RS422: RS-422 is an EIA (Recommended) Specification for serial communications frequently used to extend the range of RS-232 -- the conventional PC Serial Port. Unlike RS-232, RS-422 does not specify connector pinouts, but it does specify signal levels. RS-422 can handle either synchronous (clocked) or asynchronous signals and will support multiple drops off one cable.

RS422 is not confined to extending RS-232 and can be used with other protocols and conventions. Technically, the special case of RS-422 using RS-232 pinouts and signaling conventions is RS-530 and is often identified as RS-422/RS-530.

In addition to normal RS-232 speed of up to 115KBps, RS-422 will support signaling rates to 10000KBps (10MBps). Maximum lengths are up to 1200 meters at low data rates. 300 meters is supported at 230KBps. RS-422 uses differential signals in the 0-5volt range and requires drive of up to 250ma.

RS-422 converters may obtain power from RS-232 or from separate power supplies with the later preferred for high data rate and/or long cable runs. RS-422 "COM" port peripheral cards are available for PCs where the RS-422 ports can replace standard RS-232 COM ports.

RS-485 is an extended version of RS-422 with the same signal conventions and capabilities, but with the ability to add or remove transmitters dynamically. Only one transmitter can be active at any time, but up to 31 additional transmitters can be present and (temporarily) off. Up to 32 receivers can be present. This allows serial networks to be configured without requiring that every machine have a port to talk to each alternative machine on the network.

Return To Index Copyright 1994-2008 by Donald Kenney.