I'd like to support the cheaper 74HC165 SOP as a 4021 replacement, as it would free up a lot of space on the PCB for traces, but it's unclear if it's fully compatible. It might need current limiting resistors on the Y lines and some pull-ups on the select and enable pins (see "?" areas). The PC-engine's original controller had no autofire switches (nor does the NES pad) but I can't find PCB photos of it to work from. Hard to tell by pics of the SMD PCB but the through-hole PCB has it there. R7 in the 3 button MD pad seems to pull-up the select pin (1). I had to make some guesses with the schematics. In some cases the resistor pads could be given both SMD pad and through-holes. Suitable resistors might be 10K and 1K for pull-ups and current limiting, but the 555 might need larger values to remain compact.
I've added a minimal 555 circuit so one of the unused buttons can act as autofire of some hardwired frequency. With an Amiga/Atari/MSX/SMS, no IC or RP would be needed as it's just dumb switches, so only the relevant DE-9 wires would have to be soldered on. The MCU has a few breakouts for custom connections, USB, and some exposed ICSP stuff. if there are, I suppose a few solderbridge-jumpers could be used (1A-2A-GND in MD circuit?). Omitted ICs would leave those traces unconnected so there would be no conflicts that I can think of. The idea is that one would pretty much just solder on the required IC, a resistor network, then the wires for the new cable (likely a DE-9 or mini din). The layout needs some work and I might've made some errors on the schematics. Some of the cheaper joypads do not have it.Ī swap-in PCB for the NES joypad shell. Unsure if the pads need to be coated with the black carbon(?) stuff or not.
Custom PCBs are very cheap nowadays, unlike making plastics. I've had this idea for a while of making compatible PCB with SMD(?) pads for a shift reg (4021), MD/PCE mux (74HC157), or plain Amiga/Atari wiring. PCB traces were hand-drawn back in the day so it's rather curvy looking.
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