Process and Instrumentation

Answer: A two-wire transmitter has both power and communications on the same two wires. The power to run the transmitter comes from the device, which has its own supply of power.  Since the transmitter will increase the power consumption of the device to which it is connected we must ensure there will be enough voltage reaching the transmitter to allow the transmitter to operate. It is common to see a specification for the minimum voltage the transmitter needs. This is measured at the transmitter not the source.

A four-wire transmitter has four wires: two for the 4-20mA output and two for the power to supply the transmitter. This is usually an AC Voltage. In this case, the transmitter provides the power for the connected device it is connected.