We have a AB PLC 5/??? showing fault LED RED ON. Operator does not seem to be able to change set point on PIDs and PLC is not responding to these setpoint changes or to the ON/OFF commands from HMI display. The comms are still working because when we removed the comm cable from the back of the HMI PC, we lost what ever data were diplayed on the screen.When we put back the cable, data lost came back on the screen. Technican told me that they repalced the Processor battery and we switched from the run mode to Prog and remote but no success.