Analog Input underscale

brucechase

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Join Date
Sep 2004
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Augusta GA
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I need some help on a program I inherited. I don't usually fool around with analog inputs so I am very confused. I have a SLC 5/04 with a pressure input that is ranged 0-507.6 psi on a 4-20mA signal. The card is configured by writing 2077 to the output file. This basically sets up the card for 4-20 signal in the 1746-NI4 format with no filter and a zero for open circuit. I thought that the card could handle 3.5mA to 20.5mA. When I input a 4mA signal, I get 3077 as the raw data input. When I input a 3.8 mA signal, I still get 3077 as the raw data input. Do I need to change my scale?

scppress.jpg



I thought that I've seen an underranged input before but maybe someone needs to set me straight.

My mind is wandering too much now thinking about where I will spend Friday morning waiting in line (do I want the $189 computer at BB or the $999 HDTV at Sears?). Any hard core black friday shoppers here?
 
You dont need to write a config to an NI4 its set up for Voltage or Current input through dip switches. Only the NI8 and NI16 require a Config Write.

For current inputs

It takes 0-20mA which is 0 - 16384

You can scale this for a 4-20 signal which would be 3277 - 16384.

I:20.6 should read your raw mA data regardless of your SCP rung.
 
The card is an eight channel card. The format is the NI4 data format. The dip switch is set for current. I could use the configuartion tab, but the programmer chose to write the word on a first scan.


configpress.jpg
 
bruce,

Why not send DECIMAL"2076" in config word for that channel and read 0-20ma instead of 4-20ma. Would that clear it up? I hate shopping in the crowded stores by the way. They would have to pay me more than a hundred dollars to get me in there with those sale crazed people.
BD
 
Just scaling it from 0-20Ma would only set the zero pressure at 0mA. I guess it would work if the scale was 0-20mA and the scaled input was -xxx to 500psi. I just thought it should work otherwise and I am missing something.
 
More info on Analog signals

I just thought I would update everyone on this little problem I’ve had in case anyone else encounters this. I have a 1746-NI8 card that is configured for NI4 data format. I am inputting a 4-20 mA (3277-16384) signal and scaling the output to be 0-507 PSI. If the signal ever falls below 4 mA (3277) the card stays at 3277. Now here is the twist, at another plant, they said that when their system falls below 4 mA, the raw data goes below 3277. I set this up at my desk and I never could get it to do that (they use the same 3277 to 16384 scale that I had set up). What I did notice is that they are not using a NI8 card – they are using a NIO4I card (but the NI4 card should be the same). There really is no set up for this card, it is in NI4 data format already. The interesting thing is how different they act in response to the signal. So, if you are not using a NI8 card and the signal goes below 4mA, that card will output a true value. If you are using a NI8 card, you will always be limited to the 3277 (or 4mA).



To continue on with this, if I change the configuration of the NI8 card to engineering units and I change the scale to 4000 to 20000 (to equal the 4-20mA signal), the output will show less than the 4mA signal.



I am attaching a couple of screen shots to show the difference in the output.

385inputwithni4format2.JPG


385maengineerunits2.JPG
 
Greetings brucechase ...



here’s something that I said in this post from a related thread ...



my best guess right now (pocket change only) is that the (configurable) 1746-NI8 “works different” than the (non-configurable) 1746-NIO4I module - even when the configuration is set for NI4 operation ... at least that’s what I EXPECT to find ... anyway ... I’ll let you know what finally happens ...



my expectations were correct ... (I love it when that happens) ... here’s a quick rundown of the little experiment that I just worked through ...



Slot 0 = 1747-L542C processor - SLC-5/04 Series C - OS401

Slot 3 = 1746-NIO4I Series A combination analog module

Slot 4 = 1746-NI8 Series A FRN 1.0 analog input module - Class 3



both analog modules were set up and wired for “Current” inputs ... the 1746-NI8 module was configured with a data value of “2077” ... the analog input signal was supplied by a Fluke 787 Process Simulator ... the current signal was wired in series through both of the analog input modules ...



overview: the 1746-NIO4I module uses the “plain vanilla” scaling format commonly known as the “NI4” format ... specifically, a 4.00 mA input signal shows up as a data reading of 3277 ... and a 20.00 mA input signal shows up as a data reading of 16384 ... the 1746-NI8 module is “configurable” to handle different scaling ranges ... for the purpose of this test, the module was configured to use the “NI4 Data Format” ...



the issue which is the subject of this thread is as follows:



when the 1746-NIO4I module experiences an input signal which is LESS THAN 4.00 mA, the data reading from the module falls below 3277 ...



BUT ...



when the 1746-NI8 module experiences an input signal which is LESS THAN 4.00 mA, the data reading from the module does NOT fall below 3277 ...



similarly ...



when the 1746-NIO4I module experiences an input signal which is GREATER THAN 20.00 mA, the data reading from the module rises above 16384 ...



BUT ...



when the 1746-NI8 module experiences an input signal which is GREATER THAN 20.00 mA, the data reading from the module does NOT rise above 16384 ...



one specific cause for concern might be as follows ... suppose that a program has been written for use with a 1746-NIO4I module ... suppose that a couple of simple comparison rungs have been programmed to test the analog input signal - and to sound an alarm if the current signal is either too high or too low ... that program might work reliably ...



now suppose that a new 1746-NI8 module is to be substituted in place of the original 1746-NIO4I module ... a significant issue might develop if the “monitor/alarm” logic is not properly modified for use with the new module ... specifically, since the data from the new module will NOT fall outside of the expected range, the original comparison rungs will never detect a faulty signal ...



oops! ...



luckily there is a solution to the problem ... when the 1746-NI8 module is configured for operation in “Class 3” mode, the module returns an “extra” 8 words of data to the input table - in addition to the expected 8 words of “channel input data” ... each one of these “extra” words corresponds to one of the module’s analog input channels - and included in each “extra” word is an “Over-Range” bit - and an “Under-Range” bit ... the proper use of these two bits could serve the same purpose of the comparison rungs that we discussed earlier ...



so here’s the experiment that I ran ...



Test (1) = input signal too low ... I cranked in an analog input signal of 3.50 mA ...



Results: the data from the 1746-NIO4I module registered at 2867 ... (note that this IS below the “low-end” reading of 3277) ... the data from the 1746-NI8 module registered at 3277 ... (note that this is NOT below the “low-end” reading of 3277) ... the “Under-Range” bit (I:4.8/14) for the 1746-NI8 module had a status of 1 ... the “Over-Range” bit (I:4.8/13) for the 1746-NI8 module had a status of 0 ...



Test (2) = input signal ok ... I cranked in an analog input signal of 12.00 mA ...



Results: the data from the 1746-NIO4I module registered at 9826 ... the data from the 1746-NI8 module registered at 9828 ... the “Under-Range” bit (I:4.8/14) for the 1746-NI8 module had a status of 0 ... the “Over-Range” bit (I:4.8/13) for the 1746-NI8 module had a status of 0 ...



Test (3) = input signal too high ... I cranked in an analog input signal of 20.50 mA ...



Results: the data from the 1746-NIO4I module registered at 16786 ... (note that this IS above the “high-end” reading of 16384) ... the data from the 1746-NI8 module registered at 16384 ... (note that this is NOT above the “high-end” reading of 16384) ... the “Over-Range” bit (I:4.8/13) for the 1746-NI8 module had a status of 1 ... the “Under-Range” bit (I:4.8/14) for the 1746-NI8 module had a status of 0 ...



and I think that this answers the original question ... if not, please post again ... I’ll leave the same equipment set up for another day or two - just in case you want me to nail down anything else along the same lines ...
 
Great Ron, thanks for the help.

As you can see, I did the exact same thing using a NI8 and a NIO4I card with a Transmation (Except I had a 5/05 laying around). See post #7 in this topic. Of course, I got the exact same thing as you. That is when I figured that I could change the format to engineering units and read the raw input that way.


Wait, there is more………



But first -

I guess I might explain exactly what I am trying to do. I have a transmitter that measures pressure on a system that is 0-500 psi. This transmitter puts out a 4-20 mA signal in proportion to that pressure. After the pressure is cut off, I pull a slight vacuum and I want to verify that there is a vacuum. I can see the transmitter go to 3.85 mA when there is a vacuum and I wanted to see it in the plc. But every time that I go below 4 mA, the raw number never changes so I can never tell if there is truly a vacuum on the system.



Now for the more part, another plant that has a very similar system tells me that I am full of it. They can see the input drop below 3277 with an NI8 card configured for NI4 data format. WHAT!!!! I am no expert (I know that is pretty obvious) but for the life of me I can’t figure out what is different. I was hoping someone here can point me in the right direction. Here is a screen shot of their plc with the configured input.

I really appreciate you taking some time out of your schedule to help me on this. I might be out of touch for the next few days since we are entering a shutdown and there is plenty of work to do. We come back up on the 26th so I might not be able to respond until then.





jolplc.jpg
 
They can see the input drop below 3277 with an NI8 card configured for NI4 data format ... I can’t figure out what is different.



well, if you believe them (and it sounds like you do) then there has to be SOMETHING different ... how about the Series/FRN versions of their NI8 module? ... they’d probably have to shut down and pull the module to read the label - but MAYBE that’s where the difference lies ...



trivia side trip: on the PLC-5 analog input module side of things, the older 1771-IFE modules - Series A and Series B - will NOT go outside of their “scaled” ranges ... usually the range is set for 0 to 4095 for a 4.00 to 20.00 mA input signal ... BUT ... the newer 1771-IFE Series C module WILL go outside of the “scaled” range of values whenever the input signal leaves the 4.00 to 20.00 mA range ... so if the “new-and-improved” hardware for the PLC-5 platform can do the “out-of-range” trick, then MAYBE there’s a newer version of the 1746-NI8 module out there - and MAYBE you and I just haven’t run into it ... MAYBE your sister plant has one of those ... MAYBE? ...



I certainly don’t want to start a fight with anybody - so I’m not going to say that the guys at your sister plant are wrong ... MAYBE there’s just a “failure to communicate” somewhere along the line ... is there ANY chance that they’re simply misunderstanding exactly what you’re saying? ... is there a chance that they’ve never really actually SEEN the results that they’re reporting - and that they simply ASSUME that “it-must-work-that-way”? ... how easy/safe/simple would it be for them to absolutely/positively confirm that they actually CAN duplicate the “out-of-range” results that they’re reporting? ...



I’ll think about this over the next few days ... maybe something will pop up ... any other members out there have any experience with this particular issue? ...
 
Last edited:
Greetings brucechase ...

I think that I might have come up with the answer that you're looking for ...

NI8_a.JPG


try this setting for the "Input Type" ... when I tried it this way in the lab, the NI8 and the NIO4I modules both reported the same "BELOW 3277" data when the input signal went below 4.00 mA ... I'd be willing to bet (considerably more than pocket change) that this is the way your sister plant has their NI8 module configured ...

all of your screen shots show a "4 to 20 mA" setting ... but I think that "0 to 20 mA" is the winning ticket ... let me know how this works out for you ...
 
Not a Betting Man

I would imagine that betting against certain people here (and Ron is one of them) would be a very bad move. Of course, Ron is correct. When I configured the input for 0-20mA, I read what I wanted to read.

Now for the explanation of what I did wrong. When I looked at the configuration data, I looked at channel 6 for input I:20/6. In reality, I should have been looking at channel 7 since I:20/0 is channel 1. When I called up that configuration file, the input type is 0-20mA just like Ron said. I guess I'm a little embarrassed that I associated the wrong input with the wrong channel, but that isn't the first time. Thank goodness that there are people here that can keep some of us straight.

Thanks Ron for helping me out. It drove me crazy for quite a while and the worst thing is it was all self induced.
👨🏻‍🏫 - :geek: I am truly humbled with all the knowledge here.
 

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