Glossary · Automation fundamentals, platforms and components
Marker (bit memory)
Also known as: Bit memory, Flag, Memory bit
German: Merker
In PLC programming, a marker (bit memory, German "Merker") is a global memory area of the controller used to store intermediate results and states, addressed for example as M10.0 in Siemens SIMATIC. Other vendors speak of internal relays, flags or memory bits.
- Automation components
In one sentence
A marker (bit memory) is a global PLC memory area for intermediate results, addressed for example as M10.0 in Siemens SIMATIC.
Example
The bit M20.1 stores that the homing sequence has finished, and several program blocks read it to release automatic mode.
How it applies
- Engineering: Markers are global, so any block can write them; this makes programs hard to follow and test. Modern guidelines prefer symbolic variables in data blocks or instance data.
- Maintenance: Overlapping marker addresses written from several places cause hard-to-find errors. Cross-reference lists help.
- Documentation: If markers are used, keep a symbol table with meaningful names and comments. Programming guidelines should state where markers are allowed and whether they are retentive after power failure.
Marker vs. variable
A marker is an absolute address in a fixed memory area. A symbolic variable has a name, a data type and a defined scope. IEC 61131-3 has no marker concept of its own; it works with variables, some of which can be mapped to memory addresses (%M).