I'm interfacing a Local SCADA HMI with an existing RTU from two different Vendors but my MW and MVar reading in HMI is bigger than the Value from the RTU. How do I get a coefficient to put in my SCADA database to have the same values with RTU?
HMI's do not use I/O to get their data, an HMI reads digital data or has digital data written to it via an industrial comm protocol.
When reading or writing digital values via industrial protocols, one either gets the correct number or a communications error. Incomplete or error flagged values are not reported - only the comm error is reported.
One never gets something 'close' when there are missing, flipped or extra bits; those are comm errors.
There are known instances of reading the wrong data, for example reading one Modbus register from one variable and a 2nd Modbus register from another variable and combining the two to get a 'close-to-expected-but-wrong value', but the core problem is still reading the wrong data in the first place.
When source (RTU) and destination (HMI) values do not match it's because:
- the HMI is reading a different value than expected
- the HMI is displaying a different value than expected.
- the HMI is interpreting a correctly read value improperly, like the incorrect scaling of an integer value with assumed multiplication factor.
Assuming the RTU has the correct data, correcting the problems above is done by examining and correcting the HMI configuration.