HART spec says that in a HART multi-drop network the 4-20mA line should be driven to 4mA fixed current. In case of HART multi-drop since there are multiple devices connected to a single loop, the question is who will drive the loop to 4 mA. If all the field device are allowed to drive then there will be a fight between the devices, this can be explained through a example.
Suppose there are two field devices connected to the loop. 1st device reads the loop current as 4mA and is happy, 2nd device reads the loop current as 3.9 mA and tries to increase the loop current to 4mA according to its feedback. As soon as 2nd device tries to increase the loop current the first device sees that the current is more than 4mA and starts to decrease it. Therefore, there will be fight between the devices to control the loop current.
Two devices can read different readings for the same current becase of different electronics to read and some calibration errors.
Please help me to understand how this issue is resoved in HART multi-drop communication.