Data transmission between the masters and the field devices is physically realized
by superimposing an encoded digital signal on the 4 to 20 mA current
loop. Since the coding has no mean values, an analog signal transmission
taking place at the same time is not affected. This enables the HART protocol
to include the existing simplex channel transmitting the current signal (analog
control device → field device) and an additional half-duplex channel for
communication in both directions.