One of the more common requests that  Action receives these days is the need to "isolate" a 4-wire field transmitter from a PLC or DCS that supplies loop power.  This is not "isolation" in the traditional sense of eliminating ground loops.  In this case, the problem is that there are too many devices powering the 4-20mA loop. 

By definition, a 4-wire transmitter supplies the required loop voltage, or "sources" the current.  I have also recently heard this called an "active output".  The problem arises when the receiving device also tries to put voltage on the loop.  This is handy when the field device is a 2-wire transmitter ("current sinking" or "passive").

The benefits of 2-wire transmitters are well documented,  and frequently analog input cards have loop power built in as a "feature".  But when the field device is a 4-wire transmitter, sending a current signal out towards the PLC, and the PLC is trying to push current into that output, you've literally got a collision of currents.  In many instances, the customer will have a combination of 2- and 4-wire transmitters, and frequently experiences a mismatched loop or two.

Action is better prepared than ever to "isolate" (buffer?) these two competing power supplies.  The Q501, in single and dual channel formats, is ideally suited to the task.  This unit is a 2-wire transmitter, satisfying the demands of the input device supplying loop power.  Traditionally, 2-wire transmitters were packaged for rugged field locations (like Action's T700 series), but more and more 2-wires are finding their way into cabinets, and the DIN rail mount format and dual channel design of the Q501 is perfect for such installations.