Friday, October 7, 2016

A Support Query: Importing the Wrong Value from an M/A Station Block

Dear Support:

I was wondering if you could explain this error to me.

Dear Client:

For sure.

One place block 3088 is used is as a block imported by an AI/L.
The exception report from an AO/L block looks like this:
  • one byte - status bits, including H, L, Q
  • one real - value of the AO/L
The exception report from a STATION block looks like this:
  • one byte - status bits, including H, L, Q, Deviation
  • three real values in this order
  •     value of the PV (S1)
  •     value of the SP (output N+1)
  •     value of the CO (output N)

The AI/L block reads and supports the first two values - one byte, from which it gets the quality, and one real, which it imports as the value.

The message tells you that the value of the AI/L block imported equals that of the PV of the station block, something like 239 (S1), not the CO value of 46% (Block 3088), which is probably what is expected.

Please send the same screen shot, but also the AI/L block.

Dear Support:
The AI/L block

Dear Client:

As you see, the description PLANT AIR FLOW CONTROL OUTPUT just does not cut it. In fact, 46% would not be not high enough to trigger the 50% level in the subsequent H//L block, but the erroneously imported PV of 239 is.

You could insert an AO/L after the station block, to transmit the actual output value to the AI/L block.

However, a simpler way might be to put an H//L block on the page where the Station block 3088 exists and send the output of the H value to a DO/L, then import that DO/L here.

Does this now make sense?

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