Glossary
What is Process image?
A copy in controller memory of every input and output, refreshed once at the start and once at the end of each scan.
Your program reads the input image, not the terminal, so an input value cannot change mid-scan whatever happens in the field. That is what makes behaviour predictable and testable.
Where a live reading is needed, for fast measurement or pulse counting, controllers offer direct terminal access, but it is an exception used with care.
Related terms
- Scan cycle
- The loop a PLC repeats thousands of times a minute: read the inputs, run the program top to bottom, write the outputs.
- Programmable logic controller (PLC)
- An industrial computer built to control machines and processes: it reads sensors and switches, runs the control program, and drives motors and valves, repeatedly and reliably.