help needed with my set up

[cropwell]

Rob you are getting old like me It is the other way round, it needs the 12-24v for the inputs to work

So why bother with a 5v USB connection from the PC ? Either way you need the 12-24v and the OP from his latest post indicates he has not deemed it necessary.

Somewhere in the deep brainfog of age I have a memory of a simple BOB circuit. I must look it out on t'Interweb and try and understand it better.

http://www.embeddedtronics.com (Several points of interest in this site !)

Aha! there it is - looking at the schematic pdf, now I understand the 5v from the PC is used to lift the output of the optos which feed homes, limits and Estop info back to the PC and power the input opto LEDs.