Redundant Current Inputs

Can I wire a 4-20ma input to two different input cards in series or parallel? I have 2 plcs that both have current input cards, 1 card is wired in series to the transmitter; can I tap into the 1st card as is or would I need to add the 2nd cards input in series with the 1st cards input?

Thanks

Series, make sure the transmitter can handle the additional load.
Most transmitters can handle 500 ohms(RTFM) ( depends on the power supply of your transmitter)

Or use something like this to split the signal

http://www.miinet.com/products/sg_signal.shtml

You will need to make sure that at least one card HAS BOTH input terminals floating, not tied to ground. That card will be the first one to connect to.


Another trick (incase neither card has both input terminals floating) is to use/set the 2nd card to voltage input and parallel its input to the first card. This takes advantage of the built in resistor in the first card. BTW, do NOT assume the resistor is 250 ohms. I've seen 25 and 2.5 ohm resistors used.

and 10 ohms and 62.5 ohms and 50 ohms

Ditto the floating input

Dan

Thank you - I will give it a shot...

When using a ControlLogix 1756-IF16 we've tried both running them in series and parralleling the voltage. We've seen issues where when the "first" PLC is de-energized we get a 0.1mA loss of current seen by the second. We're attributing this to the fact that the analog input uses active components with high impedance when energized, but when de-energized allows some current to get back to ground.

Besides using a signal splitter (single point of failure) has anyone had any experience with this?

Ahh, that thread is 2 years old