Help With Analog Value Calculation

Having a brain freeze.

I am trying to figure out how to calculate an underrange RAW value for a PLC-5 1771 analog input card set for 4-20ma input range with 0(Raw Min) To 4095(Raw Max).

Instrument Range = 4 ~ 20ma (50 to 100degF)


Suppose the instrument fails and It's now outputting a 3.0ma signal instead of 4.0ma which is the low scale value. How do I calculate the PLC raw data value for this?

3.0ma is going to be a negative value since instrument 0 = PLC Raw count of zero.

Without changing the analog card set-up

Q1. Can AB's analog data reigster for a channel have a negative number (instead of 0 to 4095) if it's raw count is set as 0 To 4095 and is set-up for a 4-20ma signal?

Q2. I don't want to use the under-range bit for this but instead of looking for the actual raw value at 3.0ma. Can this be done? If it can, How would I find (formula) the actual negative value at 3.0ma?



Thanks all!

whether or not it can be done depends on what series of 1771-IFE module you have ...

if it's an "A" or "B" type then the value will NOT go below the "MIN" scaled value ... (or above the "MAX") ... but if you have a "C" series, the value CAN go "out of the scaled range" ... either above or below ...

but ... notice that the "C" series has a jumper which can be used to force the newer module to "simulate" an older "A" or "B" module ... when that jumper is properly set, the data reading won't go "out of bounds" ...

Configure the card for 0 to 20ma instead of 4 to 20ma. ( see pic below)
Then 4ma=819, 3ma=614 raw value.
Or connect 3ma to the card and see what value it gives you ( real close to -255) ,it will be negative. Then use that value to set your alarm.

Greetings Mickey ...

glad to see that you're picking up on this one ... time for me to head home for supper ...

maybe I'm wrong - but based on this:

Without changing the analog card set-up

Click to expand...

I'm not sure that he's going to be able (or willing) to change the setup ... oh, well ... let's see what tomorrow brings ...

Using Mickeys sugestion of 0~20 mA
Instrument Range = 4 ~ 20ma (50 to 100degF)
50 deg range for 16 mA

becomes
Instrument Range = 0 ~ 20ma (37.5 to 100degF)
62.5 deg range for 20 mA

4 mA input would still read 50 deg F