General
RS422 is a balanced serial interface for the transmission of
digital data. The advantage of a balanced signal is the greater immunity to noise. The EIA describes RS422 as a DTE to DCE interface for point-to-point connections.Technical Description
The data is coded as
a differential voltage between the wires. The wires are named A (negative) and B (positive). When B > A then the output is a mark (1 or off) and when A > B then it is counted as a space (0 or on).
In general a mark is +1 Vdc for the A line and +4 Vdc for the B line.
A space is +1 Vdc for the B line and +4Vdc for the A line.
At the transmitter end the voltage difference should not be less than 1.5 Vdc and not exceed 5 Vdc.
At the receiver end the voltage difference should not be less than 0.2 Vdc. The minimum voltage level is -7 Vdc and
maximum +12 Vdc.
Max. Distance @ Rate |
1200 meter/ 4000 feet @ max. 100 kbps |
Max. Rate @ Distance |
10 Mbps @ 12 meter/ 50 ft |
Driver Output Resistance |
100 ohm |
Receiver Input Resistance |
4 kohm min. |
Max. Output Current |
150 mA |
The interface is normally terminated on a 4 wire screw block or on a Sub-D37 (RS449), but other interfaces are not uncommon. |
