Namco Serial Checker Interface
The Namco Serial Checker Interface is a proprietary Famicom expansion port device used for some cartridge games developed by Namco. Supported games send and receive data using this interface to check if a cartridge self-test should be performed at power-on/reset.
As the interface hardware was never released commercially, the information on this page is based on analysis of disassembled code from supported games.
Interface
Input ($4016 write)
7 bit 0
---- ----
xxxx xxxI
|
+- Data to interface input
Output ($4017 read)
7 bit 0
---- ----
xxxx Dxxx
|
|
|
+---- Data from interface output
The interface appears to consist of a 4-bit shift register (initial contents are undefined). Data sent to the interface will appear in the output after 4 reads. The received data is logically inverted from what was sent due to how the expansion port inputs work.
Usage
Games using the interface do the following at some point in their reset handler (the specific code may differ):
- Fetch a source byte.
- Initialise a target byte.
- Read the data bit from $4017.d3 and shift it into the target byte.
- Shift one bit out from the source byte and write it to $4016.d0.
- Repeat steps 3-4 11 more times. (i.e. 4 "dummy" bits + 8 data bits total)
- Check if the target byte matches the source byte XORed with $FF. Go to step 9 if false.
- Return to step 1 and repeat for however many source bytes remain.
- If all bytes pass the check, execute a cartridge self-test (partial PRG/CHR checksum verification).
- Continue with init tasks.
Note: The source data used is commonly a 32- or 64-byte string consisting of programmer credits, sometimes XORed with $FF.
See Also
- GitHub Gist - Lua script to simulate the interface in Mesen2.
- Namco IPL Interface - A similar serial interface used in FDS games developed by Namco.
