Analog Scaling for 12Bit

People,

I am using a Micrologix 1200 controller with a 1762-IF2OF2 module, using an Analog input with 12Bit resolution from the product specs. I have read some previous posts, but I am trying to resolve something about the basic SCP command in Rslogix500.

I am not sure what the range should be set at as the INPUT MIN and the INPUT MAX, for a 12Bit card or does this really matter? My current range is 3136 to 15600, and is calibrated by our calibration tech's. I am not convinced seeing all of the ranges that people use in the programs that the range is set properly. The values are linear, and seems to be calibrated properly, but I need some basic understanding of what the range is really a product of. I have the PIC that was attached in a previous post,

(not sure how to insert image with this forum)

I am looking for accuracy, so any help would be appreciated.

Sorry, I am using a 4-20mA Input..

Depending on how you have it set up in the module configuration word, it would either be raw data (6240 to 31200) or scaled-for-PID (3120 to 15600).

I don't think that card is really 12 bit in the sense used by most PLC manufacturers. That usually means resolution of 0-4095.

Here are some examples of card set-up and scaling analog inputs on a ML-1200:

For some reason unknown to me it is in vogue these days to justify a 12-bit analog signal to the upper bits as opposed to the lower bits. So what you think should be a 0-4095 signal ends up being a 0-32760 signal with a resolution of 8 (0-4095 shifted up 4 bits). Another ineresting little twist is that they may specify the signal as a 12-bit signal because it needds 12 bits to express the highest value, but the highest value may not be 4095. Again, I'm not sure why but I have seen this.

I would recommend either looking in the appendix of the manual links Tom posted to get the design range of your analog module or just determine the values imperically by applying a signal with known engineering units and seeing what the input to the plc is. Do this at any two widely spaced points and you have the values for your SCP instruction.

Keith

Analog

This maybe what Keith is talking about look on page 109.
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1762-um001_-en-p.pdf