So if you want to use the smaller display, it's as easy as plugging that one onto the cable?

Depending on the display you'd need slightly tweaked driver code.. the 3900 (updated 3100) and 7000-series I have have mostly identical commands (but the 7000 supports a subset of the 3900's), whereas the K610a that foxtrot is using is very different (and pretty limited). It's not a big deal to change the code though, you just read the VFD documentation and adjust what it sends to, say, draw a rectangle.

Also some (like the K610a) don't accept a real RS-232 (-12 to +12) signal so you need a chip in there to convert to TTL-level (that's part of what foxtrot's board is for). Mine just passes through the out from the Empeg to the in on the display.

I'll see if I can get the 140x32 working and make a video.. the glass is a bit smaller than the Empeg's VFD, but it has a very tall control board behind it.

I'm assuming that you ignore any error codes/ready prompts from the VFD, then?

Yeah, I'm currently ignoring the display's output line (which you can read the 3900's 10 i/o pins on, read settings, etc).. it could be connected to the BASIC Stamp and then the stamp could pass the data back to the Empeg. I did have the 'busy' line connected to one of the serial control pins but it turns out it hasn't actually been needed for any of the drawing yet @ 38,400.