Emerson Exchange 365

IO_READBACK - what is the use of it?

Both PID and AO function blocks have the IO_READBACK parameter.

In the Books Online I do not see how the parameter is used.

Is IO_READBACK used only for 'statistical' purposes, or is it used to control the OUT or IO_OUT so IO_READBACK gets to the desired value?

• AO PID IO_READBACK

Maarten van der Waal

• 8 Feb 2018 11:11 AM
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2 Replies

• Matt Stoner

  

  • 8 Feb 2018 12:05 PM

  It is used to be able to show the Actual Valve Position (AVP) instead of using Implied Valve Position (IVP). For the PID block the OUT_READBACK will show what the AVP is and the AO block will set the PV when the READBACK parameters are configured to a DST.
  
  Just be aware of when the AO is set for Increase to Close, the OUT will be the inverted signal to the field but the PV will be the real value. So for the AO the IVP would be found on the SP parameter. Example if the valve is suppose to be 25% with increase to close, SP=25, OUT=75, PV will be 25 unless READBACK is configured and then it will be what the AVP is.

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• James Beall

  

  • 8 Feb 2018 1:41 PM

  In reply to Matt Stoner:

  Note that the AVP can come from an AI channel from an independent position transmitter or from a smart positioner via communication on the OUT channel such as HART or fFieldbus. For example, for HART based Fisher DVC, the position is available from the HART_F (fourth) signal communicated on the 4-20 mA wires. The AO channel must be a HART AO in this example. The availability of the AVP is invaluable in troubleshooting!

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