SLC500 5/05 Analog Inputs Bouncing

I have a SLC500 5/05 and we have an alarm setup to monitor the status of the Analog Inputs we have wired to a 1746-NI8. We are using 5 of the inputs on this module. All 5 inputs are bouncing around a bunch.

I thought maybe the module was starting to fail so I just replaced the 1747-NI8. Still seeing the same bouncing signals. When I read the signals coming in the mA signals are not bouncing they are solid.

Any suggestions on what to try next?

I have videos of the bouncing signals but I cant get them to upload.

Could there be induced voltage into the lines? Try wiring up a new test sensor to a spare input on the card without running the cable anywhere and see what happens?



Is there some kind of wiring issue? Are the shields connected to the shield terminals?

Have you tried putting a jumper across unused channel?

I am going to try unwiring one of the points and hook up and simulate a signal. Also going to check the grounding and the power source.

The unused channels are not being monitored for the status. They currently do not have jumpers on them.

Bhicks31 said:

They currently do not have jumpers on them.

Click to expand...

Well, try it.

Mickey said:

Well, try it.

Click to expand...

Will do. Its been running for 20 plus years without jumpers but I am willing to try anything!

Thanks!!

What changed recently?

motor changed to vfd?
analog wiring?
machinery moved?
machinery added nearby?
Wiring changed / rerouted?

if the machine has been running that long without problems, something has changed.
james

Really interesting failure and I am wondering what is root cause?

Maybe, there is some water or dirty on terminal connection box... maybe your 5 instrument are broken in same time.

You can also use filter to reduce the bouncing

Try to connect pt100 to direckly analog input and see there is bouncing or not...

By the way do you have fluke RTI kalibrator, maybe you can check your analog channel with this device.

James Mcquade said:

What changed recently?

motor changed to vfd?
analog wiring?
machinery moved?
machinery added nearby?
Wiring changed / rerouted?

if the machine has been running that long without problems, something has changed.
james

Click to expand...

Nothing has changed that we are aware of. The machine still runs just getting the common alarm that we have configured due to the analog inputs.

They will have it shutdown again here in about 30 minutes. So hope to have an update after more testing when they go down.

Ok machine was down long enough to perform a few tests.

Lifted the channel input wiring and applied a simulated 4-20 mA and it was solid.

Now it gets difficult the common is shared through the whole panel. The transmitters go through a 250 ohm resistor. The power supply should be 24 volts and it is setting about 19 volts.

We tried to isolate signals and ran out of time before they needed to start the machine back up. Will hopefully get another shot at things tomorrow.

It might be worthwhile to try grounding the 0V side of your 24 VDC power supply.

Bit_Bucket_07 said:

It might be worthwhile to try grounding the 0V side of your 24 VDC power supply.

Click to expand...

I checked the grounding it looked good. The negative is already grounded. I also believe the 250 ohm resistors in series with the transmitters were an old way of making the loop current limited for area classification which would not fly today.

I had a similar situation upgrading a system to CLX.


The analog input card was isolated analog inputs and all of them bounced just like you describe with a good, steady signal from signal conditioners or a mA calibrator. If the output should read 55PSI it wold read that once in a while but fluctuate from 1PSI to 54 every read of the input.



I tried jumpers to unused terminals, grounding, better shielded cable, adjusting filtering & settings for the card - nothing worked, and dampening the signal just made it unreliable always showing too low a value.



Finally bought a regular analog IO card and no problems since without all the jumpers.


Plus, I have had a lot of weird things happen in the PLC, on the rack and to the backup battery when the power was off all from analog IO on a SLC rack. If I have to troubleshoot a SLC rack the first thing I look for is any analog card to replace first.

Bhicks31 said:

Ok machine was down long enough to perform a few tests.

Lifted the channel input wiring and applied a simulated 4-20 mA and it was solid.

Now it gets difficult the common is shared through the whole panel. The transmitters go through a 250 ohm resistor. The power supply should be 24 volts and it is setting about 19 volts.

We tried to isolate signals and ran out of time before they needed to start the machine back up. Will hopefully get another shot at things tomorrow.

Click to expand...

Perhaps you have a bad filter capacitor on your 24 VDC power supply causing a ripple in it's output voltage?

I've seen that happen, but didn't see it until I put a scope on it.

Bhicks31 said:

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The power supply should be 24 volts and it is setting about 19 volts.

Click to expand...

If you are measuring 19 volts at the power supply output, something is pulling it down.

I'd start by disconnecting the positive from the supply to ensure it comes back up.

If it does, then start disconnecting the positive feeding sensors one by one until the supply comes back up and then you will have found the culprit.