I plan to release the electronic diagram for the multitap.
That is really simple, the multitap uses the 2 ports of the Neo Geo, 2 controllers being multiplexed on each port. I use the same principles as the Majhong controller, the pins OUT 1 and 2 are switched on/off to select the relevant inputs.
But I think then you can only read alternatively each pair of controllers every other frame (thus a 1 frame delay in 4P mode).
Correct me if I'm wrong.
Could even be 4 frame delay if you read all 4 controllers from port 1.