Analog input, Newbie

Hey guys, I'm trying to learn how this analog input works. Right now I have a level sensor that spits out 4-20mA depending on what it sees.

The question I have, what is the value the plc actually recieves?

Does it see the integer 4 (4mA), does it see it as .004A? Or some other value all together. I'm thinking I need use a move function to move that analog value to a register. But then do I need to scale anything? Thats where I'm lost.

I want to trigger a low level light and a high level light. So if I brought that value in, and compared it to a preset value with a greater than/less then command I could trigger a bit to turn on the corresponding light.

This is for a GE Versamax plc. Probably a pretty simple concept, just need some direction, thanks!!

It converts 4-20 to 1-5 volts. (A 250 ohm precision resistor is across the input). The scaling inside the PLC will be 0-5 volts. The A/D will read this as counts. Probably 0-32767. Check the manual for the actual number. 6553 is 1 volt, so 4-20ma will scale as 6553 to 32767.

Keith is right about reading the manual.

If you have the correct input card the 4-20 mA is scaled 0-32000 in the input register. You can then scale it to engineering units, and use compares to set your alarm. The attached may help get you started.

Thanks for the input guys.

My card is 0-32000. If I use the Scale function, I would use 0-32000 as my input scale, and lets say 0-24 as my output scale. (0-24 being the height of a tank I want to fill in inches). So I'd have 1333.33 counts/inch. Would units have to be addressed?

By using this scale function, I could then use different compare functions to turn off an on my lights? But, scaling that down to 0-24 is quite a bit, if I wanted to keep an accurate track of my level in my tank, would it be more precise to scale it to 0-24,000 taking note that you must divide by 1000 to get back to my true inch unit?

Am I thinking right?

Paully's5.0 said:

Would units have to be addressed?

would it be more precise to scale it to 0-24,000 taking note that you must divide by 1000 to get back to my true inch unit?

Click to expand...

1) I'm not certain what you are asking. Engineering units aren't required in the PLC - they are for the human beings using the system. However, there must be a register address in the function to store the engineering units.

2) Using two or three implied decimal places is a good way to do it. In your case I doubt if you need resolution to 0.001 inch, so 0-2400 would probably be fine.

I'm used to putting the units on everything I work with, not seeing them in the plc just looked weird too me.

Again, thanks for the input, I'm gonna go experiment!

The difference between 0-20ma and 4-20ma is open-loop (fault) detection.
Assuming an analog input of 1-5 volts for 4-20ma, anything below 1 volt is a fault. Testing for .5 volts or so would be a good point to eliminate drift and noise.
With this in mind, I would expect a range of 6000-32000, not 0-32000.

I'm probably adding extra confusion here. I'm at work so this is just a quick comment. Please feel free to update/correct.

Why don't you scale it to the engineering units you want, if 4mA is empty and 20 mA is 10 inches, then scale your final reading between 0 to 10, or 0 to 1000 if you want fractions, i.e. 850 = 8.5 inches.

It would then give you a reading you can visualise.