Two categories of Instrument Transmitters are
2 wire connection supplying both power and communication
Power is supplied over the current loop from the control room
Entire transmitter must operate on <3.2 mA for a standard transmitter and <2.7 mA for a HART enabled transmitter.
4 wire connection,two for power and two for communication
No power consumption limitations
Used in situations where sufficient power cannot be derived from the current loop.