RX3i Corrupt User Memory

Good Afternoon all,

We occasionally have PLC's across site fail and when powering back up on a test bench the only fault log is "Corrupt User Memory"

My question is, to maintain a highly reliable system should the PLC be replaced or is there no issue downloading the program again and putting the PLC back in service?

The fault is not isolated to a particular PLC, just a random one occasionally failing out of many across site

Kind Regards,

Kyle Donaghy

  • Good Morning, first question, are you running on 24 Vdc or AC? is the earth connection solid or does your power to the PLC float ?
  • Corrupted user memory is usually caused by either electrical noise (airborne or through the power supply) or poor earthing. Use of high qaulity isolating transformers for the AC supply to the PLC power supplies is recommended, along with checking shielding, earthing and grounding of power supplies and signal cables. PLC power, I/O and signal cables should be run away from sources of interference such as high voltage power lines and should be installed in their own shielded and grounded trunking. The PLC itself needs to be installed in a metal IP54 cabinet with adequate shielding and grounding.

    Lightning strikes cause problems and any cables run outside should have their own lightning surge arrestors installed or be replaced with fibre.

    As for the PLC itself, if it can be power cycled and it recovers then it is likley that the CPU is ok and it can be returned to service. However if this happens repeatedly in the same unit or the CPU is getting old (> 10 years) then it may be time to replace it. CPU315 was discontinued in 2017 but you can check the date code on the CPU to see when it was manufactured.

    Modern CPUs can store the application in Flash memory and be configured to load from Flash on power up. We also use ECC memory that can recover from a single bit error.

    The PACSystems RX3i documentation includes details of installation practices that should be followed.

    Steve Ward