1794-IR8 Configuration

[James_85 0]

 Hello everyone.  I have installed a remote Flex i/o enclosure with a AENTR ethernet adapter and an 1794-IR8 RTD card. I have landed 4 Rtd's on this card. The RTD's are 3 wire PT100's with a Fahrenheit range of -58 - 572. I have tried using an AOI scale block and tried using some other scaling methods we have and nothing seems to work. I have tried reading the 1794-IR8 manual, but it's not very helpful (To me anyways). Is there certain config bits on this card that need to be set? I have never used one of these cards before. Any thoughts or ideas? Anything would be appreciated.  

[Joe E. 259]

I haven't worked with this module before, but I have used other Flex IO Analog modules, so here's what I suggest. First, it's really...interesting...that the latest installation instructions are from 2022 while the latest user manual is from 1996.

Anyway...

Once you add the module to the I/O Configuration, double-click on it to open its Properties and go to the "Channels" section. Did you set that correctly? Looking at the module specifications sheet, I don't see a direct match to your RTD as far as temperature ranges, so you may be able to choose the closest. I wouldn't really expect that to work too well, though. Page 20 of the module specifications says it can handle 1 to 433Ω. Does your RTD fall within that range?

The literature talks about using high-accuracy resistors to do a calibration. Based on that, here's how I would proceed:

1) Set the channel configuration to "Resistance" since none of the RTD options seem to match yours.
2) Note the resistance values of the RTD at the extremes of its range (use the spec sheet for the RTD to find those)
3) Get your hands on some resistors with those values. **If you can't find 2 matching resistors, get some that are within that range, as close to the ends as possible and calculate the temperature that corresponds to each resistor**
4) Connect the low-temperature resistance to the input and note the value in the Ch*Data tag. *Refer to the user manual and/or installation instructions for connection information, especially the difference between 2-wire and 3-wire connections*. This raw value becomes the Input_Low value for your scaling function.
5) Change to the high-temperature resistance and note the value in the Ch*Data tag. This raw value becomes the Input_High value for your scaling function.
6) Your Output_Low and Output_High values are the temperature values that correspond to the respective input resistances.
7) In previous versions of RSLogix, I'd say use an SCP instruction, but AB didn't include that in 5000. I've written an AOI that does it and I think AB has one as well that you can download. Or, you can write your own scaling code. Here's what I do, broken down into steps to make it easier to see what's happening:
A) Input_SPAN = Input_High - Input_Low *Limit check your Input_SPAN to make sure it is not zero (so you don't fault the processor)*
B) Output_SPAN = Output_High - Output_Low
C) Input_PART = Input - Input_Low
D) Input_RATIO = Input_PART / Input_SPAN
E) Output_PART = Input_RATIO * Output_SPAN
F) Output = Output_LOW + Output_PART
That can mostly be combined into a CPT instruction as long as  you make sure the Input_SPAN isn't zero ahead of time.

Let us know how you make out...

[James_85 0]

Thank you Joe E. Yes, I noticed that the manual was very out dated as well.  So I believe its been sorted out. There was a wiring issue along with something on page 4-3 of the manual. There is a small note there that says there is an implied decimal one space to the right of the last digit, so I had to divide by 10 for the correct value and make it a REAL value.

Note to self: Read The all the small notes in the manual.

Thank you for your help with this.