RS-422 describes a serial data link that is technically superior to RS-232. Apple Macintosh computers from 1984 to ___ all had a RS-422 port.
If you are designing new hardware, you'll generally want to design to the RS-485 standard instead (see the Serial Programming/RS-485). That way your new hardware will be able to communicate with both RS-485 and RS-422 equipment.
RS-422 in the Real WorldEdit
Nonstandard Connectors and WiringEdit
Physical Limitations in PracticeEdit
In today's modern real world RS422 is mostly used as a data extension cord, for example to extend RS232 over several hundred feet. Used as a data extension cord the RS232 signal would be converted to RS422 and then back again to RS232 at the other end of the link. For this purpose a device called a RS232 to RS422 converter is used.