Necro-edit: I got into a car accident shortly after posting this and never completed the mod, there's no conclusion, only my ponderings. This is before the Arcin existed, just get one of those or one of DJ Dao's newest version PCBs, they're both great.
I've been tasked with dual-modding a friend's PS2 "DJ Dao FP-7" beatmania IIDX arcade-style controller with a Cthulhu so that he can use it natively on his PC. (for those that know these controllers, this is the first, non-flippable revision of FP-7, basically one of DJ Dao's first controllers.) I'm going to try and make this as simple as I can so you guys can follow me. TLDR: What I'm puzzling over is, for this dual-mod, do I join the Cthulhu's VCC and GND to the PS2 side, or to the USB side? Or should I join all 3 together?
The controller has 2 cables coming out. One is a PS2 cord, one is a USB cord. USB has no data in the cord, only +5V and GND.
The controller has basically 2 PCBs inside: a rotary decoder PCB (takes in turntable input from 2 optical sensors and decodes quadrature output from sensors into discrete "turning CW/turning CCW" pulses) which also provides +5V and GND for the buttons and LEDs, and a PS2 PCB which takes in input from the buttons (and output from the decoder PCB) and does the typical PS2 communication.
Back to the cables. Both of them enter the PS2 PCB, and the USB +5V and GND immediately jump to the decoder PCB, which then gets power to do its thing, and also feeds USB +5V and GND to the buttons and LEDs. The PS2 cable's VCC and GND do not seem to join the USB cable's VCC and GND, I continuity-checked, and it appears that the PS2 VCC and GND power the PS2 IC only. USB +5V powers the decoder PCB, the optical sensors, and also runs to the LEDs (wired simply, so that when you press a button, you close the LED's GND, and it completes the button's circuit and LED's circuit at the same time). USB GND is GND for the buttons, LEDs, optical sensors, and decoder PCB.
With only the USB cord plugged in, the lights light up, and for sure the turntable PCB spits out its decoded pulses (verified with my multimeter). Yet to test behavior with only PS2 cord plugged in (I had my converter with me in the garage tonight, but completely forgot to use it).
So, as I said above... What I'm puzzling over is, for this dual-mod, do I join the Cthulhu's VCC and GND to the PS2 side, or to the USB side? Or should I join all 3 together?
I had actually previously attempted a similar dual mod. I have a DJ Dao Pop'n Music Arcade Style Controller, and its original PS2 PCB I used in the dualmod is the exact same one as the PS2 PCB in my friend's FP-7, with same separate USB +5V/GND and PS2 VCC/GND and everything. In my old dual-mod, I soldered the USB pad's +5V and GND to VCC and GND on the PS2 side. Everything worked fine for a few months (plug in USB pad and USB power and you get lights and inputs working perfectly), then suddenly, neither side worked. But a few things... that Dao PCB had taken a lot of abuse (poor strain relief meant the PS2 cord was yanked a lot, tearing the wires from the PCB over and over again that I kept having to repair) so that could be a cause, that it just decided to die (my repairwork back then wasn't so good so I could have lifted a trace or something). But I also later found out that the USB pad I had padhacked into it wasn't actually common-ground, so it's possible I joined some potentials that weren't supposed to be and blew everything up (no computers/USB ports died though). So I know there's not a lot to conclude from this anecdote as to whether my previous approach was OK or not, but I felt I should mention it.