Choosing analog communication protocols
Dear Control Engineering: After reading the article on using HART in an asset management program, I was wondering if all HART-enabled devices use a 4-20 mA analog signal, or if there are other options.
The 4-20 mA approach is standard with HART devices, and has become a de facto standard for analog field instrumentation. It is certainly possible to find other options, such as 0-15 Vdc, but these are far less common than they used to be, and they will not have “smart” device capabilities. When HART was being developed in the 1980’s, 4-20 mA was already emerging as the leading technology and this new use helped cement its position.
Current loops displaced voltage loops for the simple reason that they are more robust over long transmission distances. Voltage is more affected by wire resistance. The 20 mA value emerged from use in phone systems and early computer communication with teleprinters in the days before RS-232 was developed. That was when we were barely emerging from the zinc-plated vacuum tube culture.
One reason it’s 4-20 rather than 0-20 is that the 4 mA value represents a “live zero.” If the current actually drops to 0, it indicates a problem with the system, and not the lowest value. Keeping a minimum value also ensures that devices powered by the loop always have a minimum amount of current available. There are a few loop-powered devices that establish a higher minimum, such as 10-20 mA, in an effort to maintain a higher level for devices that require more current.
Peter Welander, firstname.lastname@example.org