(PLC) Memory / Data Retention Issues

It is common for the user program and the system’s parameters to be backed up automatically in non-volatile memory such as Flash memory (NAND) or faster EEPROM memory (NOR), however many PLCs will use battery backup for their volatile memory such as their RAM in case the PLC’s power supply is disconnected for maintenance or to maintain power during a power outage or brownout.

Power interruptions during read, write, or erase operations not only result in failure of the operation in progress, but it can also corrupt data already present in the flash device and can negatively impact the integrity of future data written both immediate and long-term causing integrity issues to the device. Understanding the Impact of Power Loss on Flash Memory document is a more in-depth dive on this.

The backup battery in a PLC should be checked regularly to ensure the voltage is within specified values. Self diagnostics should give low battery indication, however the diagnostic LED on the CPU module may be hard to see, or the diagnostic may be placed at bottom of the diagnostic error list. It is common to replace the low battery within a week after the battery diagnostic is first presented. When replacing a low battery, It is common to install the new battery within five minutes after removing the old one to ensure data retention.

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Z7892-ND

The battery service life of most PLCs is typically five years max under ideal conditions, however the service life will depend based on the age and model of the system, and may be significantly shortened if one or more of the following occurs:

  • Frequent power outages or brownouts
  • Powering off the system for long durations
  • High operating temperatures -Above 25°C (77°F)

Common Storage Characteristics:

Flash memory: User programs, parameters (such as the PLC Setup), data register holding up to a set memory amount, comment data, and the entire DM Area can be saved to flash, memory as initial values.
Battery backup: The Holding Area, DM Area, and counter values (flags, PV) are backed up by a battery. The battery also keeps power to the RAM upon power outage or brownouts thus keeping the sequence pulled from non-volatile memory such as Flash or EEPROM. This helps serve as a buffer providing data retention and helps prevent data corruption especially if it is pulling from serial memory instead of parallel. Serial memory is especially more prone to data corruption during power failure without battery backup if the memory is in the read, write, or erase mode. The battery is also common to keep clock / calendar settings which may be important for keeping timestamps for system events and tracking purposes.

See also:
Controllers - Accessories (Batteries)
Batteries Rechargeable (Secondary)
Batteries Non-Rechargeable (Primary)
Controllers - Programmable (PLC, PAC)
Controllers - PLC Modules
What is a PLC and what does a PLC do?
Choosing a PLC (Programmable Logic Controller)
Hidden Programming Access on PLCs
Omron PLC still serviceable?
Programming PLCs
PLC Communication Protocols
3 Wire Connection of PNP and NPN Sensors
“Noisy Power” or Bad Power Remedies