murrbuck2

1747-NI8 Open Circuit

9 posts in this topic

I have an 8 point analog input card on a rack with a SLC 5/04 CPU. 

It has been in operation for 20 years or so.  There have been some power problems at site lately and ever since they fixed them I have been having issues with 3 of the 4-20mA circuits on this card showing open circuit. 

I have had my meter on them when they show open circuit.  The signal reads 3.96mA, just below the 4mA minimum and boom, the machine goes down.  Is there a way to adjust this tolerance??  I don't think there is anything wrong with the 3 transmitters so the error is just a nuisance.

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The module has status bits to monitor for an open circuit. Is the program using these bits to cause the machine to go down.

Check programming for this.

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I wouldnt think almost 4ma would present as an open circuit.  It may have an underrange bit that could be being used.

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https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1746-um020_-en-p.pdf

If this is the correct module, according to the troubleshooting section, the open circuit detection is not configurable. If it goes below 4ma, you get the error and there does not appear to be a way to adjust that tolerance.

Try wiring a signal generator or replacement sensor directly to the input module, and run the input up and down the 4-20 range and check module response. If all is well, do the same at the instrument end of the wires. If your test tool or replacement instrument works at the controller but drops below 4ma when wired at the sensor end, you likely have a wiring issue. Otherwise, your sensors may be bad. It's not normally the case for multiple sensors to fail simultaneously, but the fact that you've had recent power quality issues raises an eyebrow towards your wiring and/or field devices.

Hope this helps.

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I cant find in the manual where the open circuit function is looking for the input to just go below 4 ma.  That doesnt make sense.  What would the underrange bit be set for?  I think the open circuit function is just that.  It looks for a load and if it doesnt see one then it alarms.  If it was just looking for a below 4ma condition then a quick work around would be to setup the inputs as 0-20ma, but I've used this card in the past and not had this issue.  It's not unusual for a transmitter to send a value of 3.95 or such at the low end of its range.  Generally the 4-20 inputs will read slightly below 4ma and slightly above 20ma.  I would still check the logic to make sure its not programmed around an underrange bit.  If it really proves to be the open circuit function causing the problem then you may have a damaged card or field device.

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PLCMentor, I've worked with many AB products that give various *loss of signal* alarms when the signal is merely out of range. For instance, the PowerFlex 750 series VFDs will throw a loss of analog input fault if the input drops below 1 volt. Even though the literature says the drive is rated for 0 - 10v, anything below 1 is an automatic fault. Verified. Many times, lol.

The OP states he gets an open circuit fault only when the input signal drops below 4ma, only on three channels, (granted, he didn't say how many channels are actually in use), and that he does have at least 3.96ma (which is not an open circuit). So at that level of input signal the card would, in fact, be detecting load. Yes, I am extrapolating from previous experience but I don't think it's unreasonable to assume that it's possible the card is interpreting the drop below 4ma to be an open circuit. The OP asked the question, "Is there a way to adjust this tolerance??". Assuming I have the correct manual, the answer to that question is no, you can't change the tolerance, per se. The manual states that when in 4-20ma mode it's automatically checking for open circuit and unfortunately, it doesn't define for us what constitutes an open circuit. While 3.96ma is definitely a load, we're still seeing the open circuit fault at that input level. The only way to know for sure whether less than 4ma is an "open circuit" is to test this by using your suggestion of reconfiguring the card.

You also suggested looking at the logic. The OP stated that when the input(s) go below 4ma, "...boom, the machine goes down". He didn't state it specifically, but it sounds a lot like he's describing a major recoverable fault in the controller. While I don't think an I/O fault bit would be used in logic to trigger a controller-level fault, the input card fault bit going true would be able to fault the controller. In other words, we can troubleshoot this with configuration settings, but I doubt it's code related. This assumes that the power quality issue is the only thing recently worked on and that someone hasn't been monkeying around with other things in the controller. (SOOO many assumptions, lol).

According to the OP, the devices worked for many years, then began giving faults after the system was worked on. Chances are, something was damaged when the work was being done. While I think my previously posted recommendations are solid, you may have cut to the chase a bit quicker by suggesting reconfiguring the card. If a wire was pinched during a repair and now the signal is dropping below the configured threshold, that would definitely throw a fault. Changing the config to a value below that threshold would prove this out and get us closer to solving the problem.

muurbuk2, page 5-2 of the manual shows that changing bits 0, 1, & 2 in the configuration word from 101 to 100 will change the input config from 4-20ma to 0-20ma. This has to be done on each affected input channel. As PLCMentor suggests, this may be a less invasive (and possibly quicker) way of narrowing down the problem. Ultimately, something changed and your previously working system is now faulting out. Reconfiguring the I/O channels is a method of troubleshooting, and therefore probably not a permanent fix. The bad part(s), whether the card, wiring, or instrument(s), will eventually need to be found and replaced.

 

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One quick thought comes to mind reading all of this,  @murrbuck2 said he had Power Issues which were recently resolved.  I'm assuming a lot here, but what if those Power Issues degraded the DC Power Supply feeding the Analog Devices.  Now the devices are sending 3.96 ma rather than 4.00 or 4.01 because of a slight reduction in supply voltage.  Of course this line of theory only works if all three bad inputs share one Power Source and the good one another.  The suggestions from @PLCMentor.com and @ElectronGuru are probably the best place to start.

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I'm having a hard time with the open circuit with slightly under 4ma.  I reread his post and he does specifically indicate that he is receiving an open circuit condition.  That throws out any underrange condition that I veered off course on.  Maybe the power supply is wigging out (technical term) or maybe the card has problems.  I would not assume a processor fault just because the machine goes down.  The open circuit condition makes the input value fail high or low and that would very likely trip a limit in the program to make the machine stop.  If I remember right, the manual says that the open circuit function is only operational in the 4-20ma mode.  That would be another reason to try the 0-20ma just to get away from that function.  Keep in mind that you may have to do some rescaling.  I would also add some logic to check if the input is dropping out (maybe less that 2ma).  Just because you cant see it while monitoring doesnt mean that the PLC doesnt see a quick dropout that is more dramatic than 3.96ma.  A rung with a LES and a latched bit would be a simple way to check if something is happening that you are not seeing from the programming software. 

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are problems with all three channels happening at the same time? that would suggest wiring problem (loose connection).

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