I like to use two point linear equations in most industrial situations, then it is easy to build a learn/calibration screen.
In the odd case where you have one point and the slope, you can edit the calculation logic. I like to be able to "see" all four values easily on the screen too, so I will use simple math instructions grouped in series and parallel for ease of viewing, as opposed to one big simplified CPT.
If I add HMI or other form of remote teaching/editing of the scaling values, I also add extra checking to ensure a minimum resolution (counts between raw min and max) and direction (raw_max - raw_min > 0) exists on the input signal.
You want to make sure Bubba doesn't successfully re-calibrate a bad sensor or one that can't meet the needs of the controls system because Cletus forgot to tell Bubba that earlier, he turned the all the pots on the device at other end of the wire before he noticed it was unplugged....
So, to avoid doing the hand calcs, you can build your scaling math using tags for all sources, then teach two points using RSLogix to "poke" the raw and engineering values in manually.
Paul
