https://assemblergames.com/threads/ps1-pu-22-modchip-install-causes-disc-drive-to-stop-working.57545/#post-824923
Trimesh:

1 - VCC - Connect to any 3.3V supply
2 - CLK - Clock signal for MCU (4.2336 MHz from CD DSP)
3 - XLAT/ - Normally high, pulses low at the end of each command sent to the CD DSP
4 - RESET/ - Normally high, goes low when reset is asserted
5 - WFCK - Subcode write-frame clock. Synchronized with the subcode frames on the disc, used to chop the data signal up
6 - DATA - Transmitted modchip data - basically NRZ anded with the WFCK signal
7 - DOOR/ - Normally high, pulls low when the CD door is closed
8 - GND

The way the data line is handled is a bit nasty, but seems to be fairly reliable in practice - it's injected into the external passives used as part of the tracking error amplifier / filter. Obviously, this overrides the actual tracking error signal, which is why it's chopped up using WFCK so that 50% of the time the tracking error still gets through. This may sound like a problem, but it only produces a 3db decrease in servo gain, and the tracking servo has lots of margin.

The XLAT/ line is used to turn the modchip data stream off - basically, it has a timer that waits until the console should be reading the leadin then sends SCEx strings until XLAT/ is asserted - this works because at this point the mechacon isn't talking to the CD DSP because all the information it needs to build the ToC for the disc is coming over the SUBQ interface.