Allen bradley polling

How many tags can be polled per second or per minute without throwing plc in error or running into problem.

Thanks.

It depends on which PLC model, and how "loaded" it is. Large PLC's with small amount of logic, can service many more tags, than say, a small MicroLogix, with the maximum amount of logic running.

AB Polling Rate

One approach is for the polling device to only issue one request at a time and then wait for it to complete before issuing another. That will guarantee that the PLC can handle the traffic with no problem.

Other approaches may involve using multiple TCP connections so that multiple requests can be issued at the same time. There is a limit as to how much can be gained this way since there is a socket count limit and the CPU has to have some time to scan the ladder program.

It seems that the ControlLogix cpus using the PCCC protocol allow some level of 'pipelining' where you can have more than one request on the same socket at the same time. I don't know what level of pipelining is allowable and I don't know if AB encourages doing that.

In general, if you need a lot of data the register-based PCCC protocol will probably give you more performance than tag-based communication. You can get around 120 integer registers in a single exchange.


Your question is too vague to be answerable. It depends on the cpu, the protocol that is being used, what is polling it, and how well the data is packed. It is not hard to get over a thousand data items a second from a ControlLogix CPU. Poor design will not get close to that.

As many as you can read as long as the request-response-request-response .... order is respected.
If the PLC has a high workload it will simply respond with higher delay but this does not have to throw a PLC error

In addition there are many differences between SLC/MicroLogix and CLX/Micro800 in the first case you can read several consecutive registers, in the second case, which is tag-based, you can read several tags in a single request on CLX but not on Micro 800

As other people answered, it really depends on the specifics of both the PLC and how you’re requesting the data.

For example, doing things like bundling data into structures on Logix can really speed things up. If your able to read data as larger bundles and deserialize it on the client end, that’s more efficient. I know I’ve read as much as 14KB/sec for HMI displays before.

swapnil.naik said:

How many tags can be polled per second or per minute without throwing plc in error or running into problem.

Thanks.

Click to expand...

Short answer: A *LOT*

Longer anwer
On our ControlLogix, it is important to increase the 'System Overhead Time Slice' if you are running Continuous tasks

1756-L63 and 1756-L73

We do a lot of message instructions. As the number of active messages rises, you need to increase that 'System Overhead Time Slice'. The time is used for running logic if it is not used for communications. But it puts some variability in your logic execution times.

Our highest-load ControlLogix has a DeltaV VIM polling 4000 single integers (16 bit) every 400 ms. The data is sent as IO (type 1 messages). That PLC has 6 - 10 active message instructions during any given scan. It's time slice is set to 30%.