Music Synthesis led me into microprocessors in the late 70s. Around the time I built an Elf for synth control (an unrealized dream eclipsed by college then by the C64 then by Midi ) I was buying bits and pieces from PAiA and inspired by their designer John Simonton and contributor Craig Anderton. At that time they were offering a microprocessor controller add-on, very much like a trainer, for their modular synth rigs and selling bits of software for doing effects and controlling keyboards actions like portamento and the like. My recollection was that these were 6800 systems just like on the small cardboard boxed Heathkit 6800 trainer. My recollection was off. I just ran across 1978 PAiA catalog to find it was a 6502 variant with membrane keys instead of switches. http://rtellason.com/catalogs/PAIA-1978.pdf The controller is on the cover page and detailed on page 11. The whole catalog is a hoot of hobby electronic music equipment of the era. A short article on PAiA is here for orientation of folks not familiar with PAiA. https://www.revolvy.com/main/index.php?s=PAiA%20Electronics&item_type=topic