Thanks, Ross. I'm still digesting your latest brain dump, but do please keep them coming. I'm sure everyone else here also enjoys them!

Meanwhile, I decided that the only way to improve the layout of my circuit was to scrap the preassembled L293D boards. They are fun to play with, and have blinken lights for debugging and all, but they also lack any attempt at EMI suppression. Not a single cap on board either of them.

My last order from Digikey included a couple of raw L293D chips, so I just plopped them down on the breadboard, and started routing signals again.

This time I was careful to keep the logic and motor power buses completely separate, as well as maintaining the full isolation to the USB sub-assembly.

I applied a generous spinkling of little through-hole ferrite beads, and some little blue capacitors labeled "693M", as well as my limited stock of 0.1uF ceramic discs.

Woo-hoo, it works!

I can power the motor & electronics from the same ATX supply that the PC and USB end use, and have not managed to trip up the USB at all, despite furious attempts to do so.

Here are some photos of the latest layout.