I finished the control board for this yesterday, and sorted out the multiswitches today -- three out of six were defective, and I ended up salvaging two of those using parts from the third.

The completed Frankenswitch v3 has four 4:1 switches tied to a single USB interface, with a pair of 74HCT139 demuxes providing fanout of the eight available GPIO lines, and a pair of 74HCT240 buffers acting as octal inverters to fix the inverted outputs of the '139 chips. Lots of wires/soldering to tie all of that together.

The switch is now installed in the Myth cabinet and working quite well. The remaining 4:1 switch (plus the dead one) is destined for a friend's Myth system.

Photos later, when the cabinet is next opened up to remove Frankenswitch v3.

Cheers