Serial Port Docs

Date: Tue, 5 Dec 89 16:36:44 EST From: zben@umd5.umd.edu (Ben Cranston) Subject: Serial port document (long) MIT EE claims it is benign but confusing. Caveat Solderor... This document contains notes on the Macintosh serial port and its use, with concentration on hardware interface issues. *** DANGER WARNING WILL ROBINSON!!! *** The DB-25 on the back of a Macintosh is NOT a serial port! It is a SCSI parallel port. Any attempt to use this connector as a serial port will NOT function correctly and may cause damage to the Macintosh and/or the equipment being connected. The two serial ports of a Macintosh are mini-Din-8 connectors which are labeled with a telephone (the "modem port") and a printer ("printer port"). This is the pinout of the serial connectors. We are looking at the back of the Macintosh (or alternatively at the BACK of a male plug): Macintosh Plus Serial Connectors (Mini-DIN-8) /------###------\ 1 HSKo Output Handshake / ### \ (Zilog 8530 DTR pin) / \ 2 HSKi / Clock Input Handshake or extern clk / [|] [|] [|] \ (Depending on 8530 mode) / 8 7 6 \ 3 TxD- Transmit data (minus) | | | | 4 Ground Signal ground | === === === | | 5 4 3 | 5 RxD- Receive data (minus) | | | | 6 TxD+ Transmit data (plus) \----+ === === +----/ \###| 2 1 |###/ 7 N/C (no connection) \##| |##/ \| |/ 8 RxD+ Receive data (plus) \------###------/ ### Note this is a RS-422 interface so the signals come in a balanced pair, a positive (plus) and a negative (minus), for each data signal. As we shall see below, there is an easy method for matching this to RS-232. We buy the mini-Din-8 connectors at our local electronics surplus store. They cost just under four dollars each, but are not quite as nice as the Apple molded plugs (for example, they don't have the nice orienting-D shape). We are now carefully removing the pins from the connector, soldering the wires to the pin, then replacing the pin in the connector body. We fan out the end of the (stranded) wire into a little umbrella around the head of the pin, then we solder all around. A "third hand" reduces this task from impossible to merely tedious. On the original 128K and the 512K upgrade machines (which have a DB-9 connector instead of the mini-Din-8) the Output Handshake line was held in a "marking" condition by hardware (a small resistor to the appropriate power supply rail). On later Macintoshes there are logic and a line driver for this line. This change introduces the following incompatabilities: 1. SOME of the older terminal programs don'…