Famicom Network System: Difference between revisions

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(→‎Communication Between Famicom and CPU2: Added column for which modes commands are supported in.)
(→‎Communication Between Famicom and CPU2: Improvements to command descriptions.)
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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.
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, followed by a mystery byte, followed by data bytes.  The mystery byte 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 CPU2 ROM address $FBB2.  This is how CPU2 efficiently directs to a unique message handler function for each command byte.  There is also a mode byte that determines which commands are supported in different situations.  This is probably used for enforcing the correct sequence of commands, as the command handlers themselves seem to be the main thing 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.
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 Command Bytes==
==CPU2 Command Bytes==
Line 1,549: Line 1,549:
|$00
|$00
|$F43A
|$F43A
|?
|>= 0<br/><= 60
|Write-Read
|Write-Read
|0
|0
|Unknown Function.
|Unknown Function.
<pre>Message Bytes:
[00] [count=N] [xx] [...N bytes...]</pre>
*xx byte is ignored.
*Super Mario Club, Heart no Benrikun Mini, and JRA-PAT use this command when opening a connection.
*Super Mario Club, Heart no Benrikun Mini, and JRA-PAT use this command when opening a connection.
*Writes #$06 to $4100
*Writes #$00 to $4101
*Writes #$02 to $4102
*Changes to Command Mode 1 when complete.
|-
|-
|$01
|$01
Line 1,561: Line 1,568:
|2
|2
|Unknown Function.
|Unknown Function.
<pre>Message Bytes:
[01] [count=01] [xx] [parameter]</pre>
*xx byte is ignored.
*Parameter Values:
**00 = Write 00 to $0031
**01-FF = Write 02 to $0031
*JRA-PAT uses this command when closing a connection.
*JRA-PAT uses this command when closing a connection.
*Checks if the byte received is 00 and if so, acts differently.
*Checks if the byte received is 00 and if so, acts differently.
*Writes $06 to $4100
*Writes #$06 to $4100
*Writes $00 to $4101
*Writes #$00 to $4101
*Writes $02 to $4102
*Writes #$02 to $4102
*Changes to Command Mode 3 when complete.
|-
|-
|$02
|$02
Line 1,573: Line 1,587:
|0
|0
|Unknown Function.
|Unknown Function.
*Writes $06 to $4100
<pre>Message Bytes:
*Writes $00 to $4101
[02] [count=02] [xx] [parameter 1] [parameter 2]</pre>
*Writes $02 to $4102
*xx byte is ignored.
*Parameter 1 gets written to $002E.
*Parameter 2 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
|$03
Line 1,583: Line 1,604:
|0,1,2,3,<br/>4,5,6,7
|0,1,2,3,<br/>4,5,6,7
|Unknown Function.
|Unknown Function.
<pre>Message Bytes:
[03] [count=00]</pre>
*Writes to $4120
*Writes to $4120
*Reads from $4121
*Reads from $4121
*Reads from $4126
*Reads from $4126
*Writes to $412F
*Writes to $412F
|-
|-
Line 1,595: Line 1,617:
|0,2,5
|0,2,5
|Unknown Function.
|Unknown Function.
*The mystery byte has special case $10 that acts different.
<pre>Message Bytes:
[10] [count=28] [parameter] [...40 bytes...]</pre>
*Note that the 3rd byte ("parameter") is not completely ignored in this command like it is in most others, though it still does not count towards the byte count.
*Parameter Value:
**Special case $10 acts different.
**Otherwise, only bits 2,1,0 of the parameter are used.
*Writes 00 to $4137
*Writes 00 to $4137
*Writes shifting data to $4130
*Writes shifting data to $4130
Line 1,609: Line 1,636:
|0,2,5
|0,2,5
|Unknown Function.
|Unknown Function.
*Does lots of bit shifting and byte swapping then handles the rest of itself as command $10.
<pre>Message Bytes:
[11] [count=7F] [...5 byte chunk...] [...41 byte chunk 0...] [...41 byte chunk 1...] [...41 byte chunk 2...]</pre>
*Note that the 3rd byte 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
|$12
|$F773
|$F773
|?
|>= 3<br/><= 252
|Write Only
|Write Only
|0,2,5
|0,2,5
|Unknown Function.
|Unknown Function.
*Does lots of bit shifting and XORs.
<pre>Message Bytes:
[12] [count=N] [...3 byte chunk...] [...N-2 bytes...]</pre>
*Note that the 3rd byte is not ignored in this command like it is in most others, though it still does not count towards the byte count.
*Does lots of bit shifting and XORs for each byte in the N-2 byte chunk.
*Disables and re-enables interrupts.
*Disables and re-enables interrupts.
*Doesn't seem to write to any registers.
*Doesn't seem to write to any registers.
*For all commands, when command byte counts fail, the code seems to store those failed messages in a log / "black box".
**It seems this command is for manually creating a log entry.
|-
|-
|$40
|$40
|$F7AF
|$F7AF
|?
|>= 1<br/><= 252
|Write only
|Write Only
|2
|2
|Unknown Function.
|Unknown Function.
*Writes data directly into memory starting at the location pointed to by $0000:0001.
<pre>Message Bytes:
**Not known what sets that pointer.
[40] [count=N] [xx] [...N-1 bytes...]</pre>
*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.
|-
|-
|$41
|$41
|$F7D0
|$F7D0
|?
|>= 1<br/><= 100
|Write Only
|Write Only
|5
|5
|Unknown Function.
|Unknown Function.
*Similar to command 40 but writing at a lot more places with repeating data.
<pre>Message Bytes:
[41] [count=N] [xx] [...N bytes...]</pre>
*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
|$60
Line 1,644: Line 1,699:
|0
|0
|Unknown Function.
|Unknown Function.
<pre>Message Bytes:
[60] [count=06] [byte 0] [byte 1] [byte 2] [byte 3] [byte 4] [byte 5]</pre>
*Note that the 3rd byte is not ignored in this command like it is in most others.
*The 6 bytes written by this command get copied to CPU2 RAM starting at address $83.
*The 6 bytes written by this command get copied to CPU2 RAM starting at address $83.
*The mystery byte is the first of the 6 bytes in this command.
|-
|-
|$61
|$61
Line 1,653: Line 1,710:
|1,3,4
|1,3,4
|Unknown Function.
|Unknown Function.
<pre>Message Bytes:
[61] [count=00]</pre>
*This command writes #$00 to CPU2 register $4102.
*This command writes #$00 to CPU2 register $4102.
*(incomplete description)
*Changes to Command Mode 0 when complete.
|-
|-
|$62
|$62
Line 1,662: Line 1,721:
|0,2,5
|0,2,5
|Unknown Function.
|Unknown Function.
<pre>Message Bytes:
[62] [count=01] [xx] [parameter]</pre>
*xx byte is ignored.
*Disables interrupts while executing this command.
*Disables interrupts while executing this command.
*Valid values of the data byte are 00 and 01.
*Valid values of the parameter are 00 and 01.
**Other values reject the command.
**Other values abort the command.
*Appears to write to $4120
*Appears to write to $4120
|-
|-
Line 1,672: Line 1,734:
|Write Only
|Write Only
|0,1,2,3,<br/>4,5
|0,1,2,3,<br/>4,5
|Unknown Function.
|NOP Command
*Seems to be an empty/NOP function.
<pre>Message Bytes:
[63] [count=00]</pre>
*This command only validates the byte count = 00 and is then complete.
|-
|-
|$64
|$64
Line 1,680: Line 1,744:
|Write Only
|Write Only
|0,2,5
|0,2,5
|Unknown Function.
|Modem P4-27 Control
<pre>Message Bytes:
[64] [count=01] [xx] [P4-27 mode]</pre>
*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.
*PiT Motorboat Race uses this command when closing a connection.
*The data byte gets written to $004E.
*If the data byte is 00, it exits.
*Else if the data byte is 01, it clears bit 4 in register $4127 (Modem P4-27) and writes 00 to $0049.
*Else it sets bit 4 in $4127 (Modem P4-27) and writes 00 to $0049.
*Note: This description is fairly complete.
*Note: This description is fairly complete.
|-
|-
Line 1,693: Line 1,761:
|Write Only
|Write Only
|0,2,5
|0,2,5
|Unknown Function.
|Modem P4-21 and P4-19 Control
<pre>Message Bytes:
[65] [count=02] [xx] [P4-21 mode] [P4-19 mode]</pre>
*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 uses this command when closing a connection.
*Heart no Benrikun Mini uses this command when closing a connection.
*First data byte:
**Gets written to $004F.
**If it equals 00, it directly goes to check the second data byte.
**Else if it is 01, it clears bit 6 in register $4127 (Modem P4-21), then checks the second data byte.
**Else it sets bit 6 in $4127 (Modem P4-21), then checks the second data byte.
*Second data byte:
**Gets written to $0050.
**If it equals 00, it exits.
**Else if it is 01, it clears bit 7 in register $4127 (Modem P4-19)
**Else it sets bit 7 in $4127 (Modem P4-19)
*Note: This description is fairly complete.
*Note: This description is fairly complete.
|-
|-
Line 1,713: Line 1,784:
|0,2,5
|0,2,5
|LED Control
|LED Control
*The first data byte is the Red LED mode, and the second LED is the Green LED mode.
<pre>Message Bytes:
**Mode 00 = ? (Does not immediately change the LED, but saves the mode as 00)
[66] [count=02] [xx] [red LED mode] [green LED mode]</pre>
**Mode 01 = Set LED on
*xx byte is ignored.
**Mode 02-7F = Set LED off
*Red LED Mode Values:
**Mode 80-FF = Set LED to be controlled automatically by network status
**00 = Don't update but save mode as 00.
<br/>
**01 = Set Red LED on.
Original observations:
**02-7F = Set Red LED off.
*First data byte:
**80-FF = Set Red LED to be controlled automatically by network status.
**Gets written to $004C.
**Value gets saved to $004C.
**If it equals 00, it directly goes to check the second data byte.
*Green LED Mode Values:
**Else If Bit 7 = 1, it may set bit 1 (Red LED) and/or clear bit 2 (Green LED) in $4127
**00 = Don't update but save mode as 00.
***This logic depends on something elsewhere having controlled RAM $004B and $004D.
**01 = Set Green LED on.
**Else if it is 01, it clears bit 1 in register $4127 (Red LED), then checks the second data byte.
**02-7F = Set Green LED off.
**Else it sets bit 1 in $4127 (Red LED), then checks the second data byte.
**80-FF = Set Green LED to be controlled automatically by network status.
*Second data byte:
**Value gets saved to $004D.
**Gets written to $004D.
**If it equals 00, it exits.
**Else If Bit 7 = 1, it sets bit 2 of $4127 (Green LED) to the inverse value of RAM $004B bit 0.
***This logic depends on something elsewhere having controlled RAM $004B.
**Else if it is 01, it clears bit 2 in register $4127 (Green LED)
**Else it sets bit 2 in $4127 (Green LED)
*Note: This description is fairly complete.
*Note: This description is fairly complete.
|-
|-
Line 1,785: Line 1,850:
|$7B
|$7B
|$F994
|$F994
|?
|No<br/>Check
|Write Only
|Write Only
|0,1,2,3,<br/>4,5
|0,1,2,3,<br/>4,5
Line 1,797: Line 1,862:
|$7C
|$7C
|$F9AC
|$F9AC
|?
|>= 3
|?
|?
|0,1,2,3,<br/>4,5
|0,1,2,3,<br/>4,5
|Unknown Function.
|Unknown Function.
*Behaves similar to command $7D.
|-
|-
|$7D
|$7D
|$F9D9
|$F9D9
|Variable
|>= 3
|Write Only
|Write Only
|0,1,2,3,<br/>4,5
|0,1,2,3,<br/>4,5
|Arbitrary Memory Write
|Arbitrary Memory Write
*7D, Count, xx, xx, xx, Address MSB, Address LSB, ...Data to start writing at that address...
*7D, Count, xx, xx, xx, Address MSB, Address LSB, ...Data to start writing at that address...
*xx bytes are ignored.
*xx bytes are validated, maybe an unlock sequence.
*Byte count - 5 is the number of bytes to write to memory.
*Byte count - 5 is the number of bytes to write to memory.
*Writes $00 to $006E.
*Writes $00 to $006E.
Line 1,815: Line 1,881:
|$7E
|$7E
|$F9FE
|$F9FE
|?
|>= 3
|Write Only
|Write Only
|0,1,2,3,<br/>4,5
|0,1,2,3,<br/>4,5
Line 1,824: Line 1,890:
|$7F
|$7F
|$FA16
|$FA16
|?
|>= 3
|Write Only
|Write Only
|0,1,2,3,<br/>4,5
|0,1,2,3,<br/>4,5

Revision as of 05:29, 14 February 2021

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
  • 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
||||++---- 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.
  • 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
+++++++--- (unknown)

Read
76543210
++++++++-- Bits not shown to exist
  • 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)
|||||+---- EXP 4 = !($40A4.2)
+++++----- (unknown)

Read
76543210
++++++++-- Bits not shown to exist
  • Bench test found open bus when reading this register.
  • 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
  • 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
++++++++-- 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.
  • 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
  • Bench test observed value $00 with pull-ups.
  • 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
  • Bench test found open bus when reading this register.
  • Reading this register resets the value of $40A9 back to $00.
  • 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
++++++++-- Bits not shown to exist
  • Bench test:
    • Observed $00 with pull-downs and $FF with pull-ups (totally open bus).
    • Reading this register prevents timed toggle on RF5C66-69 (see notes in pinout).
  • 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:
             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
  • 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:
|||||||      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.
  • 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
+++++++--- (unknown)

Read
76543210
++++++++-- Bits not shown to exist
  • All reads reset the Kanji auto-increment counter.
  • Bench test found open bus when reading this register.
  • 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.
  • 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
||||||+--- Modem Module pin 32 = $40B1.1, rises slowly, goes low fast
|||||+---- Modem Module pin 31 = $40B1.2, rises slowly, goes low fast
||||+----- CPU2 Reset
||||         CPU2 /Reset = !($40B1.3)
||||         CPU2 runs when $40B1.3 = 0.
|||+------ Exp 15 = $40B1.4, rises slowly, goes low fast
||+------- Exp 14 = $40B1.5, rises slowly, goes low fast
|+-------- Exp 13 = $40B1.6, rises slowly, goes low fast
+--------- Exp 12 = $40B1.7, rises slowly, goes low fast

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
  • 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:
|||||||      RAM +CE Enable (1 = enabled, 0 = disabled)
||||||+--- Pin 5C66.36 = $40C0.1:
||||||       (This pin normally n/c)
|||||+---- Pin 5C66.37 = $40C0.2:
|||||        (This pin normally n/c)
||||+----- Pin 5C66.38 = $40C0.3:
||||         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.
  • Bench test observed value $00 with pull-downs, $70 with pull-ups.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
|||+++++-- Bits not shown to exist
+++------- 3-bit value read here was written by CPU2 to register $4122.
  • 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 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)
  • 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 Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
||++++++-- Bits exist but function is unknown
++-------- Bits not shown to exist
  • $40Dx registers can be monitored in real-time by statically giving the RF5A18 the bus signals for a read from this register.
  • Bits 7,6 follow pull-ups and downs on the data bus.
  • 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, measured 19.20kHz (19.6608MHz CPU2 crystal / 1024)
  • 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)
  • 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.
  • 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
  • Bits 7,6 follow pull-ups and downs on the data bus.
  • Bits 2,1,0 only observed with values '111' regardless of all registers being continuously written with inverting values on CPU2.
  • Writing a value to this register does not seem to cause the read value to change.

RF5A18 Internal CPU

Note that the RF5A18's 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 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 Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
  • Read Bit Existence:
    • Values $41, $40, and $BF from previous fetch observed unaffected.
  • ROM $E12E writes value #$06 to this address.
$4101 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
  • Read Bit Existence:
    • Values $41, $40, and $BF from previous fetch observed unaffected.
  • ROM $E131 writes value #$00 to this address.
$4102 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
  • 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.
  • ROM $F840 writes value #$00 to this address.
$4103 Yes Yes Unknown Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
|||||||+-- Bit exists but function is unknown
+++++++--- (unknown)
  • 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 address in the NMI handler and throws away the value.
  • ROM $F3D9 reads from this address and throws away the value.
$4104 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
  • Read Bit Existence:
    • Values $41, $40, and $BF from previous fetch observed unaffected.
  • ROM $F415 writes value #$FD to this address.
$4105 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
  • Read Bit Existence:
    • Values $41, $40, and $BF from previous fetch observed unaffected.
  • ROM $F410 writes value #$2F to this address.
$4106 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
  • 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 Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
|||||||+-- (unknown)
||||||+--- Bit exists but function is unknown
++++++---- (unknown)
  • 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 address and throws away the value.
  • ROM $F3E1 reads from this address and throws away the value.
$4110 Unknown Yes Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- Bits exist but function is unknown
  • 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.
  • ROM $E72F writes to this address.
  • ROM $E754 writes to this address.
$4111 Unknown Yes Yes Unknown Function. 
Write
76543210
|+++++++-- (unknown)
+--------- Pin 90 (Modem TXD) = !($4111.7)

Read
76543210
++++++++-- Bits exists but function is unknown
  • 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.
  • ROM $E151 writes to this address.
  • ROM $ECE3 writes to this address.
  • ROM $ED20 writes value to this address.
  • ROM $ED4E writes value #$81 to this address, then OR's that with #$03, then writes the #$83.
  • ROM $EE83 writes a value to this address with bit 0 set to 1.
  • ROM $F0E8 reads from this address, AND's it with #$FE, and writes it back to this address.
  • ROM $F2DC writes a value to this address with bit 0 set to 1.
$4112 Unknown Yes Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- Bits exist but function is unknown
  • 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 $E159 writes value #$80 to this address.
  • ROM $E4D8 reads from this address in the IRQ handler and uses bit 0.
  • ROM $E4F9 writes to this address in the IRQ handler.
  • 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.
$4113 Unknown Yes Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- Bits exist but function is unknown
  • 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 $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.
  • 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)
  • 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 (Modem AD0) = $4120.0
||||||+--- Pin 38 (Modem AD1) = $4120.1
|||||+---- Pin 37 (n/c) = $4120.2
||||+----- Pin 36 (Modem EXCLK) = $4120.3
|||+------ Pin 35 (Modem /WR) = $4120.4
||+------- Pin 34 (Modem /RD) = $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)
  • 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 OF $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 (Modem Data) = $4121.0 when set as output (refer to $4120.6)
+++++++--- (unknown)

Read
76543210
++++++++-- Bits exist but function is unknown
  • 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
|||+++++-- (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 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.
  • Note: 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 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.
  • Note: 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 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.
  • Note: 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 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.
  • Note: 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)
  • 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 Unknown Yes Unknown Function. 
Write
76543210
|||||||+-- Pin 56 (Modem Reset) = $4127.0
||||||+--- Pin 57 (Red LED) = $4127.1
|||||+---- Pin 58 (Green LED) = $4127.2
||||+----- Pin 59 (n/c) = $4127.3
|||+------ Pin 60 (Modem P4-27) = $4127.4
||+------- Pin 61 (Modem Audio Enable) = $4127.5
|+-------- Pin 62 (Modem P4-21) = $4127.6
+--------- Pin 63 (Modem P4-19) = $4127.7

Read
76543210
++++++++-- (unknown)
  • Read Bit Existence:
    • Values $41, $40, $FF, and $BF from previous fetch observed unaffected.
  • ROM $E237 writes value #$14 to this address.
  • ROM $E24D writes value #$14 to this address.
  • ROM $E263 writes value #$14 to this address.
  • ROM $E27D writes value #$14 to this address.
  • ROM $E29A writes value #$14 to this address.
  • ROM $E3B0 writes value #$14 to this address.
  • ROM $E987 writes value #$14 to this address.
  • ROM $EA83 writes value #$14 to this address.
  • ROM $F3BF writes value #$FF to this address, then writes again with value #$FE.
$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)
  • 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
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
  • 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.
  • 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.
  • 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
++++++++-- (unknown)

Read
76543210
|||||||+-- Bit exists but function is unknown
|||||++--- (unknown)
||||+----- Bit exists but function is unknown
|||+------ $412F.4 = !(Pin 33) (Modem /INT)
+++------- Bits exist but function is unknown
  • 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 $E31C writes to this address.
  • ROM #E33B writes to this address.
  • 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 to this address.
  • ROM $F99C writes value #$00 to this address.
$4130 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

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

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

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

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

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

Read
76543210
++++++++-- (unknown)
  • Read Bit Existence:
    • Values $41, $40, and $BE from previous fetch observed unaffected.
  • ROM $F74A writes value #$00 to this address.
$4137 Unknown Unknown Yes Unknown Function. 
Write
76543210
++++++++-- (unknown)

Read
76543210
++++++++-- (unknown)
  • 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 Command Bytes

Note that most descriptions are incomplete.

Command
Byte 		CPU2
Handler
Address 		Byte
Count
Expected 		Type Modes
Supported 		Description
$00 $F43A >= 0
<= 60 		Write-Read 0 Unknown Function. 
Message Bytes:
[00] [count=N] [xx] [...N bytes...]
  • xx byte is ignored.
  • Super Mario Club, Heart no Benrikun Mini, and JRA-PAT use this command when opening a connection.
  • Writes #$06 to $4100
  • Writes #$00 to $4101
  • Writes #$02 to $4102
  • Changes to Command Mode 1 when complete.
$01 $F46C 1 Write Only 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 uses this command when closing a connection.
  • 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 Write Only 0 Unknown Function. 
Message Bytes:
[02] [count=02] [xx] [parameter 1] [parameter 2]
  • xx byte is ignored.
  • Parameter 1 gets written to $002E.
  • Parameter 2 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]
  • Writes to $4120
  • Reads from $4121
  • Reads from $4126
  • Writes to $412F
$10 $F71A 40 Write Only 0,2,5 Unknown Function. 
Message Bytes:
[10] [count=28] [parameter] [...40 bytes...]
  • Note that the 3rd byte ("parameter") is not completely ignored in this command like it is in most others, though it still does not count towards the byte count.
  • Parameter Value:
    • Special case $10 acts different.
    • Otherwise, only bits 2,1,0 of the parameter are used.
  • 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 Write Only 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 3rd byte 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 		Write Only 0,2,5 Unknown Function. 
Message Bytes:
[12] [count=N] [...3 byte chunk...] [...N-2 bytes...]
  • Note that the 3rd byte is not ignored in this command like it is in most others, though it still does not count towards the byte count.
  • Does lots of bit shifting and XORs for each byte in the N-2 byte chunk.
  • Disables and re-enables interrupts.
  • Doesn't seem to write to any registers.
  • For all commands, when command byte counts fail, the code seems to store those failed messages in a log / "black box".
    • It seems this command is for manually creating a log entry.
$40 $F7AF >= 1
<= 252 		Write Only 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.
$41 $F7D0 >= 1
<= 100 		Write Only 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 Write Only 0 Unknown Function. 
Message Bytes:
[60] [count=06] [byte 0] [byte 1] [byte 2] [byte 3] [byte 4] [byte 5]
  • Note that the 3rd byte is not ignored in this command like it is in most others.
  • The 6 bytes written by this command get copied to CPU2 RAM starting at address $83.
$61 $F839 0 Write-Read 1,3,4 Unknown Function. 
Message Bytes:
[61] [count=00]
  • This command writes #$00 to CPU2 register $4102.
  • Changes to Command Mode 0 when complete.
$62 $F849 1 Write Only 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 are 00 and 01.
    • Other values abort the command.
  • Appears to write to $4120
$63 $F887 0 Write Only 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 Write Only 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.
  • Note: This description is fairly complete.
$65 $F89D 2 Write Only 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 uses this command when closing a connection.
  • Note: This description is fairly complete.
$66 $F8B7 2 Write Only 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.
  • Note: This description is fairly complete.
$67 $F8D1 2 Write Only 0,5 Unknown Function.
  • If the first data byte is 00:
    • Writes lots of stuff to $4127.
    • Writes 02 to $4114.
    • Writes to $4111, 4112, 4113.
    • Writes to $4120, 4121, reads from $4121.
    • Writes 00 to $0051, 0076, 0077.
  • Else if the first data byte is not 00:
    • Writes the first data byte to $74
    • Writes the second data byte to $75
    • Writes lots of stuff to $4127.
$68 $F912 1 ? 0,5 Unknown Function.
$69 $F951 10 Write Only 0 Unknown Function.
  • PiT Motorboat Race uses this command when opening a connection.
  • The 10 bytes includes the mystery byte.
  • The data starting at the mystery byte is copied to CPU2 RAM starting at $89.
  • The 7th byte of the message ($0409 -> $8F) may get overwritten by a 01 depending on its original value.
$6A $F96F 80 ? 0,2,5 Unknown Function.
  • Takes the first 40 bytes and copies them starting at RAM $035F
  • Takes the negative (2's complement) of the second 40 bytes and stores them starting at RAM $038F
  • Writes 00 to $0388 and $03B8.
  • Writes 10 to $0055.
  • Note: This description is fairly complete.
$7B $F994 No
Check 		Write Only 0,1,2,3,
4,5 		Software Reset
  • Updates the green LED (turns it ON when set to automatic control??).
  • 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 >= 3 ? 0,1,2,3,
4,5 		Unknown Function.
  • Behaves similar to command $7D.
$7D $F9D9 >= 3 Write Only 0,1,2,3,
4,5 		Arbitrary Memory Write
  • 7D, Count, xx, xx, xx, Address MSB, Address LSB, ...Data to start writing at that address...
  • xx bytes are validated, maybe an unlock sequence.
  • Byte count - 5 is the number of bytes to write to memory.
  • Writes $00 to $006E.
$7E $F9FE >= 3 Write Only 0,1,2,3,
4,5 		Unknown Function.
  • Writes a lot of stuff to $4120 and $4121
  • Writes to $4127 bit 5 (Modem Audio Enable)
$7F $FA16 >= 3 Write Only 0,1,2,3,
4,5 		Unknown Function.
  • If the mystery byte is $00-7F, write value $0C out the P5 Expansion Port.
  • If the mystery byte is $80-FF, dump RAM $0240-$0280 out the P5 Expansion Port.
    • The P5 Expansion port sequences out 1 nybble at a time.

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        Orientation:
    CHR RAM /CE (input) -> \ 34                                 67 / <> Exp P3-12           --------------------
                 RAM +CE <- \ 35                               66 / <> Exp P3-13                80         51
            (n/c) $40C0.1 <- \ 36                             65 / <> Exp P3-14                  |         |
             (n/c) $40C0.2 <- \ 37                           64 / <> Exp P3-15                  .-----------.
                CHR RAM /CE <- \ 38                         63 / <- $40B1.3 (n/c)            81-|O Nintendo |-50
                         GND -- \ 39                       62 / <> Modem P4-31                  |  RF5C66   |
Built-in RAM /CE ($6000-7FFF) <- \ 40                     61 / <> Modem P4-32               100-|  GCD 4R  O|-31
      (n/c) ? /CE ($4xE0-4xEF) <- \ 41                   60 / <> Modem P4-29                    \-----------'
       5A18-85 /CE ($4xD0-4xDF) <- \ 42                 59 / -- +5Vcc                            |         |
                         (GND) ? -> \ 43               58 / -- n/c                              01         30
                          (GND) ? -> \ 44             57 / -> Kanji ROM A17
                           (GND) ? -> \ 45           56 / -> Kanji ROM A4         Legend:
                            (GND) ? -> \ 46         55 / -> Kanji ROM A3          ------------------------------
                           CIRAM A10 <- \ 47       54 / -> Kanji ROM A2           --[RF5C66]-- Power
                              PPU A11 -> \ 48     53 / -> Kanji ROM A1            ->[RF5C66]<- RF5C66 input
                               PPU A10 -> \ 49   52 / -> Kanji ROM A0             <-[RF5C66]-> RF5C66 output
             Kanji ROM /CE ($5000-5FFF) <- \ 50 51 / -- n/c                       <>[RF5C66]<> Bidirectional
                                            \     /                               ??[RF5C66]?? Unknown
                                             \   /                                    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 \ -> Modem TXD
                            CPU2 A10 <- / 12                       89 \ <- Modem 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
         Modem DATA <> \ 32                                     69 / <- Tone Rx D1
          Modem /INT -> \ 33                                   68 / -> Tone Rx GT         Orientation:
            Modem /RD <- \ 34                                 67 / -> Exp P3-19           --------------------
             Modem /WR <- \ 35                               66 / <- Exp P3-18                80         51
            Modem EXCLK <- \ 36                             65 / <- Exp P3-17                  |         |
           (n/c) $4120.2 <- \ 37                           64 / -- +5Vcc                      .-----------.
                Modem AD1 <> \ 38                         63 / -> Modem P4-19              81-|O  RF5A18  |-50
                 Modem AD0 <> \ 39                       62 / -> Modem P4-21                  |  Nintendo |
                    CPU2 D0 <- \ 40                     61 / -> Modem Audio Enable        100-|  GCD 8C  O|-31
                     (n/c) ? <- \ 41                   60 / -> Modem P4-27                    \-----------'
       (4.9152 MHz) Exp P3-16 <- \ 42                 59 / -> $4127.3 (n/c)                    |         |
                           GND -- \ 43               58 / -> Green LED, active low            01         30
                19.6608MHz Xtal -- \ 44             57 / -> Red LED, active low
                      1k to Xtal -- \ 45           56 / -> Modem Reset          Legend:
                              GND -- \ 46         55 / <- Modem P4-23           ------------------------------
                   (+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.

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
Modem Reset -> | 2   20 | <- Tone Rx GT
  Modem AD0 <> | 3   21 | <- 5A18-62
        GND -- | 4   22 | -> Tone Rx DV
  Modem AD1 <> | 5   23 | -> 5A18-55
  Modem RXD <- | 6   24 | <- Tone Rx Xin, from 5C66-26
 Modem DATA <- | 7   25 | -> 5A18-53
  Modem TXD -> | 8   26 | -- GND
  Modem /WR -> | 9   27 | <- 5A18-60
  Modem /RD -> | 10  28 | <- 5A18-54
Modem EXCLK -> | 11  29 | <> 5C66-60
      +5Vcc -- | 12  30 | <- Modem Audio Enable, 1 = enable
 Tone Rx D1 <- | 13  31 | <> 5C66-62           __________________________
 Modem /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

Retrieved from "https://www.nesdev.org/w/index.php?title=Famicom_Network_System&oldid=2731"