Thanks for your reply m_c
Are you (or other forum members) familiar with the EuroStep I/O controller? If yes does a limit error stop the serial data link from operating (i.e. a displayed L with flashing dot on the controller status display). I spent all day today messing with no progress. The PC DOS message I get is error 50 Mint controller disconnected. I will check all the limit switches. I suspect the Z Axis is at its lowest limit.
I have checked the RS232 cabling and set-up of the IRQ and com port addressing.