Glossary · PLC and control programming
Output image table
Also known as: Process image output table (PIQ), Output process image
German: Prozessabbild der Ausgänge
In PLC technology, the output image table is a memory area in which the program writes its output values during a scan; the controller transfers these values to the physical outputs at the end of the scan.
- PLC programming
In one sentence
The output image table collects the PLC program's output values during a scan and writes them to the physical outputs at its end.
Example
If a program sets a valve output TRUE in one network and FALSE in a later one, only the final FALSE reaches the valve, because the output image is written once per scan.
How it applies
- Engineering: Because outputs are written once per cycle, intermediate values within a scan never reach the field. This hides some programming errors, such as writing the same output in two places, which only the last write wins. A Cross-reference check helps find them.
- Timing: An output changes physically only after the image transfer. Together with input filtering, input image update and program execution, this sets the reaction time. See Latency.
- Fault behavior: When the controller stops or communication to remote I/O fails, output modules take a configured substitute behavior, such as switching off or holding the last value. This choice is part of the machine's defined behavior on failure; see Safe default.
- Documentation: Document the configured output behavior on controller stop and communication loss for each I/O station. Service staff need it when they stop a controller for maintenance.
Output image table vs. input image table
The Input image table is a snapshot of the field at the start of the scan; the output image table carries the program's results to the field at the end.