One 4-20mA Transmitter to 2 Analog Input Cards

rajy2r

Member
rajy2r
Join Date
Nov 2006
Location
Canada
Posts
167
I am working on a SIL2 system with a control logix, and refering to control logix documentation, to achieve SIL2 each Input Discrete or Analog needs to be wired to two input modules.

Further it goes into 4-20mA transmitters to be wired with a 249 Ohm resistor and then wire them as Voltage input to the transmitters (Tech Note 71027).

Is there a way around to wire these as 4-20mA, as i would like to keep the ability to detect open wire.

Capture.jpg
 
To answer your specific question, 4-20 mA across a 250 Ohm resistor will give you a 1-5 VDC signal, so you can still detect loss of signal at less than 1 VDC.

However, the diagram you posted isn't illustrating one transmitter to two voltage input channels. It is showing two transmitters to two current input channels. Everything to the right of the screw symbols (IN-0 etc.) is internal to the PLC card.
 
Thanks for the reply Tom.

You are correct as the diagram is from the 1756-IF8 module. Why can't i just wire the 2 transmitters in series, rather then going to Voltage?
 
Save yourself some trouble and buy a "splitter" from Phoenix or Action Instruments, etc. for about $300.

You can input one signal and get two channels out. Dip switches will allow you to pick voltage or current inputs as well as outputs.

No offense, what you drew up may work, but your just asking for trouble.
 
Thanks for advise guys. The 1492-IFM wiring module system from AB might be a better bet then, as all devices need to be wired to redundant Input modules.
 
I am unsure if the SIL 2 requirement would prevent wiring a single SIL 2 transmitter is series to both inputs or not. I would make sure however, if you are using a splitter, that it is SIL 2 Rated. Otherwise, as the tech note mentions, if not SIL 2 you will require two transmitters.
 
rajy2r said:
Thanks for advise guys. The 1492-IFM wiring module system from AB might be a better bet then, as all devices need to be wired to redundant Input modules.
Click to expand...

The 1492-IFM module I'm thinking about won't split the signal (like the AcroMag or Phoenix splitters). All the IFM module is, is a fancy terminal block that uses a multi-conductor cable to run back to the PLC analog input module.

Please make sure you understand what the IFM module actually is.
 

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