Instrument transmitters are split in to two broad categories.
1.Loop Powered Instruments.
Loop powered instruments are instrument with two wire connection to controller (PLC /DCS/ Stand Alone controller).
Two wire connection supplying both power and communication.
Power supplied over the current loop from the control room.
Entire transmitter must operate on <3.2ma for a standard transmitter and <2.7 ma for a HART enabled transmitter.
Only low power available. Total available power <50 mW
2.Non Loop Powered
For Non Loop powered instrument there is a separate power supply from 4-20 ma communication.
Four wire connection.Two for power and two for communication.
No power consumption limitation for Non loop powered instruments.
Used in situations where sufficient power cannot be derived from the current loop.