1746-NT8 Analog Scaling

Hi all,

I am messing with an existing system that is operated with a PLC5-60. The system has remote I/O with the 1771-ASB module. The system use to have a rack that had a 1771-IXE thermocouple card in it. They recently wanted to upgrade that rack, so they bought some SLC cards/chasis/ASB module and replaced it.

I have the 1746-NT8 card in this rack and I can not for the life of me get correct readings from what the operators say was displaying. There is a temperature gauge on the outside of a freezer and when the freezer is at 46 F my reading on the NT8 is 46 F. However, when the value gets lower than about 20 F my temperature slowly stops being accurate. The value on the gauge will read about 11 F and my reading will be about 20, it could be -10 and I'll be getting a reading of +11.

I'm using 100 mV ICTDs for the inputs and the card is configured for the 100 mV signal. The 1771-IXE card that was in the old rack was configured for 2's compliment. Which from my understanding was returning a value of 0-8190. There was then a calculation of ((((x|24.9)-273)*9)|5)+32) to give the value in degree F.

I configured my 1746-NT8 card to receive 100mV signal and scale it at 0-16383. Then I did the exact calculation just divided it by 2 or 49.8 and carry on. However, something is not clicking.

Any insight on this would be more than helpful. It seems like something simple but I'm missing something and I'm beating my head against the wall.

I've attached the manuals for both if needed.

NT8: http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1746-um022_-en-p.pdf

IXE: http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1771-um658_-en-p.pdf

Thanks!

From a quick glance i had on the manual go to page 47 of the first manual you posted and the one you want to look at is :

Proportional Counts to Engineering Units
Equation:
Engineering Units Equivalent = SLOW+ {(SHIGH-SLOW) x [(Proportional Counts value displayed +
32768)/ 65536]}
Data:Assume type E input type, proportional counts display type, channel data = 21567.
Want to calculate °F equivalent.From Channel Data Word Format table, SLOW= -454°F and SHIGH=1832°F
Solution:Engineering Units Equivalent = -454°F + {[1832°F -(-454°F)] x [(21567 + 32768)/65536]} = 1441.3°F

So make sure you pick the correct thermocouple type that you are using then for the parameters SLOW and SHIGH those are the scaled min and max values that you want. If that doesn't work check your wiring and make sure the shielding cable is terminated correctly then if that still doesn't work try wiring the transmitter to a different channel in the module and see if that works.

To scale properly we need sensor information, not just PLC card information. Can you give us the range specs on your ICTD? There are different models, for example, one typical ICTD ranges from -40 to 100C. Also let us know the output, is it 1mv/K or 10mV/K, and let us know if it is normalized to 0°K (my assumption but you know what they say about assumptions), 0°C, or 25°C.

The sensor information is 1mv/K. As far as the range, I do not have that and from what I understand from the electrician he does not have that either. The only information I have on this ICTD is that it is 1mV/K and it was going to a 1771-IXE card. The 1771-IXE card was configured for two's comp and that's all. Then in the code the original programmer did a CPT instruction to get real data back ((((x|24.9)-273)*9)|5)+32).

Can you post the program for the PLC5/60? We can then see how the card (1771-IXE) was configured. The .RSP file, zip it first.

Refrig1 is the original code. LAD 17 - Analog is where they did the BTW/BTR instruction. Rung 207 is the one that I'm dealing with.

Refrig1_9_28_13_Online is the newest code. LAD 38 is the file where I am doing my BTW/BTR instruction.

Thanks!

Edit: Rung 60 not 207 is where the IXE card is.

Sorry monkey082506, I don't understand what your ICDC device is or what the millivolt values represent.

You might try configuring your 1746-NT8 card for engineering units (data format) to see if this gives you the same values as the 1771-IXE card did.

I'll be on site in the morning, I'll keep the post updated if I figure it out.