Glossary · Fault behavior and system states
Remanence (retentive memory)
Also known as: Retentive data, RETAIN variables
German: Remanenz
In PLC programming, remanence is the property of variables or memory areas that keep their values after a power failure or controller restart, as opposed to non-retentive data that are reset to initial values; IEC 61131-3 declares such variables as RETAIN.
- Data and interfaces
In one sentence
Remanence is the property of PLC variables that keep their values after power loss or restart, declared as RETAIN in IEC 61131-3.
Example
Piece counters and recipe parameters are declared retentive, so production data survive a power failure, while motion commands are reset on restart.
How it applies
- Engineering: Decide deliberately which data are retentive: counters, parameters and calibration values usually are; commands, step states and outputs usually are not.
- Safety: Retained commands or sequence states can cause unexpected motion after power returns; restart behavior must be analyzed together with Restart prevention and Unexpected startup.
- Documentation: The documentation team should describe which settings and states survive a power failure and what the operator must check before restarting.
- Testing: Test the restart behavior by switching off power in different machine states during commissioning.
Retentive vs. persistent
Retentive (RETAIN) data survive a power cycle or warm restart; persistent data (PERSISTENT in IEC 61131-3) can also survive a cold restart or program download, depending on the implementation.