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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.