When talking to a ModBus device; how does the different tag group settings, Polled vs Subscribed, behave?
In a opc ua device, the differences are very apparent; but as ModBus is, by its very nature, Polled; what does setting it to subscribed do?
It modifies the behavior between OPC UA client/server.
Polled: periodically call the Read service for all the tags belonging to that server.
Subscribed: create a Subscription for the Tag Group and then a MonitoredItem for each tag belonging to that server.
For our server, with our Modbus driver, there is no fundamental difference in behavior - it's just polling like you mentioned.
The difference in behavior you might be seeing is in how requests end up being "optimized", i.e. what requests are made to the device for what addresses. Tags in a Subscribed Mode group wouldn't get optimized together with tags from a Read Mode group, for example.
How much of a time variance would the optimizations cause?
I am working with a Modbus device; and I could not get it to stay connected. It keeps disconnecting because I am not sending it a Modbus command. I was able to fix it by putting a tag in "Polling" mode.
After playing around with the rate, It disconnects bettween 3000 - 4000 ms with Polled mode selected.
But my original tags were 1000ms Subcribed.
Would the optimizations cause a 3x delay in the update rate?
No.
Maybe you can look at a Wireshark capture to try and spot a difference in the traffic patterns.