openHistorian Modbus connection - Maximum number of registers per second


I am trying to transfer the maximum amount of Modbus points between an RTAC and the openHistorian.

Can I set only one sequence with 500 registers or should I split in many sequences with a maximum of 125 registers/pool (recommended is 100)? Does “sequence” has the same meaning as the Modbus “pool” in this case?

In the “Inter Sequence Group Pool Delay”, the minimum time of that field is 100ms. That means 10 sequences in one second.

If I understood it right, I can create a Modbus connection between the RTAC and the openHistorian that can pool up to 10 sequences of 125 registers (16bits). In theory, I could read up to 1250 registers every second with that Modbus Link. Is that right?

Can we increase that, considering the RTAC has the registers aligned in sequence from 1… end? Will it be better if I use DNP3 or any other protocol?

If I have 2 devices, will the historian read all the sequences of the first, and then go to the next device ? or it will read the 1st sequence of all the devices and then go to the next sequence?

Thanks again for the help!

These timing restrictions are not because of openHistorian, but more based of the nature of the Modbus protocol.

FYI - two openHistorian devices will read in parallel. If you are connecting to the same device for a different set of registers, it will all depend on how well the RTAC responds to simultaneous Modbus connections.

We can also certainly try easing off some of those screen restrictions too - but it will be trial and error based on the RTAC Modbus performance.

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Hi @ritchiecarroll, Thanks for the repply and for the help!

The fastest processing rate is 4 ms for the main task and 1 ms for the automation task in the RTAC. I would like to check if i can reduce the inter sequence time to 20 to 50ms. It would great if we could easing off the restriction in the screens so then I could test the minimum time I can get between readings. But I understand that i will have a limitation related to the nature of the Modbus.

This part was an easy change - I am also implementing the ability to select primary historian. Right now I am testing edit mode operations before I check-in change…

OK - this change should do it:

This change updates minimum accepted intervals to 1 millisecond and sets any added measurements to the selected historian (new drop-down top right).

The new build should be ready around 1:00 AM Eastern tomorrow morning after the nightly build runs. After that you can download form here:

Since some of the changes were in an associated Javascript file and the browser always caches these, make sure to do a hard reload on the Modbus page once it loads after you install the new version (Ctrl + F5 on most browsers).

Best of luck!

Thank you again @ritchiecarroll!

I will test to check the minimum time between poolings and I will reply with the results.