Emerson Exchange 365

DI CHARM: Bad Hardware Error

Hi all,

We have a DI charm (24 VDC low side) connected to a pressure switch, but in diagnostics it is giving me a bad hardware error.

We have:

Replaced the CHARM

Replaced the terminal block

Tried this on a different channel

Removed the end device and checked the wiring.

I'm having a hard time grasping what causes this CHARM to see an error. If the device had failed, would it not just always sense an open circuit or closed circuit?

What causes a hardware error on a DI CHARM? We don't have line fault detection enabled. I've also come across this with two temperature switches in our plant.

Any input would be appreciated!

Ryan Severson

• 28 Apr 2015 2:57 PM
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3 Replies

• RickV

  

  • 28 Apr 2015 5:49 PM

  Hello Ryan,
  
  Couple of questions and comments. First, can you isolate the problem to DeltaV or the field? (Remove field wires from the CHARM, short across terminals 1 and 2 if CHARM is good and detects change in logic 0/1 problem is in the field)
  1) I am assuming that the CHARM term block is a standard term block and is wired to terminals 1 and 2.
  2) I am assuming that the CHARM is configured correctly in DeltaV.
  3) When you removed the end device and checked the field wires, did the CHARM read good?
  4) Are the switches you have issues with dry contact or solid state devices?
  
  Possible problems.
  What could be happening is that the common of your switch is tied to ground in the field. The DeltaV DI dry contact CHARM uses the 24 VDC power supply ground. You could be seeing ground issues. Might want to check if the switches are being used for multipurpose, as example there is not another connection on the common of the switch. i.e. DeltaV reading across common and a N.O. contact and another system or device reading across common and N.C. contact.
  
  Could be getting induced voltage from the field wiring. Remove field wires at the CHARM and check for voltage AC and DC. Last if the field wiring is a long distance to the field end device could be a wiring issue (inductance/capacitance type issues).
  CHARM Specifications
  Detection level for On > 2.25 mA (<5.3 kΩ)
  Detection level for Off < 1.75 mA (>8.2 kΩ)
  Wetting Voltage 22.5Volts (± 5%), current limited to 12.5 mA nominal

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• Ryan Severson

  

  • 28 Apr 2015 8:01 PM

  In reply to RickV:

  Hi Rick,
  
  1)We removed the end device and shorted out terminals 1 and 2 and the CHARM seems the change.
  2) Yep, I double checked all the configuration
  3) Yes, it read status change when we closed the 2 wires with no end device
  4) I believe that these are dry contacts. We have another one of these, identical and working.
  
  I will have to look and see about the induced voltages, that could very well be the issue.
  
  What causes the CHARM to see device failure though? It doesn't mention in BOL what causes these errors (transient voltages, short circuit current, etc. )
  
  Thanks for the reply!

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• RickV

  

  • 29 Apr 2015 1:33 PM

  In reply to Ryan Severson:

  Ryan,
  
  Sorry as I do not know what the “technical specifications or conditions” are for the CHARM displaying a device failure. So any additional information would be speculation on my part. Maybe someone with a better knowledge of the product can chime in.
  
  Cheers and good luck

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