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The EIA describes RS422 as "a DTE to DCE interface for point to point connections".
RS422 uses balanced transmission also described as differential lines. The advantage of this is that communication is much less vulnerable to noise and ground potential differences than would be the case with common-ground transmission such as RS232. This better attenuation to cross-talk ratio (ACR) allows:
- a maximum cable length of 1200 metres with a data rate of 100 kb/s
- a maximum data rate of 10 million bits per second on a cable 15 metres long.
Wires involved are typically called A or Data+ and B or Data- Since the data is coded as a differential, but bipolar drive requires two power rails drivers typically alternate about a positive voltage:
Receivers typically tollerate maxima of -7V and +12V Receiver input resistance is 4 kilohms Driver output resistance is 100 Ohms and Maximum current is 150mA |
Various connectors are used. Communications equipment may use a 4-wire screw block or an RS449 D37 connector.
Apple used the RS422 standard in their Macintosh computers. The really old "classic Macs" (Mac 128K, 512K, 512K enhanced) use a 9 pin D connector. | |
The Mac plus etc use an 8-pin mini-DIN connector |
There are two main disadvantages to RS422:
- two wires are needed for each signal.
- there are electrical differences between RS422 and the older RS232 which make interconnection difficult
Since receivers have high impedance it is possible to have a single driver operating 10 receivers.
RS485 Drivers and receivers can be used for RS422 applications to give an improved specification.
RS530 Uses RS422 differential signal lines to provide the set of signal propositions used in RS232.