4-20mA XMITTIER INTO 2 INPUTS?

[DAVID45]

i need to feed a signal from a single pressure transmitter into 2 inputs on an allen bradley plc.
can this be done?
if so, how?
thanks for any suggestions.

 

[Skogsgurra]

A true 4-20 mA transmitter has a large compliance range. That means that the source is more or less floating - and that also applies to a true 4-20 mA receiver. In other words: the inputs are isolated and can easily be connected in series. Like this:

-----------------------> 4-20 mA
+ -------RCVR1--------RCVR2------- -

The 4-20 mA loop was once designed for just this, connecting several devices in series. In the older days (when all devices were moving coil type recorders and indicators) the insulation came naturally. Today, one has to look carefully to find true floating circuits, but I am convinced that AB has them.

Just connect and watch it work! You can not destroy anything, really.

 

[peebee]

You should be able to series the inputs as mentioned.

But a PLC program change might make sense instead. Just a thought.

 

[hacksaw]

if you have isolated 4-20mA inputs series them, otherwise bring your ma signal to a common sense resistor and monitor the voltage with your two inputs.

Some input cards allow universal (mA/vdc) monitoring.

 

[DAVID45]

the inputs are configured as single ended with a common power supply and common connection so i can't really find a way to series them into the input board.
a software change is definetly the prefered solution but no one here is willing to buy the required software(anyone know of a lower cost program to access allen bradley controllers).
i found a work around by changing the order of the inputs which exploits a software quirk.
thanks for all your help!

 

[hacksaw]

some mA input cards are actually voltage inputs with a sense resitor in place. what has the A/B supplier advised?

 

[peebee]

Re: "the inputs are configured as single ended with a common power supply and common connection" -- that doesn't sound like 4-20mA to me, which is typically a 2-wire loop powered at the transmitter.

But maybe I'm the one that's confused. Can David45 or anyone illuminate this for me?

 

[DAVID45]

my "common connection" remark was misleading. all 3, 2 wire transmitters are powered by the same 24v power supply. they then return individual mA signals to the inputs.
a common for the entire input card is tied to the power supply ground.
the allen bradlety appears to let you set up 16 inputs/card this way or 8 inputs/card if you need isolated grounds.