PLC5 4-20ma input Scaling

[WildBill49]

I have a 4-20ma input to a 1794-IE8 Flex I/O module. The Flex I/O is talking to a PLC5/40. What is the correct value to use when scaling the raw value. This circuit is measuring water flow in gal/min, and has a max value of 1000 gal/min.

 

[Binaural]

I recently had to make some modifications to an existing PLC5 system to include some analogue values and unless I am very much mistaken, there is no scale instruction. You will probably have to use a compute instruction with good ol' y=mx+b to calculate the scaled value, with a bit of clipping added in just in case.

As an example, assume the 4-20mA input is configured to give 0-4000 counts, and let's assume the input is a pressure transmitter that outputs 12mA at normal atmospheric pressure, 4mA at -1bar and 20mA at +1bar (0 counts, 2000 counts and 4000 counts). This gives you at least two known points on a line that you can use to calculate the presssure given a number of analogue counts.

So, we have the following, assuming pressure is on the y axis:
m = y2 - y1 / x2 - x1 = 2 - 0 / 4000 - 0 = .0005
b = 0 (b is y axis crossing point, but 0 bar = 0 counts)
x = measured input (let's say 3000 counts for first calc)

so y = .0005 * 3000 + 0 = 1.5 bar, which is what you'd expect. Substituting 2000 = 1 bar, 0 = 0 bar.

Hope that helps.... does anybody know a better way to do this? I had no time to reasearch anything better...

 

[Contr_Conn]

Like other 1794 analog modules, IE8 uses only most significant bits.

Look at this manual http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1794-um002_-en-p.pdf

Page C-2 gives you unscaled values for all modes and page C-3 gives you example of scaling

For 4-20mA:
4 ma= 0000
20mA= 7878 HEX
21mA= 7FFF HEX