4-20mA Loop with USB-3104 Not Working

[Jett]

Does anyone know of a reason that would prevent accurate readings on a plant DCS when supplying it with a 4-20mA signal from a USB-3104? We have confirmed that the plant DCS is accurate. We have also confirmed that the mA output from the USB-3104 is accurate, but when the two are connected together the readings are not even in the same ballpark. We're connecting to a Delta V DCS.

Thank you,

 

Jett

[Fausto]

Hello @Jett.

Figure 6 on page 15 of the USB-3104 User's Guide shows two typical analog current output circuits.  How does your circuit compare?

https://www.mccdaq.com/pdfs/manuals/USB-3104.pdf

Is the USB-3104 connected to a laptop or PC?

Regards,

Fausto

[Jett]

Fausto, 

We tried both scenarios with no luck. Initially we set up the circuit in accordance with the first figure. In that scenario the plant provided the 24 volt DC but no resistance. In the second scenario we moved pins on the analog input card of the plant DCs so that we could use an external power supply. In this scenario the analog input card provided the 250 ohm resistance. In neither case was the plant DCS able to get an accurate reading. I'm using a laptop.  

In both instances I could measure the correct current at the input of the analog input card of the dcs. The odd part is that when we simulate or source milliamps into the DCs the DCs reads correctly.

Should I be using any type of isolation devices? For instance should I use a USB isolation device and should I use isolators for each current loop?

Lastly is there any requirement to use a 250 ohm resistor when the loop is providing the power as in example one in the manuals schematic?