Cylic and acyclic communication.

So I've already searched if there was a question like this posted before, but I wasn't able to find the answer I liked.

I've been working with some PLCs and variable frequency drives lately and thought it was about time I finally found out what cyclic and non-cyclic communication is.

So correct me if I'm wrong, but when I think of cyclic data, I think of data that is continuously being updated and is able to be sent/sampled to other devices. With relation to what I'm doing, I'm thinking that the variable frequency drive is able to update information such as speed and frequency that can be sampled/read from a PLC. This is what I would consider cyclic communication, something that is always updating a certain type of information that can be sent as data.

So I might be completely wrong with this assumption, and that leaves me with the question of what exactly would be considered non-cyclic or acyclic communication.

Any help?

You are pretty close. As the name implies, cyclic communication happens every cycle, scan, time slice... whatever you mayt call it. It is the data exchange that occurs at the regular pre-determined time intervals, has higher priority and is being used to transmit the most time-critical data (in your example, the VFD's speed command/actual speed feedback).

Anything else that can wait, so to speak, is transferred via non-cyclic communication. For instance, the PLC program needs to update a VFD's parameter value. In that case, the PLC sends the request and expects some acknowledgement in return; this happens only once and not every cycle continuously.