Improper 4-20 mA Reading on a 1746-NO4I

[JKS]

Greetings;

We have a flow meter measuring our Process Water flow, and it sends a 4-20 mA signal to a SLC 5/05 through a 1746-NI8. The head engineer wanted to connect a data logger to it to gather some readings, so I simply wrote a move that passed from the Input signal on the NI8 to an Output on a 1746-NO4I. Should be easy-peazy, right?

Of course not. While I have ~9.5 mA signal coming into the NI8, I have ~5.5 mA signal coming from the NO4I. From what I can see, everything is configured correctly. Tried moving the output, but no-go - more of the same. Anyone have any ideas as to where I should be starting when I get back in tomorrow morning?

Any ideas are greatly appreciated! Thank you much for your time!

- Jeff

 

[OkiePC]

Double check the configuration words. Sometimes when dealing with these, I start a new RSLogix500 file and use the Adv. Config. button to create the configuration words for me and then compare the numbers it generates with the "real life" machine.

At one time I had a comment database in MS Excel for all the analog cards that use configuration so I could type in the base address and get comments created for me to import, but I left that behind at a previous employer. It is really sweet when every bit of every configuration detail is commented. Somewhere, some day, someone will be grinning when they drill into one of those files and find all my efforts to tell them exactly what is going on.

The NI8 card can be configured for a number of different input types and is probably not scaled the same as the NO4I. Maybe it is scaled 0-20mA, and your logic "thinks" it is 4-20mA.

Another approach would be to scale the input to milliamps in the SLC, and then scale it back to raw for the output.

Or do none of this, and just wire the data logger in series with the input.

 

[mppenno]

Are you sure you are moving the raw input and not a value that has been scaled/manipulated to reflect the 16 mA of the transmitter range? With it being exactly 4 mA different that would be the first thing I would check. Did you check the difference in mA at different flowrates? Again if it is exactly 4 mA I would think it would point toward some sort of scaling problem.

 

[tarik1978]

The value you moved is it scaled or unscald?

 

[Mickey]

Post your .RSS file. Zip it first.

Tell us what input you are trying to pass.

 

[BryanG]

Before you starting questioning your code, check what the output is connecting to. Is its input resistance low/high enough for the 4-20mA output to drive. Do you have an offset caused by different 0V voltages between the PLC and Data Logger. Do you have an active or passive output, is your loop driving voltage correct. You are far more likely to find the error in the 4-20mA loop than in your coding.

 

[JKS]

The value is unscaled, simply: Mov: I:5.1 Dest: O:12.2, as can be seen in the screen grab (Rung 0007):

I've also attached the full program (zipped, of course), but you will find the associated code in Process TK (LAD 14), Rung 0007.

I've looked through the config, but not completely sure as to what I'm looking for, so... still looking. Wanted to post a reply before the complete day was shot...

Thanks much for your help guys - it's very much appreciated! Hopefully I can get a handle on this...

- Jeff

 

[OkiePC]

I quickly looked at the file. The NI8 card in slot 5 has all channels configured for engineering units.

So, 4mA will show up at the input word as 4000, 20mA will be 20000. This is not going to match up with the way the output card scaling works.

Change your MOV to a SCP and use 4000 and 20000 for input min and max, and ...well I would have to look up what the output card calls for 4 and 20mA...I work with too many Micrologix now and can't trust my memory on the SLC card details...

 

[JKS]

Ok, I'm seeing that for the Input card. I'll look up what it wants for the Output.

But, wouldn't I need to rescale it before sending to the output? Or am I over thinking things?

-Jeff

 

[OkiePC]

Yes, you need to rescale it from 4000 to 20000 as whatever represents 4 and 20mA in a 1746-NO4I
EDIT:
Which is 6,242 to 31,208 according to the manual I just found.

 

[JKS]

Ah-ha! I hadn't gone quite far enough in the manual, tried using the wrong numbers. These make much more sense!!

- Jeff

 

[JKS]

Sweet! Works perfectly now! Thank you very much, my boss is now very happy!

- Jeff

 

[DwSoFt]

Okie PC has always been helpful on here, Good Job!