AB Flex IO 1794-IF8 Input Fluctuate?

sutton

Lifetime Supporting Member
Join Date
Mar 2003
Location
Singapore
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74
Dear all,

I am experiencing temperature fluctuation problem in our new IO Input RTD module, 1794-IF8.
Previously, the temperature sensors are connected to Yamatake Honeywell DCP200 and the reading shows stable process variable shown in the display and chart recorder.

We migrated the temperature sensor to the new Flex IO input RTD module, but the temperature fluctuate at 5-8 oC variance.

When we put back the temperature sensor back to the Yamatake Honeywell, the temperature reading is stable. FYI, we already set the resistance configuration on the module.

Please refer to the attached file in excel format.

Anyone please advice how to deal with this error?

Thanks.




 
Are you using shielded cable to from your RTD's to your input module? I had a similar problem previously. The flex IO modules maybe more prone to interference than a dedicated controller. We had readings floating all over, after we replaced the cable with shielded cable (only ground one end) the problem went away.
 
Thanks Keystone for experience sharing.
We are not using shielded cable. We doubt of the cable problem. However its costly to replace all the screen cable while we are unsure what could be other reason that cause the reading floating.
Instead of the cable, what could be other reason that we need to look at? Thanks a lot.
 
sutton said:
Thanks Keystone for experience sharing.
We are not using shielded cable. We doubt of the cable problem. However its costly to replace all the screen cable while we are unsure what could be other reason that cause the reading floating.
Instead of the cable, what could be other reason that we need to look at? Thanks a lot.


This has worked many times for me.
The cables that go to the RTD's have a look at them and any unused cores in the cable should be earthed, this should be done at one end only.
 

Thanks Gil47,

Yes, we will go ahead with a trial with the screen cables, and make sure of the grounding.


I have a reply from Rockwell distributor technical specialist.
  • In this case, most probably is due to the instable power, or the module problem,So I suggest you do the following:
    • Please check all 24VDC voltage level when in operation.
    • Use a fix value resister to confirm the input signal, also check is the shielding of cable connected properly.
  • Isolate power supply for different module, e.g. adaptor& IR8 module use one power supply and DI/DO/AI/AO use another, hardwire the COM of these 2 power supply,
Technical Specialist


He said that 24VDC level could be a problem as well. Do you think it will be better if I use RTD input card at main PLC rack?

Best regards.
 
Last edited:
sutton said:

Thanks Gil47,

Yes, we will go ahead with a trial with the screen cables, and make sure of the grounding.


I have a reply from Rockwell distributor technical specialist.
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He said that 24VDC level could be a problem as well. Do you think it will be better if I use RTD input card at main PLC rack?

Best regards.




Having read your post I will add that I never allow analog inputs to share the same power supply as digital inputs and outputs, as relay switching causes spikes on the dc that will really disturb the analog readings.

I always put in a separate power supply to drive analogs
 
Had this once called in rockwell, they could'nt understand it either, I noticed that the 24v supply was not grounded so I grounded the -ve at the input card this cured all the fluctuations in values.
 

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