PLC Fault Mode

did you check your ground?

Please refer to cards not as cards but as slots. Check your I/O map and let us know what type of card is in what slot. Remember to count your CPU. Better yet, can you get that map out here where we can look at it with you?

Guest,

I agree with RD... Allen-Bradley PLC's, like most of them, are very demanding. Their CPU's map of the I/O slot usage must exactly match the modules in each slot. The programming software will allow you to read into the CPU the current configuration of modules in the slots. In some cases, a minor mis-match will pass (such as a generic input or output module in the place of a specific one with the same number of points). With some of the specialty modules (such as RTD), slight differences in module number are important. Finally, NEVER EVER do a "hot swap". The power MUST be off when changing modules. Doing otherwise is a guaranteed way to fault the PLC.

In the programming software, you should see a link to the actual fault code and its explanation (as well as a short fault history). I normally do this by going to the Status file and selecting the tab for faults. This should also have a button to clear the fault. If the underlying cause was corrected (such as an incorrect I/O map), you should then be able to return it to Run Mode.

--JMM

You are fortunate to have the RSLogix software. Hopefully you have a copy of the program as well. If you can get a copy of the program (from a backup, an m/c that does the same operation or the OEM installer) then if all else fails, clear down the memory and reload the program.

First thing I would look at is the hardware tree on the left in your RSLogix500 program and specifically the module in question. I would guess that the replacement module you put in was a different revision then the old one you had. You will want to verify that whatever module you want to work in that slot that the hardware tree shows the correct model number and revision number. Sometimes you can get into a problem if it's a recently purchased card on an old system where the firmware in the processor doesn't support the rev of the new card and you may have to update the processors firmware.

As JMM stated you do not want to swap SLC modules under power but an interesting FYI, AB ControlLogix PLC modules can be removed and inserted under power (RIUP).

Shawn

I guess, I have a made mistake by hot swapping them.

Guest,

Such open honesty--very good. The hot swapping does not normally cause any physical damage to the module. But, it causes a fault within the PLC processor.

Reason: A PLC program is a sequence of logic instructions that are intended (normally) to be run over and over and over. When conditions on the inputs change then the most recent running of the logic will respond to these changes to do some control action (such as turning on a motor or changing a valve position, etc.). In general, the processor does four things in a sequential loop:

• Read the status of the inputs to an internal register.
• Use this internal input status to solve the logic in the program, rung by rung, with all the possible jumps, subroutines, etc. that the logic has; the logic updates the internal values that the outputs will become.
• Copy these internal output values to the output modules.
• Check for any input/output module faults and problems with the hardware.

What you did was to cause a fault at the moment in this looping process when the processor checked for module faults---it saw no module where one was expected. It then responded in the only way it was built to respond--stopped running and turned on the fault light.

Since the program refers to each input and output by the address of the slot it is wired to, if you move a module to an empty slot, the program will not be able to use it because the program is using a former address. This will also cause the processor to fault.

Therefore--it is best to get the module back into the slot it originally occupied, unless you are experienced in programming and are ready to edit every program reference to the former slot's address and change it to the new slot's address! After you get the input/output slot assignments correct, you can clear the fault and set the processor back to the run mode.

--JMM