Famicom Network System

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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
$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
||||++---- Bits not shown to exist
++++------ Bits exist but function is unknown
  • Reading this register acknowledges /IRQ.
    • Observed inconsistent behavior acknowledging, possibly suggesting multiple IRQ sources.
$40A3 Unknown No Yes RF5C66 Unknown Function. 
Write
76543210
|||||||+-- EXP 6 = $40A3.0 (POR value = 0)
+++++++--- (unknown)

Read
76543210
++++++++-- Bits not shown to exist
  • Various values written to this register affect the behavior of pulses on pin 79.
$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
++++++++-- Bits not shown 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.
$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
|++++----- Bits not shown to exist
+--------- Input value of EXP 11
$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
++++++++-- Bits not shown 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.
$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.
$40AB Yes No Yes RF5C66 Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- Bits not shown to exist
  • Reading this register resets the value of $40A9 back to $00.
$40AC Yes No Unknown RF5C66 Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- Bits not shown to exist
  • Reading this register prevents timed toggle on RF5C66-69 (see notes in pinout).
$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
|+++++++-- Bits not shown to exist
+--------- Present value of CIRAM A10
$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
++++++++-- Bits not shown to exist
  • Refer also to $40C0.0 for built-in RAM enabling.
$40B0 Yes No Yes RF5C66 Kanji Graphic ROM Control 
Write
76543210
|||||||+-- Kanji ROM Bank Select (POR value = 0)
+++++++--- (unknown)

Read
76543210
++++++++-- Bits not shown 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.
$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
$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
|+++------ Bits not shown 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.
$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.
$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.
$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.
$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
|||+++++-- Bits not shown to exist
+++------- 3-bit value read here was written by CPU2 to register $4122.
$40D4 Unknown Yes Yes RF5A18 Unknown Function. 
Write
76543210
|||||||+-- RF5A18 Pin 65 (Exp 17) = $40D4.0 (enables internal pull-up?)
||||||+--- RF5A18 Pin 67 (Exp 19) = $40D4.1 (output value is driven)
|||||+---- RF5A18 Pin 66 (Exp 18) = $40D4.2 (enables internal pull-up?)
+++++----- (unknown)

Read
76543210
|||||||+-- $40D4.0 = input value of RF5A18 Pin 65 (Exp 17)
||||||+--- $40D4.1 = output value of RF5A18 Pin 67 (Exp 19)
|||||+---- $40D4.2 = input 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)
$40D5 Unknown Yes Unknown RF5A18 Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
||++++++-- Bits exist but function is unknown
++-------- Bits not shown 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.
$40D6 Unknown Yes Unknown RF5A18 Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
|||||+++-- Bits exist but function is unknown
||||+----- Bit 3 = A complicated logic involving at least 3 of the bits written by CPU2 to $4113
|||+------ Bit 4 = Inverse of value written by CPU2 to $4113.7
||+------- Bit 5 = Inverse of value written by CPU2 to $4113.6
++-------- Bits not shown to exist

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. 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 Unknown Unknown Yes NMI timer low byte. 
Write
76543210
++++++++-- NMI timer low byte.

Read
76543210
++++++++-- (unknown)
$4101 Unknown Unknown Yes NMI timer high byte 
Write
76543210
++++++++-- NMI timer high byte

Read
76543210
++++++++-- (unknown)
$4102 Unknown Unknown Yes Unknown Function. 
Write
76543210
|||||||+-- (unknown)
||||||+--- NMI timer enable?
++++++---- (unknown)

Read
76543210
++++++++-- (unknown)
  • Bit 1 of this register is set at 1 place in the code, always in connection with writes to $4100 and $4101.
$4103 Yes Yes Unknown NMI Acknowledge 
Write
76543210
++++++++-- (unknown)

Read
76543210
|||||||+-- Bit exists but function is unknown
+++++++--- (unknown)
  • Reading from this register likely acknowledges the NMI.
    • Theory: $4103.0 read value could be the NMI flag.
$4104 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4105 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4106 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4107 Yes Yes Unknown IRQ Acknowledge 
Write
76543210
++++++++-- (unknown)

Read
76543210
|||||||+-- (unknown)
||||||+--- Bit exists but function is unknown
++++++---- (unknown)
  • Reading from this register likely acknowledges the IRQ flag $412F.6.
    • Theory: $4107.1 read value may be a global IRQ flag, i.e. $412F.6 OR $4112.0. (No particular evidence to support this theory.)
$4110 Unknown Yes Yes Modem Rx/Tx Data Byte 
Write
76543210
++++++++-- Modem Tx Data Byte

Read
76543210
++++++++-- Modem Rx Data Byte
  • 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 in modes 0 and 5.
$4111 Unknown Yes Yes Unknown Function. 
Write
76543210
|+++++++-- (unknown)
+--------- Pin 90 (Modem TXD) = !($4111.7)

Read
76543210
++++++++-- Bits exists but function is unknown
$4112 Unknown Yes Yes Unknown Function. 
Write
76543210
|||||||+-- Bit exists but function is unknown
|++++++--- (unknown)
+--------- $4112.7 = $4110 Modem Rx Data IRQ Acknowledge, 1 = Acknowledge IRQ.

Read
76543210
|||||||+-- $4112.0 = $4110 Modem Rx Data IRQ Flag, 1 = IRQ Pending.
||||||+--- $4112.1 = $4110 Modem Tx Ready Flag, 1 = byte can be written.
++++++---- Bits exist but function is unknown
  • When the IRQ flag is set, a new modem Rx data byte is available in $4110.
$4113 Unknown Yes Yes Unknown Function. 
Write
76543210
||++++++-- (unknown)
++-------- Bits exist but function is unknown

Read
76543210
++++++++-- Bits exist but function is unknown
  • The value written to this register affects the value read by Famicom register $40D6.
$4114 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4120 Unknown Yes Yes Unknown Function. 
Write
76543210
|||||||+-- Pin 39 (Modem AD0) = $4120.0
||||||+--- Pin 38 (Modem AD1) = $4120.1
|||||+---- Pin 37 (n/c) = $4120.2
||||+----- Pin 36 (Modem /RD) = $4120.3
|||+------ Pin 35 (Modem /WR) = $4120.4
||+------- Pin 34 (Modem EXCLK) = $4120.5
|+-------- Pin 32 (Modem 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 (Modem /INT)
$4121 Unknown Yes Yes Unknown Function. 
Write
76543210
|||||||+-- Pin 32 (Modem Data) = $4121.0 when set as output (refer to $4120.6)
+++++++--- (unknown)

Read
76543210
++++++++-- Bits exist but function is unknown
$4122 Unknown Yes Yes Famicom CPU <-> CPU2 Interface, Data Acknowledge 
Write
76543210
|||+++++-- (unknown)
+++------- 3-bit value written here can be read by Famicom CPU from register $40D3.

Read
76543210
|||+++++-- (unknown)
+++------- 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.
$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.
$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.
$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.
$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.
$4127 Unknown Unknown 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
++++++++-- (unknown)
  • 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.
$4128 Unknown Yes Yes Unknown Function. 
Write
76543210
|||||||+-- Pin 68 (Tone Rx GT) = $4128.0
+++++++--- (unknown)

Read
76543210
|||||+++-- (unknown)
||||+----- $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)
$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.
$412F Unknown Yes Yes Unknown Function. 
Write
76543210
|||||||+-- Bit exists but function is unknown, related to interrupt.
||+++++--- (unknown)
++-------- Bits exist but function is unknown

Read
76543210
|||||||+-- Bit exists but function is unknown
|||||++--- (unknown)
||||+----- Bit exists but function is unknown
|||+------ $412F.4 = !(Pin 33) (Modem /INT)
||+------- Bit exists but function is unknown
|+-------- $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.
$4130 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4131 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4132 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4133 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4134 Unknown Yes Unknown Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
|||||||+-- Bit exists but function is unknown
+++++++--- (unknown)
$4135 Unknown Yes Unknown Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
|||||||+-- Bit exists but function is unknown
+++++++--- (unknown)
$4136 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
$4137 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)

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.)
$92 This command is received by the Famicom in response to writing to command $12. (See next section for details.)
$C0 Unknown function.

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.
  • Theory: 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 Unknown Function. 
Message Bytes:
[$00] [count=N] [xx] [...N bytes...]

Response:
[$80] [count=$01] [$00] [status byte]
  • [xx] byte is ignored.
  • All observed software uses this command when opening a connection.
  • Theories:
    • The N bytes data of this message appears to be completely ASCII text format.
    • The data is sent as a command to the Oki MSM6827L modem chip.
    • The data is similar to a Hayes modem command.
    • The data contains the phone number to be dialed.
  • Changes to Command Mode 1 when complete.

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.
    • Found these status bytes in CPU2 ROM: 01, 02, 03, 04, 05, 06, 07, 08, 0B, 0C, 0D, 0E
  • This response can only happen in mode 1 via NMI.
$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.
  • Writes #$06 to $4100
  • Writes #$00 to $4101
  • Writes #$02 to $4102
  • Changes to Command Mode 3 when complete.
$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.
  • Writes #$06 to $4100
  • Writes #$00 to $4101
  • Writes #$02 to $4102
  • Changes to Command Mode 4 when complete.
$03 $F4BB 0 0,1,2,3,
4,5,6,7 		Unknown Function. 
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...]
  • 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.
    • 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.
    • 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.
    • 5 static bytes from ROM are copied starting at RAM address $0320: 01 40 50 00 60
  • [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 shifting data to $4130
  • Writes shifting data to $4131
  • Writes shifting data to $4132
  • Writes shifting data to $4133
  • Writes #$00 to $4136
$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...]
  • 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.
  • The 41 byte chunk 0 gets copied to RAM starting at $0330.
  • The 41 byte chunk 1 gets copied to RAM starting at $0360.
  • The 41 byte chunk 2 gets copied to RAM starting at $0390.
  • Shares the remaining code in common with command $10.
$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.
$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.
$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.
  • 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 P4-27 Control 
Message Bytes:
[$64] [count=$01] [xx] [P4-27 mode]
  • [xx] byte is ignored.
  • [P4-27 mode] Values:
    • $00 = Don't update but save mode as $00.
    • $01 = Set $4127.4 (Modem P4-27) = 0. (Also stores #$00 to $0049)
    • $02-FF = Set $4127.4 (Modem P4-27) = 1. (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 P4-21 and P4-19 Control 
Message Bytes:
[$65] [count=$02] [xx] [P4-21 mode] [P4-19 mode]
  • [xx] byte is ignored.
  • [P4-21 mode] Values:
    • $00 = Don't update but save mode as $00.
    • $01 = Set $4127.6 (Modem P4-21) = 0.
    • $02-FF = Set $4127.6 (Modem P4-21) = 1.
    • 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 based on its mode.
    • Zeroes out RAM $76,77,37,39,38.
    • Refreshes P4-21 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.
  • 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 keys.
  • If [address MSB] >= #$80, exits early with no response. (Prevent ROM dump?? :) )
  • If Read Count >= #$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 keys.
  • 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 key bytes: 00 07 60
    • Starting address: $0104 (farthest reaches of the stack)
  • 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 key bytes: 00 EC AD
    • Starting address: $0102 (farthest reaches of the stack)
$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 keys.
  • Writes a lot of stuff to $4120 and $4121
  • Writes to $4127 bit 5 (Modem Audio 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 keys.
  • 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 \ -> MSM6827L TXD
                            CPU2 A10 <- / 12                       89 \ <- 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-DFFF) <- / 22                                     O       /
      P5 /CE ($4129 Only) <- / 23                                          80 / <> Card D5
     (GND) CPU2 +Reset 1 -> / 24                                          79 / <> Card D4
    (GND) CPU2 +Reset 2 -> / 25                                          78 / -- GND
       (GND) CPU2 +IRQ -> / 26             Nintendo RF5A18              77 / <> Card D3
(5C66-68) CPU2 /Reset -> / 27      Package QFP-100, 0.65mm pitch       76 / <> Card D2
          (10k up) ? -> / 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         Orientation:
         MSM6827L /RD <- \ 34                                 67 / -> Exp P3-19           --------------------
          MSM6827L /WR <- \ 35                               66 / <- Exp P3-18                80         51
         MSM6827L EXCLK <- \ 36                             65 / <- Exp P3-17                  |         |
           (n/c) $4120.2 <- \ 37                           64 / -- +5Vcc                      .-----------.
             MSM6827L AD1 <> \ 38                         63 / -> Modem P4-19              81-|O  RF5A18  |-50
              MSM6827L AD0 <> \ 39                       62 / -> /Phone Audio Enable          |  Nintendo |
              (n/c) CPU2 D0 <- \ 40                     61 / -> /DTMF Output Enable       100-|  GCD 8C  O|-31
                     (n/c) ? <- \ 41                   60 / -> /Phone Off Hook                \-----------'
       (4.9152 MHz) Exp P3-16 <- \ 42                 59 / -> $4127.3 (n/c)                    |         |
                           GND -- \ 43               58 / -> /Green LED                       01         30
                19.6608MHz Xtal -- \ 44             57 / -> /Red LED
                      1k to Xtal -- \ 45           56 / -> MSM6827L +Reset      Legend:
                              GND -- \ 46         55 / <- Audio from phone line ------------------------------
                   (+5Vcc) $4126.0 -> \ 47       54 / <- Modem P4-28            --[RF5A18]-- Power
                 (+5Vcc) !($4126.1) -> \ 48     53 / <- Modem P4-25             ->[RF5A18]<- RF5A18 input
                             5C66-69 -> \ 49   52 / <- Switch SW1-4             <-[RF5A18]-> RF5A18 output
                                  n/c -- \ 50 51 / <- Switch SW1-2              <>[RF5A18]<> Bidirectional
                                          \     /                               ??[RF5A18]?? Unknown
                                           \   /                                    f      Famicom connection
                                            \ /                                     r      ROM chip connection
                                             V                                      R      RAM chip connection
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

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 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

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