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Old March 13th, 2011, 07:06 AM   #1
mureed
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AB PLC 5 Processor fault

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.
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Old March 13th, 2011, 07:19 AM   #2
daba
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Originally Posted by mureed View Post
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.
The PLC has a "Major Fault", and you will need to get online and investigate what that fault is.

Because you can see data, I don't think you have lost the program.

Short-term solution if you can't get online.... try resetting the Major Fault by toggling the keyswitch back and forth several times through all 3 switch positions rapidly - this may clear the fault and allow the PLC to re-start.
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Old March 13th, 2011, 11:37 AM   #3
dmroeder
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Originally Posted by daba View Post
Short-term solution if you can't get online.... try resetting the Major Fault by toggling the keyswitch back and forth several times through all 3 switch positions rapidly - this may clear the fault and allow the PLC to re-start.
Watching the LED's on the newer Logix platform, it appears that one key switch cycle (run > program > run) will clear the fault, then the next cycle puts it back into run mode. Of course, this will not work if the fault condition remains.

The trick is (as daba mentions) going to all 3 positions, program to remote just leaves it in program mode, it has to go back to run mode. Doing it 3 times will make sure that you did at least 2 complete cycles.
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Old March 13th, 2011, 06:10 PM   #4
daba
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But of course, the PLC has major faulted before, it may well do it again immediately, tomorrow, next week, in 6 months time.....

It will be far better to find out NOW what the fault is and deal with it. Resetting the fault (and its info) now, will make it very hard to remove the cause of the fault.
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