PLC 5 analog input scaling question

Hello everyone,

I have a plc 5 input card receiving an input from a conductivity
Transmitter where 0ma=0 uhms and 20ma= 20,000 uhms
The raw min is 0 and Max is 4095, I’m not very familiar with plc 5 but I understand there’s no SCP option.

What would be the best way of scaling this in the ladder logic?



Thank you for your help.

I believe you have to do this manually using a y=mx+b equation. So I believe your equation would be y = (20000/4095)x-5000 . How to achieve this is to take your analog input (I:0.0 for example, not too familiar with PLC-5), multiply it by 4.884 (this is 20000/4095) in a MUL instruction, putting the result in a floating point (F8:0.0 maybe) and then using a SUB instruction to subtract 5000 from your floating point, placing the result of that in a second floating point (F8:1.0). This should be your conductivity reading.


If I messed up the general procedure or addressing syntax, someone else may be able to correct me, but that's what I've had to do in some cases.


EDIT: I've usually done this with 4-20mA not 0-20, so that may affect something, but I believe the math is correct.

Screenshot of how I do it. This is done in Logix5000 for a CompactLogix, but the process is the same.


Note that your b value will always be -1/4 of your max measurement. In your case, -1/4 of 20000 is -5000.


I'll do the math for my example for clarity.


y=mx+b
y=([measurement max-measurement min]/[raw data max-raw data min])x + (-[55.36/4])
y=([55.36-0]/[20000-4000])x + (-13.84)
y=(55.36/16000)x - 13.84
y=0.00346x - 13.84

Scaling formula

A few watch-outs:
1. Being a PLC-5, the input will be coming through a block transfer from the analog module (assuming a 1771-IFE). Make sure to have the correct address of the integer containing the raw value.
2. Since the input sensor is specified as 0 - 20 ma input = 0 - 20 K-uhm, the conversion will just be raw input times 4.884 (20000/4095).
3. Make sure the channel is set up for 0 - 20 ma, and not 4 - 20 ma.
4. Make sure input is wired for current and not voltage. EDIT: jumpers are set for current mode and single-ended/differential as appropriate.
5. You might be able to view the channel configuration via I/O Configuration -> Chassis -> Module. This makes the channel setup easier than editing the BTW to the module, and you can watch the live data update while online.

I believe the PLC5 scale the analog inputs in the module read the scaled input value in the block transfer read data table.
you have to find the block transfer write to the module
you will need to read the manual to understand how to scale the module. in most cases the module receives the scaling in the first block transfer it is usually only transferred in on the first scan only I dose not need to be rewritten unless you want to change something

Thank you!

Learn something new every day, sorry for the misinformation in my posts.

mylespetro said:

Learn something new every day, sorry for the misinformation in my posts.

Click to expand...

Your information is still valid. The scaling done by the card will generate an integer result. You could change it from the default of 0 to 4095 in the card configuration, but in most programs I have seen, separate math is done to generate a value that is in engineering units and usually a floating point number like in your example.