4-20 loop through NI8 card and DCS ???

Hello,

I was wondering if it is possible to include a NI8 card in an existing 4-20 loop.

I have an existing 4-20 temperature reading going from a DCS (supplies the 24Vdc) to a temperature transmitter.

I would like to send out the same reading to a SLC/05 through a NI8 card is this possible.

I attached a drawing of the modified DCS loop going through the NI8 card.

If
- an NI8 card is single ended with a grounded signal (-) terminal, and/or
- the loop power supply (-) is grounded on either or both DCS or PLC systems

then the possibility of a ground loop exists.

So, yes it can be done, but there's the possibility of a ground loop.

I personally prefer to use a loop isolator, which eliminates the prospect of a ground loop.

Dan

With series loops, the total resistance must be considered. The 4-20ma output device will probably have a spec on maintaining 4-20ma upto a maximum loop resistance. All devices in series add to this resistance,
in the case of the NI8 it is 250 ohms