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Analog Input readings are high on DCU's

Issue

All or a group of Analog Inputs on my Controller (DCU 7700) are reading high (flooding), in alarm and / or displayed as "OLD'.

Environment

I/NET

All model series DCU 77xx

Cause

The inputs on a DCU 7700 are 1-5 Volt DC inputs.

If the voltage of an input goes above 5 Volts DC the voltage will flood across the other inputs and they will all go out of range or OLD.

Resolution

To diagnose the problem sensor perform the following:

  1. Measure all inputs (Between terminal 9 and the respective input terminal) one at a time and identify the input that is the highest voltage (above 5 Volts.)
  2. Disconnect this input and then place your meter between terminal 9 and the identified input terminal.
  3. The excess voltage will slowly drain away and all the other points will come back into normal operating range.
  4. If the voltage does not drop repeat the above steps.
  5. Replace / Repair the fault sensor(s).

Also refer to Lessons Learned Article #1684.

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