Glossary · Industrial communication
Acyclic communication
Also known as: Acyclic data exchange, On-demand communication
German: Azyklische Kommunikation
In industrial communication, acyclic communication is the exchange of data only when requested or triggered, outside the fixed update cycle of the process data. It typically carries parameters, diagnostics, identification data and alarms.
- Industrial communication
In one sentence
Acyclic communication exchanges data on request or event, outside the fixed cycle, for parameters, diagnostics and alarms.
Example
An engineering tool reads the firmware version and the parameter set of a drive via acyclic PROFINET services while the drive keeps receiving its cyclic setpoints.
How it applies
- Engineering: Fieldbuses and industrial Ethernet systems separate time-critical Process data from less urgent traffic. Acyclic services are used for Service data such as parameters, device identification and Diagnostics, so that they don't disturb the cyclic update.
- Commissioning: Device parameters are often written acyclically at startup. If a write fails, the device may run with default values, so startup routines should check the response.
- Operation: Acyclic requests have no guaranteed update time. Programs that read values acyclically must handle delays and a Timeout.
- Documentation: The documentation team lists which data a device provides cyclically and which only acyclically (with index, slot or object address as the protocol defines it), so integrators don't poll parameters that are meant to be read on demand.
Acyclic vs. cyclic communication
Cyclic communication repeats the same data at a fixed interval, which makes it predictable and suitable for control loops. Acyclic communication happens only on request or event and is suitable for large, rarely changing or diagnostic data. Most industrial protocols use both in parallel.