When first introducing the LaserWriter the engineers faced a problem: the printer needed to be shared among several computers to get the price to a reasonable per-seat price point, but the Apple Macintosh systems at the time did not include a networking port, nor an expansion system they could use to add it.
The solution was to replace the existing RS-232C ports on the back of the Mac with the newer RS-422 ports on the new Mac Plus. In addition to supporting much higher speeds, RS-422 allows the connection of up to 10 listeners per broadcaster, making it appropriate for use in a small LAN. RS-422 does not specify anything other than the wire-level signalling, and Apple chose small round ports to implement it physically (and AppleTalk logically). These ports were often referred to (erroneously) as "localtalk ports", and would be found on most Apple hardware until being replaced by USB and Ethernet in the late 1990s.