Quick Micrologix 1200 Questions

I'm considering buying a Micrologix 1200 and 1762-IF4 analog module. My question concerns the operating range of the module.
The normal voltage is listed as -10 to +10 and current is listed as 4-20 mA.
The "Full Scale" range for current is -21 to 21 mA. The catalog then states "the over or under range flag is set when the normal operating range is exceeded. The module continues to convert the analog input to the maximum full scale range"
I take this to mean that -21 mA will give me a value of -32768 and +21 mA would give me +32767. Is this correct? What exactly is the over or under range flag for?

If I am incorrect, what value will -21 mA give me?

Thanks,
Josh

Must... control... fist of Death.....

I don't know if the Installation Instructions are misleading in some subtle way or merely erroneous. As far as I know, the 1762-IF4 does not measure reverse-polarity current flow; the current loop mode only measures a full-scale range of 0 to 21 milliamperes. The Over or Under value flags turn on at 20 and 4 milliampere thresholds but the module will convert A/D over the full range of 0 to 21 mA = 0 to 32767.

To expand on the Installation Instructions, A-B published Knowbase document G19893.

In the Raw Data mode, here's what you will see in the Input Data registers of the 1762-IF4:

Register Value      Voltage

32767			10.5  Volts

31206			10    Volts

0			0     Volts

-31206			-10   Volts

-32767			-10.5 Volts



Register Value      Current

32767			21 mA

31206			20 mA

6241			4  mA

0			0  mA

This is what I am looking at from Publication 1762-TD001A-EN-P

I know that some simplistic current to voltage conversions are done simply by inserting a 500 ohm resistor into the current line so that a 20 milliamp current yields a 10 volt drop. If the final measuring circuitry is the same for both the current and voltage mode (i.e. measuring voltage) then why not a valid measurement from a negative current?

See how I left myself an out ? "...as far as I know..."

The Installation Instructions and the Technical Data both imply that the 1762-IF4 will read a -21 mA to +21 mA signal. But those same booklets omit the simple Value = Current and Value = Voltage range tables that would make them complete. So frankly I don't trust 'em.

We can answer one of Josh's questions; what is the value returned by +21 mA. It's 32767. The Over-Range bit turns on above 20 mA, so you can use that instead of a comparison or limit instruction in the controller. Same for the Under-Range bit.

There's nothing left for us but good old empirical testing. Anybody got a Micro1200, a 1762-IF4, and a 4-20mA loop tester handy ?

I don't think there is a real world need for -21 mA. I have yet to see a transmitter that gives a -20 mA to +20 mA signal. The only way to get -21 mA is to wire the transmitter output backwards. I'm not sure about the 1200, but many AB modules and many other manufacturer's analog cards will give you an error bit if you reverset the polarity of a current signal.

I suspect that this is a case of a tech writer including info that was theoretically possible but pragmatically useless. (Like Ken, though, I'm going to leave myself an out and say it is possible that some transmitter does have a +/-20 mA output - I've just never seen one!)

i dont have a real world need for -21 mA. I do have a real world need for about -14 to +14 mA (After a 100 Ohm Resistor). I'm working on an automatic battery tester (AA batteries), and the company working on the bowl feeder can't get the batteries oriented polarity wise. So I am looking into other options to see if we can get it to work the way the bowl feeder works now.

I'm open any other possible suggestions.

Thanks for everyone's help

How about just running the battery leads through a 'bridge' (using the 'ac' inputs). You will have the correct polarity no matter which way they are arranged.
Just a thought, its been a long day....

If all you need is a polarity check and a go/no-go test, why not just use a +/- 5VDC or +/- 10VDC input card and skip the 100 Ohm resistor. This should have sufficient resolution for your purposes. Even at a 12 bit resolution you can discriminate at +/- 0.005 volts.

I do not need a polarity test, I need to be able to test both current and voltage...preferrably without worrying about polarity. The requirements are 1.5 Volts minimum and I can't think of the current off the top of my head but its 80% of a new battery.