One Input Two Different Values

goodoboy

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Join Date
Aug 2010
Location
Houston
Posts
86
Hello,

I'm using AB Control Logix 5580 with rslogix 5000.


I have one transmitter, that is wired to 2 AI modules (both on channel 3), just for redundancy reading. There is a resistor on the inputs. There is other transmitters on the AI modules as well.

We have an alarm if the inputs differ by 200. We are constantly getting this alarm. We checked the wiring connections and ohm the wiring. All is good.

The other transmitters is setup the same, yet we don't get this difference alarm.

Any reasons for the difference if both AI is coming from the same transmitter?

Thanks,
 
Can you help clear up the fog a bit? Please specify:
Transmitter output (units & range).
AI module part numbers
single-ended or differential wiring on the inputs

I'd think you're dealing with a 4-20mA signal wired in series to the two inputs but wouldn't want to assume anything.
 
There is a resistor on the inputs.

This suggests that you're using a current loop transmitter but that your analog modules are voltage inputs.

What is the exact value of the resistor ? What is its wattage and tolerance ?

My guess is that one of the resistors is heating up more than the other one and that's contributing to the difference in measured voltage on the analog input channels.

Describe as much as you can about the components, their wiring, and your measurements.
 
To add to Ken's questions above, has this only just started happening, or has it been like this since you installed it? Have you checked the hardware configuration on both input cards to make sure they match? Is this sensor wired exactly the same way as the correctly-functioning sensors, and is it the same type of sensor?

The more detail we have, the more focused our questions (and hopefully, answers) can be.
 
Thank you for the response.

Both AI cards are 1756-IF16.

Its one 250ohm resistor.

From the transmitter the signal wire terminate at a terminal block, then one signal wire goes to one AI card ch 3, and another set of wires jumper from the same termination to the other AI card ch 3.

Range of transmetter is 0 to 15000
 
Hello,

I'm using AB Control Logix 5580 with rslogix 5000.


I have one transmitter, that is wired to 2 AI modules (both on channel 3), just for redundancy reading. There is a resistor on the inputs. There is other transmitters on the AI modules as well.

We have an alarm if the inputs differ by 200. We are constantly getting this alarm. We checked the wiring connections and ohm the wiring. All is good.

The other transmitters is setup the same, yet we don't get this difference alarm.

Any reasons for the difference if both AI is coming from the same transmitter?

Thanks,

Any help on this please?

I just bought two new AI cards and installed but problem still exist.

I know the problem exist cause I have an alarm that notifies me once the two AI cards channels are different values by 150.

For example if AI module 5 is reading 1500 and the other AI module 6 is reading 1300, I will get an alarm.

The technicians have checked the wiring back to the transmitter as well.

Maybe the 250 ohms resistor is going intermittently bad. I can try to replace the resistor and see if that helps.

Thanks,
 
Have you put a voltmeter at the terminals on each input card and convinced yourself that the voltage values are truly the same? Both cards have channel 3 configured as a voltage input with the same scaling?

Keith
 
My guess is that one of the resistors is heating up more than the other one and that's contributing to the difference in measured voltage on the analog input channels.

Describe as much as you can about the components, their wiring, and your measurements.

Thank you for response,

I am using 1 resistor for the setup.

The signal wires come to a terminal block for termination. The other side of terminal block is where the resistor, sets of wires go to AI module 5 and another set of wires go to AI module 6 inputs.

My alarm in PLC configuration is setup that if the two inputs differ (absolute difference) by 150, I get an alarm.

This problem has been happening for about 4 weeks now. The other channels on the card are setup the same way and measure the difference of values. I do not have this problem and I am measuring the difference with the other transmitters on the same two cards.

We replaced both AI modules and still the same problem.
 
Have you put a voltmeter at the terminals on each input card and convinced yourself that the voltage values are truly the same? Both cards have channel 3 configured as a voltage input with the same scaling?

Keith

Thank you Keith,

Yes both cards are setup for channel 3 the same way.

The flow meter is powered from the 24V in the PLC cabinet. No, I have not put a volt meter on both channels.

Thanks,
 
1) The 1756-IF16 can be wired several different ways.

I'm still not clear on how the circuit is wired.

It would really help if you would sketch and label the wiring for the transmitter, power supply terminal blocks, and the two AI cards and post the sketch here.

2) Do I understand correctly that there are other circuits like this one (on this same PLC), with a single field transmitter and two AI's, one on one AI card and one on another AI card?

3) Does only this channel report a difference of 200, or do other channels report the difference of 200?

4) >We are constantly getting this alarm.

What do you do to eliminate the alarm condition?
How quickly does the alarm condition return?
How long does this 200 units difference alarm last if you do nothing? 1 second? 1 minute?, 1 hour? 1 day? forever?
 
Just a suggestion, as I have seen this before.
If the terminal blocks that the transmitter is landed to are of the thin high density din rail type. Make sure that the terminal blocks are not "backwards".
What I mean is, you can flop them terminal blocks around next to each other and the contacts on each terminal block would be touching or intermittently touching. Silly as it sounds, there has been sillier.

As in, when the tech landed the wires, he did not strip the wire at all/enough and the screw terminal is clamped on the insulation.

Just a thought...K.I.S.S.
 
1) The 1756-IF16 can be wired several different ways.

I'm still not clear on how the circuit is wired. I can sketch something. But its 2 wires from the transmitter. Those two wires go to a terminal block in the panel, where it another 4 wires terminate as well with a 250 ohms resistor. 2 of those wires go to one AI card, the other 2 to another AI card. its for redundancy reading because the system is SIL rated.

It would really help if you would sketch and label the wiring for the transmitter, power supply terminal blocks, and the two AI cards and post the sketch here.

2) Do I understand correctly that there are other circuits like this one (on this same PLC), with a single field transmitter and two AI's, one on one AI card and one on another AI card?Yes this is correct. In theory, both AIs should read the same thing.

3) Does only this channel report a difference of 200, or do other channels report the difference of 200?Correct, this problem is related to a flow transmitter. There are 4 other transmitters (3 pressure and temperature) when the same exact setup and we never receive difference alarm from them

4) >We are constantly getting this alarm.

What do you do to eliminate the alarm condition? The difference is intermittently, it comes and goes quickly. Once the difference is less then 200, the alarm goes away.
How quickly does the alarm condition return?It comes and goes, I can not say for certain, but about 20-30 times a day. If I watch the PLC AIs channels live/online, I can see the difference fluctuate up to 200 quickly. Its flow reading and I know it vary, but shouldn't vary by that much.
How long does this 200 units difference alarm last if you do nothing? 1 second? 1 minute?, 1 hour? 1 day? forever?
Once the difference is less then 200, the alarm goes away.
[/QUOTE]
 
Thank you for the sketch - a picture is worth 1000 words.

Which terminals are connected for the pressure signal on the AI modules?
 
Thank you for the sketch - a picture is worth 1000 words.

Which terminals are connected for the pressure signal on the AI modules?

You're welcome.

The pressure and temperatures AIs are also configured like the picture.

The other signals, pressures and temperature are on the same terminal block as the flow we have issues with. They are right above or below the flow terminals.

Thanks
 

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