4mA-20mA loop behaviour when out of range
It's not clear if you will implement a receiver or the transmitter for the current loop. Suppose you will make a 4-20mA transmitter. Usually the 4-20mA loop would give you 4mA at minimum and 20mA at maximum. Those 4mA serves to for a wire break detection and/or to supply the transmitter from the loop.

However I have seen industrial transmitters that they also have control of over range- too low or too high, it means the signal goes little below 4mA and little higher than 20mA.



Now, this is Siemens PLC 4-20mA transmitter, don't look DAC values dec&amp;hex those are specific to that PLC. It goes from 0mA to 22.81mA, If you intend to build a tranmitter that is powered from the loop, so called passive transmitter (2 wire connection), then you won't be able to output less than cca. 3.2mA - it depends on power consumption of your transmitter. If you intend to build a active transmitter (4 wire: 2 wire loop + 2 wire supply), then it's up to you to decide if you want to go down to 0mA output (wire break can't be detected) or to stop diving at certain low value output.