The joystick mod was incomplete – there are some cut wires and an unused DPDT switch nearby that is unused. It’s possible that the mod was completed or stopped working and was disabled.
Commonly, a joystick was added by using the switches to tug down on D[4:0] using the joystick switches. Apparently, sometimes this wasn’t even gated with an address, but it looks like this mod had some address decoding.
Reverse engineering it shows three signals ANDed together. One of these would have been IN_L and the other two would have been address lines (both 0). This doesn’t fully decode the I/O space but does restrict it to a quarter of the space. This at least gets it away from the cassette interface and the high-res graphics at 255 and 254.
I found an instruction sheet for the “Alpha” joystick. It has the same data line assignments as the mod:
D0 UP
D1 DOWN
D2 LEFT
D3 RIGHT
D4 FIREThe instructions indicate the address is 0 which is again consistent.
It notes that some games expect FIRE to be indicated by D0 and D1 being low simultaneously (STICK-80 vs TRISSTICK). I expect that the toggle switch was used to configure the interface accordingly.
I wired it all up. Being something of a tangled mess of flying leads makes it very easy to break wires, so I’ve added an inline connector.
I used the alpha test program:
Alas – no joy. The scope showed that there was no IN strobe and this was because of a dwarf pulse at Z23-8 (74LS32). I replaced the IC and it came good.
Scarfman played ok!
