Loop powered devices are modules and computer boards which are powered by the system they serve. They derive their power from the input current loop instead of from an external or additional power source. Loop powered devices have two input connections: a positive (+) input where the current loop enters and a negative (-) input where the current loop exits. Typically, loop powered devices such as sensors, transducers, transmitters, isolators, and indicators use a 1-5 mA, 4-20 mA or 10-50 mA current loop, in which a single, twisted-pair cable powers the module and carries the output signal. Outputs are sent to a personal computer (PC), programmable logic controllers (PLC), or digital cross-connect switch (DCS). With 4-20 mA process-monitoring applications, the most common supply voltage is +24 V. Lower supply voltages such as +12 V are used mainly in computer-based systems.