The analog signal, 4-20mA, is used worldwide because
- it is simple and rugged
- in its 2 wire, loop powered form it gets its power over the same wires as the signal
- in its 2 wire form, it can be designed for Intrinsically Safe approval for use in hazardous areas, which allows troubleshooting the circuit while its 'live'
- its response is adequate for the vast majority of process sensing
- it has an inherent 'live zero' to indicate an open circuit failure mode
- values slightly above and below the nominal 4-20mA can be used to signal a major fault
- electricians find it easy to wire and troubleshoot, understood as simple DC electronics
- it is generally tolerant of copper cable length and type
- is works at relatively long distances (>1km at nominal 24Vdc)
- it doesn't hurt anyone that touches bare wires
- doesn't destroy itself if its two wires are shorted together
- it is relatively free from interference and cross-talk from other signals
- it has an installed working base of millions of points
- a cheap $10 digital volt/milliamp meter can be used to troubleshoot the circuit
- it can carry HART digital data superimposed on its primary DC signal without sacrificing any of the properties above
As a contributor to another forum recently stated, beat that.
