Wrong data between OPC Server and Labview Tags

[Gwattz]

I have an application using NI OPC drivers talking with an AB SLC504 over RS232. I keep finding the PLC data is different than the tag value in LabView. I have my Poll Rate set to 0:02 (two sec.) and the tag dead band set to .01%. Any ideas?

Thanks,

[Khalid]

Hello Gwattz,

Some questions may help us find the issue here:

1. How different are the two data? Some small decimal number? Or are the two numbers totally different? Is it a fixed offset always? Can you give some real examples of the two values.

2. Does LabVIEW seem to "lag" the PLC? I.e., the current data in LabVIEW was the data in PLC some time ago? Or is there no such pattern?

3. What register in the PLC are you reading? A 16-bit or a 32-bit? Is it possible that in LabVIEW we are reading only the 16-bit of a 32-bit number, for instance?

4. Are we looking at the same register in both the cases? Lookout OPC Server has a peculiar addressing format for AB PLCs.

5. Can you check this value in another OPC Client, like ServerEx
plorer?

6. For testing purposes only, make the update deadband zero. Any difference?

Hope this gives you some ideas.

Regards,

Khalid