Data retrieval delay from Modbus devices

T3000 Software
1

This topic has some similarity to post “T3-TB Periodic BAcnet MS/TP read/write” in Aug 2025 (Philip_Butler), but I think it best to start a new topic to avoid confusion. I have read that post.

I am working on a T3-BB project which will have 4 remote instruments connected via RS485-Modbus RTU, so I want to understand how it will work.

I will be inserting into my program various Modbus “read register” requests to “fetch” data from the remote devices.

Will these data-fetches occur in real-time as the program is executed, OR will all the data-fetches in my program be retrieved asynchronously compared to program execution?? If asynchronous, what is the typical “refresh-rate” for these fetches?

Asynchronous data fetching would mean that whenever my program requires the “current” value of a particular Modbus-device register, a “recent” value will be available without having to request (and wait for) an instantaneous value. Is this right?

In many cases this will be ok, but in a few cases proper control depends on having an instantaneous/very recent (<3-5sec) value of a particular data point - just as if it were a wired Input. Is there a way to “force” Modbus data retrieval in these cases, and not in others??

I dont expect/want the T3 to pause the program while data is retrieved, but how long would it take (compared to program scan-time) until an instantaneous value is available?

I would appreciate your insights.

2

As I have mentioned here a few times, for now there’s no mechanism to programmatically control the refresh rate of the network points. Its handled automatically as a background task. Its not deterministic when that refresh will happen.

And as I’ve mentioned here before, some users were talking about their use case where it would be helpful to have control over when network points get refreshed, we have added to the task list for a future update.

3

Thank you. I have seen some of those mentions - hence my enquiry

Approximately, what is the interval between “background refreshes”?
Is the time consistent or does it depend on what else the processor is doing?

4

Its not deterministic, on the order of a few seconds but it would depend on the number of network points and what the T3 controller itself has going on with the programs, etc.

This hasnt come up as a show stopper for our system in any previous discussions.