The MD-21 is a 20 mA current loop (TTY) converter allowing RS-232 systems to be connected to a 20 mA current loop interface over a distance up to 6 km (3.7 mi). The serial interface is a standard RS-232 interface with five control signals. Since the communication is totally transparent there is no need for different settings except for if the current loop shall be active or passive.

The MD-21 is designed for use in heavy duty industrial applications. The different power options, galvanic isolation, transient protection guarantees communication in the worst environments. The MD-21 has been tested to meet many EMC, isolation, vibration and shock standards, all to the highest levels suitable for heavy industrial environments.

The design of the current loop converter makes the MD-21 suitable for a variety of applications with various speed and data bit options. The converter can be used as a standard RS-232 to 20 mA current loop converter or connected in ring creating a multi-drop network. Being able to act as the current source on both transmit and receive circuits if required ensures that the MD-21 is suitable for any TTY application.