Thanks for that, Clive. I'm just finalising my choice which motion controller to use. Too many choices! However my current plan is to make a kind of motherboard with one of those 96-way connectors on it, that replaces the existing top board. The motion controller will then sit on this. I can then provide any BoB type functions on my mini-motherboard - like voltage level shifts or opto isolation. Feels like quite a neat way to tackle the problem, but time will tell. As you say, we sometimes start with a plan and then throw it all away and do it differently!!

Just out of interest, what electronics did you use? I can see a BoB on your photo, and maybe a couple of stepper drives? But not anything I could identify....

All the best
Graeme