Allen Bradley AI Card 1756-IF16 Not Reading Full Scale


Thread Starter


Dear All

I'm facing a problem with Allen Bradley PLC System. We are using a redundant I/O and redundant controller configuration of Allen Bradley ControlLogix 5000 series. In one of the AI card, on channel 0, following configuration is made:

Input type: 0 to 5V
High Range: 5V
High EU: 20,000
Low Range: 1V
Low EU: 4000

There is a gas detector connected to this channel which is supplying 4-20mA. What I observe is that when we simulate this channel using our mA calibrator, the raw counts does not increase more than 10,000. Why it is happening? Why the raw counts are not going upto 20,000 as configured in the channel configuration. Any help will be appreciated.
I can think of 2 things.

1) A current signal needs a dropping resistor to create a voltage. If the input already had a 250 ohm resistor internally, but a 2nd 250 ohm resistor were wired across the input, the two in parallel is 125 ohms, which would create a 0-2.50V range or a 0.50-2.50V signal for a 4-20mA input. 2.5V would be half scale at 10,000 counts.

2) Less likely is that the power supply just can't drive more than whatever 10,000 counts worth of current through whatever value dropping resistor is in place. If the battery on your simulator is not charged or barely functional, it could fail to drive sufficient current through the input resistance.
Hi sbinstrument,

You said your gas detector is 4-20 mA type but your channel configuration (input type) is 0 a 5 V. please make sure your input type is 0-20 mA. low range should 4 mA and high range should be 20 mA.

Other thing that need to be checked, most of Gas Detector is 3 or 4 wire types. make sure your AI wiring to that particular channel is correct.

If problems still persisted, please check actual mA by setup mA meter serially installed within the loop so you will be able to check actual mA flowing into channel in problem.