Emerson Exchange 365

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Programming a timed response in a CALC expression

I'm currently working on programming a CND for a pump start permissive. So far, it looks like this:

IF '^/PERMISSIVE_1.CV' >= 8.0 THEN '^/PERMISSIVE_1.CV' =  TRUE

ELSE '^/PERMISSIVE_1.CV' < 8.0 AND '^/PERMISSIVE_1.CV' >4.0 ....this is where it gets funky.....I need the permissive to turn a pump off for 30 minutes, then back on for 30 minutes...

ELSE '^/PERMISSIVE_1.CV' < 4.0 then pump off for 60 minutes and then on for 15 minutes

I know this can be done with function blocks...but is it possible in an expression? I realize it's essentially structured text, but do all FBs have a related ST instruction set?

• CND
• timer
• expression

Barrick_JA

• 13 Sep 2017 7:57 PM
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3 Replies

• Brian Atkinson

  

  • 13 Sep 2017 8:09 PM

  It seems like the function blocks are probably an easier to maintain and troubleshoot in the future. Why do you want to push all of this into a CND block?

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• Barrick_JA

  

  • 13 Sep 2017 8:21 PM

  In reply to Brian Atkinson:

  To make it a permissive prior to motor start.

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• Brian Hrankowsky

  

  • 14 Sep 2017 5:00 PM

  In reply to Barrick_JA:

  Items to note:
  A permissive is only going to let you control when the pump will start. It doesn't cause the DC block to transition from active to passive.
  Calc blocks cannot do timing - timing has to be offloaded to a timer or block with timing (like CND).
  If you want the SP of the DC block changed based on an algorithm - you need to write sequencing or other logic to do that without trying to use the interlocks and permissives for that purpose.
  I have not played with it yet, but the block introduced in version 13 that replaces the old CND/BFI approach to interlocks and permissives includes forced setpoint changes as well that may help you do what you are trying to do.
  
  Good Luck!

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