1734-IT2I module showing Overrange

Hi guys,

I was hoping somebody in the Forum happens to be working tomorrow and has an application with Allen-Bradley 1734-IT2I thermocouple modules they could do a quick check for me.

The ones on the machine I'm working on are showing the "Overrange" fault bit, and giving me the maximum Type J temperature (2972.0 F) in the Data field.

The punchline is that they do this when they're not connected: we have thermocouple jacks in the wall of the enclosure, and most of the time they're not plugged into actual TC's. When an actual TC is plugged in (and the quality control guys didn't leave the screws loose....) I get a realistic process temperature.

I would expect that a disconnected thermocouple would show an Under-Range fault.

I didn't work on this part of the machine during development and testing, so my onsite experience is all I've got to go on. If somebody else can say "yeah, I disconnect mine and they go to Overrange too", then I know I'm less likely to be sitting on a few dozen damaged or defective modules.

I've got a case in at RA Tech Support, but I'm +14 hours from them.

Thanks in advance,

Ken Roach

If somebody else can say "yeah, I disconnect mine and they go to Overrange too"

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I have not used that module but others do the same when not connected to anything, floating.

I just short the terminals if it causes a problem.

Thanks for the information, Mickey.

I tried your suggestion and found that the modules still indicate the maximum measurement value and set the Over-Range Fault bit when the input terminals are jumpered together.

Either every single one of my 1734-IT2I's is defective or these modules simply react to an open circuit with this behavior.

Following Hickam's Dictum, it turns out I also have some damaged modules. When I jumper Channel 0 to Channel 1 and give them both an identical mV reference input, one channel shows the supplied number of mV, and the other remains at 78.02 mV (the maximum theoretical A/D limit of the module circuit).

I've even identified two different failure modes for the damaged modules; four of them always read full-scale, but performing a calibration procedure on one of them results in a slow draining of the over-range value (past zero and into negative values) over the course of 5 minutes. If you perform the Accept Low Limit while the value is falling through zero, you can actually get the module to appear to function correctly again. Trusting it with my application is another matter; these were branded as defective with my Sharpie of Wrath.

The first issue remains a little perplexing to me, so if anyone has a 1734-IT2I they can just pull the wiring arm off for a few seconds to check how it behaves, that would be awesome.

when the input terminals are jumpered together.

Click to expand...

Are you shorting all unused channel together? If you just put a jumper on each channel
where you would normally connect the TC it should then read a m b i e n t.

RA Technical Support got back to me today and confirmed that I'd overlooked this information in the User Manual.

Setting the Over-Range bit and the full-scale value are normal behavior for open-wire detection on these modules.

As the good book says, "I repeat, we have normality. Anything you can't cope with is therefore your own problem."