What is cyclic and acyclic communication?

Question

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?

Answer 1

Forenote: This is mostly a programming based site, and while your question does have an answer within the contexts of programming, I happen to know that in your industrial application, the importance of cyclic vs acyclic tends to be very hardware/protocol specific, and is really more of a networking problem than a programming one.

Cyclic data is not simply "continuous" data. In industry, it refers to data delivered on a guaranteed (or at least highly predictable) schedule. If the data stream were to violate the schedule, it could have disastrous consequences (a VFD misses its shutdown command by a fraction of a second, and you lose your arm!).

Acyclic data is still reliable for machine control, it is just delivered in a less deterministic way (on the order of milliseconds, sometimes up to several seconds). When accessing a single VFD with a single PLC, you will probably never notice this bursting behavior, and in fact, you may perceive smoother and quicker data transmissions. From the hardware interface perspective, acyclic data transfer does not provide as strong of a guarantee about if or when one machine will respond to the request of another.

Both forms of data transfer deliver data at speeds much faster than humans can deal with, but in certain applications they will each have their own consequences.

Cyclic networks usually must take the form of master/slave, where only one device is allowed speak at a time, and answers are always returned, even if just to confirm that the message was received. Cyclic networks usually do not allow as many devices on the same wire, and often they will pass larger amounts of data at slower rates.

Acyclic networks might be thought of as a bit more choatic, but since they skip handshaking formalities, they can often cheat more devices onto the network and get higher speeds all at the same time. This comes at the cost of occasional data collisions/bottlenecks, and sometimes even, requests for critical data are simply ignored/lost with no indication of failure or success from the target ( in the case the sender will likely be sitting and waiting desperately for a message it will not get, and often then trigger process watchdogs that will shutdown the system).

From a programmer perspective, not much is different between these two transmission types.

What will usually dictate a situation,

1. how many devices are running on the wire (sometimes this forces the answer right away)
2. how sensitive/volatile is the data they want to share (how useful are messages if they are a little late)
3. how much data they might be required to send at any given time ( shifting demands on a network that already produces race conditions can be hard to anticipate/avoid if you don't see it coming before hand).

Hope that helps :)