Rockwell Automation Analogue input card 1771-IFE/C - 250 ohm load for HART

Hi,

Does anyone have experience with this card. Looking to find out if the card has required resistance ( 250ohm ) built in for HART or if an external resistor has to be wired in? It does mention 250 ohm input resistance but not clear whether its just stating the specification of the connected input or that is the card itself. Thanks
 

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Hi,

Does anyone have experience with this card. Looking to find out if the card has required resistance ( 250ohm ) built in for HART or if an external resistor has to be wired in? It does mention 250 ohm input resistance but not clear whether its just stating the specification of the connected input or that is the card itself. Thanks

It is written input impedance, which means it is the resistance which the card will offer to the loop. We also have Rockwell Systems,though not this card specifically. In the datasheet of the cards which we have in our plant,similar input impedance specs are provided and also we do not connect any resistors when using HART.

Thank You.
 
Quoting the 1771-IFE manual, Appendix A, Specifications
Input Impedance = ">10 Megohms for voltage ranges, 250 ohms for current ranges"

I first started working with the 1771-IFE in around 1988 where we had hundreds of them at the brewery, but did not venture into Hart until sometime in the 90's and I'm not really that experienced with it.

Allen Bradly started developement work in the late 70's, long before HART and did not get any external digital communications features. (Think about that, the big clunky, rock solid 1771 family is around 40 years old. Wow!)

Rosemont developed (or adopted) what became the HART protocol around 1988. The 1771-IFE module never did get HART.

That being said, at it's core the 1771-IFE is a voltage input card with, for all practical purposes an input resistance, or if you prefer impedance that is infinite; it provides no burden on the signal.

In order to read current signals there is a jumper on the card to put a 250 ohm resistor across the input lines which converts, for the card, a current signal to a voltage signal for the card's AD convertor. You can leave this jumper off if desired and use an external 250 ohm resistor (or one of a different value) to accomplish the same effect. I have do this in an application where I have a 0-0.5 mA analog signal.

Essentially all (electronic) 4-20ma devices need up to 4mA, but no more at some minimum voltage to operate. For an analog signal we look at the current from 4 to 20 mA, or rather the voltage drop the 4-20 mA signal creates across our resistor, the supply voltage being irrelevant.

If we had two or more 4-20 mA transmitters in series the analog signal would be bonky but since HART is addressable, the digital signal from each device could still be read with no loss of confidence. Most 4-20 loops are not configured with more than one transmitter so we don't think of that configuration but if it's only a HART system with no analog it is possible.

Bottom line, yes, the 1771-IFE when set for current mode has a 250 ohm resistor. It can only have 250 ohms for a 4-20 mA loop, no more and no less or the signal will be out of standard calibration. The HART hardly cares, it just needs enough signal (voltage) deviation to detect the digital data.

I hope my above explanation helps. (I also pray that it's accurate and if not, someone will correct me ;)

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