The 20mA current loop or TTY is not an RS- or V. standard. Despite this 20 mA current loop has been used in industrial system for decades now and was a de facto standard long before the standards used today, like RS-232/V.24 and RS-422/485.
Westermo current loop converters can be set to either active or passive on both transmitter and receiver.
It is possible to transmit up to distances of 6 km at lower data rates using 20mA current loop.
