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DeltaV Diagnostics Output Value

I am attempting to troubleshoot an analog output that is not working as expected. When I drive the output from the associated PID function block and check the output value using diagnostics I am finding a mismatch between the values. For example, when I set the PID output in manual to 0%, diagnostics tells me that the VALUE parameter for the AO channel is 4.64. 100% on the PID block output equates to 7.2 on the AO channel output.

What does the VALUE parameter that appears in diagnostics actually show? Is it the value in % of 4-20mA that the card is attempting to send to the field or is it a readback of what is actually being sent to the field? I have search books online but haven't found a clear explanation of what this parameter actually shows.

Conor McPhee

• 16 Feb 2023 5:03 PM
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3 Replies

• Rebecca Ferrell

  

  • 16 Feb 2023 5:51 PM

  Have you checked the card configuration to make sure it is configured correctly as 4 to 20mA? Also check the AO block configuration as the XD_SCALE parameter is 4 to 20 mA? I've had that bite me before. As for the answer to your actual question, my suspicion based on looking at my system is that it is the Value parameter is what is being sent to the field in mA since the channel would not have the ability to get a readback without HART.

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• Conor McPhee

  

  • 20 Feb 2023 3:25 PM

  In reply to Rebecca Ferrell:

  Thanks for the suggestion to check the XD_SCALE parameter on the AO block. The problem seems to have been related to the fact that a PID block was being used to drive the output channel directly rather than via an AO block. Your comment prompted me to build a new AO block to drive the output channel which caused the scaling to start working as expected and I could drive the valve scross the full 0-100% range. I am still confused as to why this did not work with the PID block but my problem is resolved nonetheless!

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• Rebecca Ferrell

  

  • 20 Feb 2023 5:05 PM

  In reply to Conor McPhee:

  Glad you got it working! Yes, I always use a designated output block which would be the AO block in this case so you the the 4 to 20 mA scaling parameter. I think the IO_OUT parameter in the PID block doesn't do any 4 to 20mA scaling and just sets a % output which might not match what you need.

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