Glossary · Industrial communication
Process data
Also known as: Cyclic data, I/O data, Process image data
German: Prozessdaten
In industrial communication, process data are the time-critical values exchanged cyclically between a controller and field devices, such as input states, measured values, output commands, setpoints and actual values.
- Industrial communication
In one sentence
Process data are the time-critical input and output values, setpoints and actual values exchanged cyclically with field devices.
Example
The process data of a servo drive include the control word and target position from the PLC and the status word and actual position back to it.
How it applies
- Engineering: Process data are mapped into the controller's process image. Their size and cycle time determine network load. Only data that are needed every cycle belong in process data; everything else is read acyclically as Service data.
- Commissioning: The process data mapping (offsets, data types, byte order) must match on both sides. Tools usually generate it from device description files.
- Operation: When communication fails, process data freeze or fall back to substitute values. Programs must check validity flags or connection status to avoid acting on Stale data.
- Documentation: The documentation team documents the process data image of each device: direction, offset, data type, unit, meaning of each bit in status and control words.
Process data vs. service data
Process data are small, time-critical and cyclic. Service data (parameters, diagnostics, identification) are exchanged on demand. CANopen formalizes the split as PDO and SDO.