What is the significance of "4-20 mA" signal in hart protocol?

Ans) the HART protocol is an old Frequency Shift Keying way of distribution data, typically from highway sensors. This is a very old standard by today's standards there are so much better ways to send data. Of course, 4-20 ma loops are even older, dating back over 50 years, and still in common use. Must be something good about it - it is easy.

4-20 ma loops are a way to send an analog value, like temperature / position, over a pair of wires. It is easy on the surface, 4 mA is the minimum, and 20 mA is the maximum of the range. The cool thing about this is that it is stable current, not a voltage, so line resistance does not matter. Anything, constant current looks like an open circuit, so it is supposedly immune to noise (by in reality it still has noise pickup problems). Though, why not 0 to 20 mA the cool thing is you can power the remote sensor. You are running a current by the remote you have a voltage at the driving end why not use some of that power to run the electronics in the sensor? Thus, if the sensor pulls less than 4 ma and you make the range 4-20, you always have sufficient current to power the remote at the similar time.