1756-IF16 input range conversion

On one of my machines there is a 1756-IF16 analog input module in use. A gas detector that has a 4-20mA output is to be wired to it. When we checked the range in Studio5000 the scale of the numbers goes from -32768 to positive 32768 from 0 - 20mA.

Does anybody have a good way to go about converting the negative side of the range to a positive so that i can do scaling and alarms. I was hoping to get it to read 0 at 0mA and around 65536 at 20mA after any conversion is done. Ive played around with some logic to try and scale it but have been unsuccessful.

Not familiar with AB, but why not copy the analogue input register to a standard register and add 32768 to that value, then if it was at 0mA it would be 0(-32768+32768).

Thanks for the reply Jonnie_R. I deal with so much on a day to day basis here that I tend to overthink the simple stuff. I read your reply and couldn't believe I didnt think of that.

No worries, glad I could help.

Find the SCP add on instruction and add it to your project (if it is not already in use in your code). It does exactly what you want. This instruction was left out of the base instruction set for RSLogix/Studio 5000 but it is probably the most useful instruction for any PLC that uses analog I/O.

You could use a CPT block to do exactly the same thing, but the SCP is so much easier to work with that it is well worth the effort to add it.

You can change the scaling on the IO module to something easier to work with.

Ken Moore said:

You can change the scaling on the IO module to something easier to work with.

Click to expand...

I like to set them up to read 4000-20000, for a 4-20mA signal. You lose some resolution, but I can tell what the mA reading is without having to put my meter on it.

Ken Moore said:

You can change the scaling on the IO module to something easier to work with.

Click to expand...

I like to set them up to read engineering units directly, whatever the transducer measuring range is, is what I set the module to produce.


For example, if the measuring device has a 4-20mA signal representing -20 to +125 whatever units, you scale the module to give you -20 to +125 for the 4 to 20mA input.

No need for any scaling in the PLC, the module does it for you.

You need to set the Comm Format "Float Data" to do this.

Well i figured out where all my confusion started. Whoever added the card into Logix had set it us as a Data Format Interger instead of Float. That made it where the scaling and Egineering units and whatnot could not be adjusted. I deleted the card and added it correctly and now i have it scaled the way i needed it.

Daba i didnt see your reply until i posted this which is what i had to do.

Thanks for all yalls input.