Brian,
Thanks for the suggestion. I think you are on the right track but so far grounding changes have not made a difference. Mr Rutex thinks I need to ground to the Campbell board as well. I still get good step pulses that look clean and the correct levels. I have tried switching the BOB with the jumper so that com is 0, connected to PC ground and to the limit switch ground but no joy. Any other ideas?
Thanks
Keith