Glossary · PLC and control programming
Program memory
Also known as: Load memory, Code memory
German: Programmspeicher
In PLC technology, program memory is the memory area of a controller that holds the executable user program, as distinct from the data memory that holds variables and process images.
- PLC programming
In one sentence
Program memory is the area of PLC memory that stores the user program, separate from data memory for variables.
Example
Adding a large recipe library fills 85 percent of the controller's program memory, so the team plans a CPU with more memory for the next machine version.
How it applies
- Engineering: Controllers state separate limits for code and data memory, and some distinguish load memory (for example on a memory card) from work memory used during execution. Memory use is checked when selecting the CPU, with reserve for later changes.
- Maintenance: Many controllers keep the program on a removable memory card. Replacing a CPU then means moving the card, while firmware compatibility still needs to be checked.
- Online changes: Online changes often need free memory, because the new code is loaded alongside the old before switching. A nearly full memory can block them.
- Documentation: Document where the program is stored, how to back it up and restore it, and the memory reserve at delivery. Service instructions for CPU replacement should cover memory cards explicitly.
Program memory vs. data memory
Program memory holds instructions; data memory holds variables. Retentive and nonretentive behavior applies to data memory, see Retentive memory and Nonretentive memory.