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I am attempting to add a remote hand wheel to a Bridgeport 610 XP3 CNC machine while leaving the local hand wheel in tact. I have added a selector switch that will select either the local or remote unit. I can use either the local or remote hand wheel depending on the position of the switch on power up. It also works when I switch from local to remote but when I switch back to local control, I get this alarm on the screen:

SYSTEM ALARM:
950 PMC SYSTEM ALARM
PC050 IOLINK CH1
4401-C401:4000

I have to cycle power to the machine to reset the fault.

Can anybody tell me what causes this fault? What can I do to prevent it?
Any help will be greatly appreciated.