HMI and SCADA Process Value not corresponding

IdealDan said:

Hi Friends,

A quick one please:
What could cause HMI and SCADA Process value/data NOT corresponding?

A temperature value having up to 5 degree Celsius difference between the HMI (Panel View Plus) and SCADA (Vijeo Citect SCADA).
PLC is CompactLogix
Ethernet Network Communication.

Your inputs are immensely appreciated.
Regards

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kwade said:

Is this a new tag? Has it ever been correct? What datatype?

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aabeck said:

the first thought that comes to mind is, as others here have posted, - "a hmi should be a window into the plc."

my thought is that either the hmi or scada is doing some internal calculations, maybe adding a temperature offset value to compensate for an off reading, that got fixed when a thermocouple was replaced but the offset was never removed. I always do these things in the plc.

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kwade said:

Do other REALs correspond in the HMI and scada?

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Aabeck said:

The first thought that comes to mind is, as others here have posted, - "a HMI should be a window into the PLC."

My thought is that either the HMI or SCADA is doing some internal calculations, maybe adding a temperature offset value to compensate for an off reading, that got fixed when a thermocouple was replaced but the offset was never removed. I ALWAYS do these things in the PLC.

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cardosocea said:

Are you sure the SCADA and the HMI are reading the same tag? I wouldn't say that it's common practice, but where I am at the moment, the developers were so poor at creating a SCADA system that it's common to see what effectively should be the same value being different because it's linked to different places in the PLC.

Failing that, and I've never used Vijeo, some SCADA packages have the option of mapping values to ranges. This isn't common on real values, but it's one potential source.

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