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Electronic Theatre Controls Inc

Rack Communications Failure Error on DRd


The information in this post is provided to assist in troubleshooting. Perform work at your own risk. ENSURE ANY POWER FROM DEVICES HAS BEEN DISCONNECTED BEFORE SERVICING ANY EQUIPMENT. If you do not feel comfortable performing the work, please contact us or your local service center. Be aware that ETC and its Affiliates are not responsible for any damage or injury caused by service of our products by anyone other than us or our authorized service providers, and such damage is excluded from the product’s warranty.


A "Rack Communication Failure" is an error message that can show on a Architectural Control Processor (ACP) installed in a DRd rack.

Explanation of Issue

This error indicates that the ACP cannot communicate with the dimming engine in the rack, which is at the very top.  Communication between the ACP and the dimming engine is via a grey ribbon cable and then the signal distribution card in the rack.


  1. Power off the rack entirely by placing the main feed breaker in the OFF position and using proper Lockout Tagout procedure.
  2. Remove the top 4 dimmer modules from the rack, keeping track of the order they were in.
  3. Loosen the two black thumb screws at the top front of the rack until the dimming engine slides down and folds out.
  4. Disconnect and reconnect the grey ribbon cable on the dimming engine side. 
  5. Slide the dimming engine back up and into place, tightening the thumb  screws.
  6. Check the rack side of the ribbon cable connection.  This is just behind the 5th lug in the rack as shown below.  This can often be done by feel better than by sight.
  7. Reinstall the 4 dimmer modules in the order they were removed to ensure proper module locations.
  8. Power the rack back on.

DRd Lug Locations.jpgDRd Ribbon Cable Signal Distro.jpg
Left: Signal Distro card from front.  The ribbon cable connects behind these copper lugs.
Right: Ribbon Cable connected to signal distro card.  This is a side view with cabling held out the way.

If this does not resolve the error, please contact ETC technical support as there may be damage to the dimming engine or ribbon cable.



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