Analog Card (1769-IF4) not reading 4-20ma signal anymore.

I need some help.

I have a 1769-IF4 analog card that is hooked up to strain gages, the gages give a 4-20ma signal to the card.

We had an outside guy do the programming. At the time he set it up only one strain gage was wired into one channel on the card. Everything worked like it was suppose to.

Later, I hooked up another strain gage to another channel. Now the analog card is not reading any signal. In rslogix the tag for the channel displays 0 and does not change, it should show values corresponding to the changed 4-20ma signal.

I checked with a multimeter, the card is getting a good signal.
I tripled checked that everything is wired up correctly, Its wired single-ended.
The status light on the card module is green.
I am not getting any errors/faults/status bits in rslogixs.


I have no idea why it is not working now.
Thanks



Edit: I will also mention that in the program you can select between all four channels with buttons on the hmi and it only activates one channel at a time. I confirmed this works as it is suppose to and no matter what channel is selected, it still does not work.

Have you check that the IF4 module properties channel configuration in that the channel enabled is checked marked?

Put it back the way it was and see if its still not working, if not then I would say something in the card failed, if it starts working again then look at your wiring quadruple check

KuulKuum said:

Have you check that the IF4 module properties channel configuration in that the channel enabled is checked marked?

Click to expand...

Yes, the channel configuration functionality is fine. No matter what channel it is on it does not work.

geniusintraining said:

Put it back the way it was and see if its still not working, if not then I would say something in the card failed, if it starts working again then look at your wiring quadruple check

Click to expand...

Yes, I tried this and it still does not work.

Could be something in the card I would just think that the card would have given some error or something. I do not have a card to swap out to check it right now.

BrockSamson said:

I do not have a card to swap out to check it right now.

Click to expand...

I have some in stock

https://www.plccable.com/allen-bradley-micrologix-1769-if4-analog-input-1769-1f4-used/

https://www.plccable.com/allen-bradley-micrologix-1769-if4-analog-input-1769-1f4/

If you order before 1pm eastern it ships today...

There is a high probability of excessive common mode voltage through a ground loop.


Your description of the problem is a classic - one or more channels connected and work properly, when an additional channel, every signal goes off-scale. The difference in ground potential drives the signal offscale.


Can you check the reading of the "good" channel while you connect/disconnect the errant channel? If it's a ground loop, then the good channel will read correctly when the errant channel is disconnected.


Solutions:
If the AI module can be wired/configure for differential, sometimes the common mode rejection is sufficient to fix it.


If not, then the solution is a 4-20mA signal isolator.

danw said:

There is a high probability of excessive common mode voltage through a ground loop.


Your description of the problem is a classic - one or more channels connected and work properly, when an additional channel, every signal goes off-scale. The difference in ground potential drives the signal offscale.


Can you check the reading of the "good" channel while you connect/disconnect the errant channel? If it's a ground loop, then the good channel will read correctly when the errant channel is disconnected.


Solutions:
If the AI module can be wired/configure for differential, sometimes the common mode rejection is sufficient to fix it.


If not, then the solution is a 4-20mA signal isolator.

Click to expand...

Thank you,

I will look into this and yes I can configure it to differential.

danw said:

There is a high probability of excessive common mode voltage through a ground loop.

Click to expand...

Hi Dan, this was the reason I ask if they could put it back the way it was... if it started working again then they had a issue with the wiring but since it still did not work I think thats pointing to a issue with the card, but just a WAG without being there

geniusintraining said:

Hi Dan, this was the reason I ask if they could put it back the way it was... if it started working again then they had a issue with the wiring but since it still did not work I think thats pointing to a issue with the card, but just a WAG without being there

Click to expand...

You're right, but it could have been fried from the common mode.

Excessive common mode manifests in 3 ways

- an artificial offset on each channel
- each channel driven off-scale, up or down, depending on the polarity of the common mode
- card gets fried because common mode is excessive.

Putting a voltmeter between the ground/negative terminal on the AI card and the negative terminal on the incoming AI signal (check both AC and DC ranges) will show the amplitude of the common mode.