Weird Analog Input Scaling

I was going over a SLC500 program that soneone else wrote and the analog inputs (all 10) were scaled from 4000 to 20000 instead of the 3277 to 16384 that I'm used to seeing. Does anyone know why someone would use that weird scaling? The inputs were a variety of 4-20ma instrumentation.

I've seen that on GE Fanuc 9070. The scaling can be declared in engineering units in the configuration; it was done this way to give a direct read in the tables of 4-20 mA. I've never been sure of why someone would want to do that, except to remind the person looking at the program that it is a current loop. In this way, the 4000 - 20000 would then be re-scaled in the program to be pressure or speed or weight or whatever.

I assume we are talking about a 1746-NI8 input module? Check the module configuration; it is most likely configured for Engineering Units (4000 - 20000) for 4 - 20 mA.

You are right. The module was configured for engineering units. I am assuming that 4000 to 20000 is the only option since the engineering units are defined as 1 microamp per step. So,

4ma = 4000 microamp etc.

I get it. Thanks.

I actually prefet the engineering units (4000 to 20000) as I can look at it and immediately get useful info on the output signal of the transmitter. If my input data is near 12000 I know I am at 50% of range. If my input data is 9830 I have to do a calculation or at least some mental interpolation to find out I am at 50%.

4-20 is always 4-20. We work with a lot of PLC brands, and it seems like no two handle input raw data the same. Sometimes there are even variations for different model cards from the same supplier. I can't keep that all straight, but after a few decades of 4-20 mA I know what 0%, 25%, 50%, 75%, and 100% signals are!

I like it like this. When troubleshooting or testing you know what the mA are without having to calculate and you can quickly tell if what the PLC thinks it is seeing matches what your meter says. You can use PLC logic to convert to engineering units to use in your program.