Glossary · PLC and control programming
Retentive memory
Also known as: Retain memory, Non-volatile data memory
German: Remanenter Speicher
In PLC technology, retentive memory is a memory area whose contents are preserved when the controller loses power or is restarted, implemented for example with battery-backed RAM, capacitors or non-volatile memory.
- PLC programming
In one sentence
Retentive memory keeps PLC data through power loss and restarts, using battery-backed RAM, capacitors or non-volatile memory.
Example
The controller writes its retentive data to non-volatile memory during power-down, so batch counters are still correct after the power returns.
How it applies
- Engineering: The size of the retentive memory is limited and differs between controllers. Engineers decide which data is retentive and check that the reserve is sufficient.
- Maintenance: Battery-backed designs need battery replacement at defined intervals; a dead battery can lose retentive data at the next power failure. Other designs use supercapacitors or flash memory without batteries.
- Operation: Retentive data must be consistent after an unexpected power loss. Programs sometimes verify retained data with checksums or plausibility checks after startup.
- Documentation: The maintenance manual should list battery type and replacement interval where applicable, and the procedure to back up and restore retentive data. Describe the effect of losing retentive data for operators.
Retentive vs. nonretentive memory
Retentive memory keeps values; Nonretentive memory returns to initial values at restart. A Retain variable is how a program places data into retentive memory.