Glossary · PLC and control programming
Nonretentive memory
Also known as: Volatile memory, Non-retain memory
German: Nichtremanenter Speicher
In PLC technology, nonretentive memory holds variables that lose their current values and return to their initial values after a power cycle or restart of the controller.
- PLC programming
In one sentence
Nonretentive PLC memory resets variables to their initial values after a power cycle or restart; retentive memory keeps them.
Example
After a power failure, all nonretentive variables of a filling machine start at zero, so the program begins in the state IDLE.
How it applies
- Engineering: Most variables are nonretentive by default. That is desirable for states that should restart cleanly: sequences, commands, timers. Engineers decide explicitly which values must survive a restart and declare those as retentive.
- Safety: A defined, nonretentive start state supports predictable behavior after power returns. Commands that could restart motion should not be retained, see Safe default.
- Commissioning: Test a power cycle and a warm and cold restart deliberately and check that the machine comes up in the documented state.
- Documentation: The operating manual should describe what happens after a power failure: which settings and counters are kept, which states are reset, and what the operator must do to resume production. See Recovery behavior.
Nonretentive vs. retentive memory
Nonretentive memory is reset at restart; Retentive memory keeps its values. The exact behavior at warm start, cold start and program download differs between controllers.