Glossary · Automation fundamentals, platforms and components
Process image
Also known as: Process image table, I/O image
German: Prozessabbild
In PLC technology, the process image is a memory area in which the controller stores a consistent snapshot of all input signals at the start of a scan cycle and collects all output values, which it writes to the physical outputs at the end of the cycle.
- Automation components
In one sentence
The process image is the PLC memory holding a consistent snapshot of inputs and outputs for each scan cycle.
Example
A sensor changes state in the middle of a cycle, but the program still sees the old value from the input process image until the next cycle starts.
How it applies
- Engineering: Working with the process image keeps signals consistent during one cycle: every rung sees the same input value. Fast signals may need direct (peripheral) access or interrupts instead, because the process image adds up to one Scan cycle of delay.
- Commissioning: When forcing or monitoring, know whether the tool shows the process image or the physical I/O; they can differ within a cycle.
- Documentation: Response time calculations should include the process image update. Describe in the software documentation which signals bypass the process image and why.
Process image vs. physical I/O
The process image is the PLC's copy; the physical I/O is the actual terminal. With a process image, output changes take effect only at the end of the cycle, and inputs are read only at its start, which is a frequent cause of apparently missed short pulses.