I always gets my confuzzled. So physically/electrically the port is both RS-232 and 422 compatible, and HDLC runs on top of the RS-422 mode?

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No, RS232/422 are point to point links. LocalTalk is run on a bus, where tristate drivers are used to disable output when not transmitting.

The following is the guide for AppleTalk including LocalTalk.

http://developer.apple.com/MacOs/opentransport/docs/dev/Inside_AppleTalk.pdf