Hori Fighting Edge dual mod power issues

To mods: This is a double-post because my last one was auto-hidden after I edited one line for clarity, and now it’s been over 40 hours… Please forgive me…

So I popped a Brook Retro Board (for those with a UFB, it’s nearly identical) into my new Hori Fighting Edge PS4 version. Drilled a new hole for a Neutrik RJ45 on the back, because I want to keep the original PS4/PS5 functionality from the Hori board. Connected all the button and direction wires from the Hori board to the screw terminal block on the Brook Retro, and the 20-pin connector on the Brook is going to the actual buttons/stick. Both boards are common ground and the grounds are tied together.

Everything works great except: I need to have the USB from the original Hori board plugged into a power source for the Retro Board to work.

This doesn’t make any sense to me. Testing with a PS1, the Retro Board is getting power on its own, the LED lights up, but no buttons do anything. I have to plug the USB into my computer, restart the PS1, then the Retro Board works.

I don’t have their power lines tied together, but I gave that a shot as well, connecting a line from the Retro Board’s VCC to first a VCC pad on the Hori board, and then the 5V pin on the bottom of the USB connector. Neither made the Retro Board work on its own, and when the USB was plugged in there was some nice power leakage into the controller port of the PS1, causing the LED to be on when it was off. I never wanted to tie the power together on these, especially since I’m not sure what’s 5V and what’s 3.3V.

I don’t see why the Hori board would need any power for the Retro Board to operate, especially when the buttons aren’t even connected directly to it. Everything I read for a dual mod said only ever plug in one cable at a time, so I’m operating completely counter-intuitively.

Here’s a pinout I made for myself for the Hori board: https://imgur.com/a/K16pM6b

I can post some pictures of my rat’s nest inside later if that’s helpful. Don’t see how anything could be hooked up wrong considering it works perfectly when the Hori board has USB power…

EDIT: This also occurs with Saturn and SNES now that I have cables for them as well. I also tried unplugging everything connected to the Hori board internally with no difference, and have confirmed that I can start the retro console, then unplug the Hori’s USB cord, and everything will also work fine.

EDIT2: Redid my block terminal wiring and determined that if ANY wire is connected to the Start terminal on the Retro Board, the Hori board suddenly requires power. So I’m going to probably have to do an RMA on it.

EDIT3: Well this forum seems dead but I might have a solution… For one thing I didn’t read enough about dual modding and you’re supposed to tie power together for all boards, or button logic won’t be driven correctly and you’ll get nondeterministic results.

With power tied together, the Hori board works for a second or two, then all the signal wires drop to being permanently low, messing up all input. If the two data pins are shorted on the USB connector, the Hori board will power back up and drive the logic properly… Looking into it this is actually what USB chargers do so that they don’t have to do any sort of data transfer. Another option that works and I’ll be trying to hook up to a switch tomorrow, is driving one of the USB data pins low and keeping it there (ie shorted to ground). The USB standard is supposed to be able to withstand shorts between any pins, so I believe electrically this will be fine. If it ever goes high the Hori board “powers down” so the switch would merely need to be on prior to plugging into a retro console.