)
     (CPU A6) A11 -> \ 26               41 / <- A4 (5C66-56)         Orientation:
     (5C66-50) /CE -> \ 27             40 / -- n/c                   --------------------
                GND -- \ 28           39 / -- n/c                        33         23
        (CPU A7) A12 -> \ 29         38 / -- +5Vcc                        |         |
         (CPU A5) A10 -> \ 30       37 / <- A17 (5C66-57 Bankswitch)     .-----------.
                   n/c -- \ 31     36 / <- A15 (CPU A10)              34-| Nintendo O|-22
                    n/c -- \ 32   35 / -- n/c                            |  CCR-01   |
             (CPU A4) A9 -> \ 33 34 / <- A14 (CPU A9)                    | LH5323M1  |
                             \     /                                  44-|O 9528 D   |-12
                              \   /                                      '-----------/
                               \ /                                        |         |
                                V                                        01         11
                                
Notes:
- 6 & 28 are connected together internally.
- 17 has no measurable connection to 6 & 28.
- All logic pins have protection diode from pin 17, suggesting this is the true GND.
- Pin 25 also appears as a logic pin with respect to pin 17.
- When pins 25 and 27 are both driven low, the data bus becomes an output.  Otherwise it is hi-z.
- Pins 13, 9, 8, 7, 41, 37 are controlled by the RF5C66.
  - Pins 13, 9, 8, 7, 41 are controlled with auto-increment function.
  - The value of these pins increments each M2 falling edge when the CPU is in range $5000-5FFF.
  - Pin 37 is a bankswitch, controlled by register $40B0.0
  - At reset and when reading from register $40B0, these pins reset to 0.
  - The conditions resetting or maintaining the bankswitch pin to 1 are still unknown.

8633 CIC Host

                         _______   _______
                         |      \_/      |
 (Guest CIC-2) Data 0 <> | 1          18 | -- +5Vcc
 (Guest CIC-1) Data 1 <> | 2  O       17 | -- n/c
                  n/c -- | 3   8633   16 | -- n/c
                  n/c -- | 4          15 | -> /Fail (5C66-32)
                  n/c -- | 5    CIC   14 | -- n/c
                  n/c -- | 6   Host   13 | -- n/c
      (5C66-26) Clock -> | 7          12 | <- 5C66-30
(Guest CIC-11) /Reset -> | 8    U8    11 | -> +Start (5C66-29)
                  GND -- | 9          10 | -> /Reset (5C66-31)
                         |_______________|

8634A CIC Guest

                         _______   _______
                         |      \_/      |
  (Host CIC-2) Data 0 <> | 1          18 | -- +5Vcc
  (Host CIC-1) Data 1 <> | 2  O       17 | -- n/c
                  n/c -- | 3   8634A  16 | ?? GND
                  n/c -- | 4          15 | -- n/c
                  n/c -- | 5    CIC   14 | -- n/c
                  n/c -- | 6   Guest  13 | ?? +5V
      (5C66-26) Clock -> | 7          12 | ?? Card-33, n/c in Famicom Network System
   (Cap to 5V) /Reset -> | 8          11 | -> +Start (Host CIC-8)
                  GND -- | 9          10 | -- n/c
                         |_______________|

- Note: Some assumptions made on CIC chips based on similarity to F411A from Super NES.

8kByte CHR-RAM

                   _______   _______
                   |      \_/      |
           n/c? -- | 1          28 | -- +5Vcc
        PPU A12 -> | 2  O       27 | <- PPU /WR
         PPU A7 -> | 3          26 | <- +CE: U3=RF5C66 34/38, U4=PPU /A13
         PPU A6 -> | 4          25 | <- PPU A8
         PPU A5 -> | 5  LH5268  24 | <- PPU A9
         PPU A4 -> | 6    CHR   23 | <- PPU A11
         PPU A3 -> | 7    RAM   22 | <- /OE: PPU /RD
         PPU A2 -> | 8   U3/U4  21 | <- PPU A10
         PPU A1 -> | 9          20 | <- /CE: U3=PPU A13, U4=RF5C66 34/38
         PPU A0 -> | 10         19 | <> PPU D7
         PPU D0 <> | 11         18 | <> PPU D6
         PPU D1 <> | 12         17 | <> PPU D5
         PPU D2 <> | 13         16 | <> PPU D4
            GND -- | 14         15 | <> PPU D3
                   |_______________|

8kByte W-RAM

                   _______   _______
                   |      \_/      |
           n/c? -- | 1          28 | -- +5Vcc
        CPU A12 -> | 2  O       27 | <- /WR: Card R/W (P6-2 Lid Switch)
         CPU A7 -> | 3          26 | <- +CE: RAM +CE
         CPU A6 -> | 4          25 | <- CPU A8
         CPU A5 -> | 5  LH5268  24 | <- CPU A9
         CPU A4 -> | 6 Built-in 23 | <- CPU A11
         CPU A3 -> | 7  W-RAM   22 | <- /OE: GND
         CPU A2 -> | 8    U5    21 | <- Card A10
         CPU A1 -> | 9          20 | <- /CE: Built-in RAM /CE
         CPU A0 -> | 10         19 | <> Card D7
        Card D0 <> | 11         18 | <> Card D6
        Card D1 <> | 12         17 | <> Card D5
        Card D2 <> | 13         16 | <> Card D4
            GND -- | 14         15 | <> Card D3
                   |_______________|

8kByte CPU2 RAM

                   _______   _______
                   |      \_/      |
           n/c? -- | 1          28 | -- +5Vcc
       CPU2 A12 -> | 2  O       27 | <- /WR: CPU2 R/W
        CPU2 A7 -> | 3          26 | <- +CE: +5Vcc
        CPU2 A6 -> | 4          25 | <- CPU2 A8
        CPU2 A5 -> | 5  LH5268  24 | <- CPU2 A9
        CPU2 A4 -> | 6   CPU2   23 | <- CPU2 A11
        CPU2 A3 -> | 7    RAM   22 | <- /OE: GND
        CPU2 A2 -> | 8    U6    21 | <- CPU2 A10
        CPU2 A1 -> | 9          20 | <- /CE: CPU2 RAM /CE
        CPU2 A0 -> | 10         19 | <> CPU2 D7
        CPU2 D0 <> | 11         18 | <> CPU2 D6
        CPU2 D1 <> | 12         17 | <> CPU2 D5
        CPU2 D2 <> | 13         16 | <> CPU2 D4
            GND -- | 14         15 | <> CPU2 D3
                   |_______________|

P4: Modem Module Edge Connector

       Famicom     | Modem  |    Famicom
    Network System | Module | Network System
                   __________
                   |        |
          +5Vcc -- | 1   19 | <- 5A18-63
MSM6827L +Reset -> | 2   20 | <- Tone Rx GT
   MSM6827L AD0 <> | 3   21 | <- /Phone Audio Enable
            GND -- | 4   22 | -> Tone Rx DV
   MSM6827L AD1 <> | 5   23 | -> 5A18-55, Audio from phone line
   MSM6827L RXD <- | 6   24 | <- Tone Rx Xin, from 5C66-26
  MSM6827L DATA <- | 7   25 | -> 5A18-53
   MSM6827L TXD -> | 8   26 | -- GND
   MSM6827L /WR -> | 9   27 | <- /Phone Off Hook
 MSM6827L EXCLK -> | 10  28 | -> 5A18-54
   MSM6827L /RD -> | 11  29 | <> 5C66-60
          +5Vcc -- | 12  30 | <- /DTMF Output Enable
     Tone Rx D1 <- | 13  31 | <> 5C66-62           __________________________
  MSM6827L /INT <- | 14  32 | <> 5C66-61           | Modem Module           |
          +5Vcc -- | 15  33 | <- Audio from 2A03   | Orientation            |
     Tone Rx D2 <- | 16  34 | -> Audio to RF       |                        |
     Tone Rx D8 <- | 17  35 | -- GND               |    19 _____________ 36 |
     Tone Rx D4 <- | 18  36 | -- GND               |     1 |___________| 18 |
                   |________|                      |________________________|
 
Note: The modem module uses modem chip Oki MSM6827L and Dual Tone Receiver MC14LC5436P.

P2: Game Card Connector

Card |  | Famicom Network System
-----+--+-------------------------
  1  |--| +5Vcc
  2  |--| +5Vcc
  3  |??| n/c in JRA-PAT, n/c in host
  4  |??| n/c in JRA-PAT, Host has 10k pull-up only.
  5  |<>| Card D0
  6  |<>| Card D1
  7  |<>| Card D2
  8  |<>| Card D3
  9  |<>| Card D4
 10  |<>| Card D5
 11  |<>| Card D6
 12  |<>| Card D7
 13  |<-| Card R/W (P6-2 Lid Switch)
 14  |<-| M2
 15  |<-| /ROMSEL
 16  |<-| CPU A0
 17  |<-| CPU A1
 18  |<-| CPU A2
 19  |<-| CPU A3
 20  |<-| CPU A4
 21  |<-| CPU A5
     |  |
     |  |
 22  |<-| CPU A6
 23  |<-| CPU A7
 24  |<-| CPU A8
 25  |<-| CPU A9
 26  |<-| CPU A10
 27  |<-| CPU A11
 28  |<-| CPU A12
 29  |<-| CPU A13
 30  |<-| CPU A14
 31  |??| n/c in JRA-PAT, Host has 10k pull-up only.
 32  |??| n/c in JRA-PAT, Host has 10k pull-up only.
 33  |??| connected to Guest CIC-12 in JRA-PAT, n/c in host
 34  |??| n/c in JRA-PAT, n/c in host
 35  |->| Guest CIC-11 -> Host CIC-8
 36  |<>| Guest CIC-1 <> Host CIC-2
 37  |<>| Guest CIC-2 <> Host CIC-1
 38  |<-| CIC Clock
 39  |??| n/c in JRA-PAT, Host has 10k pull-up only.
 40  |<-| RAM +CE (n/c in JRA-PAT)
 41  |--| GND
 42  |--| GND

P3: Expansion Connector

                 Outside    |  FNS  |    Outside                   _____________________
                            _________                             / Orientation        /|
                            |       |                            /____________________/ |
                    /IRQ -> | 1  20 | -- +5Vcc                   |o|_| ==      |_||_|o|/
(95.94kHz Clock) 5C66-79 <- | 2  19 | -> 5A18-67                 \_  _  _ _||_  _  _ _/|
                 5C66-78 -> | 3  18 | -> 5A18-66                  |-| |    || HVC-050| |
                 5C66-77 <- | 4  17 | -> 5A18-65                  |-|_|    ||        | |
                 5C66-76 <- | 5  16 | -> 5A18-42 (4.92MHz Clock)  |        ||        | |
                 5C66-75 <- | 6  15 | <> 5C66-64                  |  20 __/__\__ 11  | |
                            |       |                             |o  1 |______| 10 o| |
                 5C66-74 -> | 7  14 | <> 5C66-65                  | ________________ | |
                 5C66-73 -> | 8  13 | <> 5C66-66                  |/_______________/|| |
                 5C66-72 -> | 9  12 | <> 5C66-67                  ||______________|/ | |
                     GND -- | 10 11 | <- 5C66-71                  |                  | |
                            |_______|                             |      ______      | |
                                                                  |o    | |    |    o| |
                                                                  \_____|/     |_____|/

P5: Expansion Connector

Note: This connector only exists on old revisions of Famicom Network System. Expansion P5 /CE is activated low specifically at CPU2 address $4129.

Outside |  | Famicom Network System
--------+--+-------------------------
    9   |--| GND
    8   |<>| CPU2 D5
    7   |<>| CPU2 D4
    6   |<>| CPU2 D3
    5   |<>| CPU2 D2
    4   |<>| CPU2 D1
    3   |<>| CPU2 D0
    2   |<-| Expansion P5 /CE
    1   |--| +5V

See Also

• https://forums.nesdev.org/viewtopic.php?f=9&t=18530