Glossary · PLC and control programming
Input image table
Also known as: Process image input table (PII), Input process image
German: Prozessabbild der Eingänge
In PLC technology, the input image table is a memory area in which the controller stores the states of its inputs at the start of a scan, so that the program works with a consistent snapshot rather than reading the physical inputs directly.
- PLC programming
In one sentence
The input image table holds a snapshot of all PLC inputs taken at the start of a scan, so the program sees consistent values.
Example
Although a sensor switches in the middle of the program cycle, all networks see its old value until the next scan updates the input image table.
How it applies
- Engineering: The process image makes program execution deterministic within one cycle: every read of an input returns the same value. Many controllers support several process image partitions linked to different tasks, and direct access to the physical input for time-critical cases.
- Timing: A signal change becomes visible to the program only at the next image update. The worst-case reaction time therefore includes input filter time, up to one cycle for the update, program execution and output update. See Latency.
- Commissioning: Signals shorter than the cycle time can be missed entirely. Fast signals need interrupt inputs, high-speed counters or hardware latching.
- Documentation: When a function's response time matters, for example for a stopping distance calculation, document which process image, task and cycle time it relies on. The reaction time of safety functions is determined by the safety system and its own documentation, not by the standard process image.
Input image table vs. output image table
The input image is read at the start of the scan; the Output image table collects the program's results and is written to the physical outputs at the end.