Why ignition values from PLC differ slightly from local HMI?

Hi guys,

We have some locations (Thermal points) which are connected to our SCADA via Modbus TCP/IP or Modbus RTU. And there are local HMI’s connected to the PLC.

My questions is why the values received by Ignition slightly differ from the local HMI values. Here it’s an example:
image - this is the local HMI values
image - this is the value from Ignition

Thank you a lot!


You mean the fact that ignition is displaying an extra digit after the decimal? You just need to change the format you’re displaying to 1dp

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No, i think he mean the fact that in HMI the value displayed is 10298 while on ignition is 10286.

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Sample times could be different (Don’t know how much the values actually fluctuate).

More likely that it’s a float conversion.

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Is the scaling on the tag set to the same values in both HMI systems?


It seems unlikely it’s the cause (probably not set that high), but another possible source of variance is the deadband in Ignition tag numeric properites:
If it’s set as high/higher than the variance you’re seeing, it can cause that kind of variance in the display by not updating the tag value until the source varies from current tag value by more than the deadband amount…

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There’s a lot of possible causes for why your values differ. Your system topology isn’t clear for me. Do you have Modbus TCP/IP and RTU and TCP/IP over RTU?.
How the PLC is involved in this topology? Consider the baud rate from modbus and mbps from tcp may be conflictive. This is import for Ignition because the poll rate should be based on that criteria.
Ignoring the above and assuming the data provider is from the same spot. As @witman said,

maybe Ignition it’s been more sensitive to data change compared to other panels, try increacing the deadband. In addition, make sure to create an appropriate scan class for your opc tag subcriptions. Play with this values and you should see the same values maybe with small errors

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Unfortunately I don’t have acces to the local HMI settings.

The configuration for this mentioned PLC is Modbus TCP/IP.
I will try to increas the deadband to see if somethings changes.
Thank you for your help!

Thanks a lot for suggestions guys!