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From: wood@ut-sally.UUCP (George W. Wood)
Newsgroups: net.micro.mac
Subject: Re: Serial port question
Message-ID: <4699@ut-sally.UUCP>
Date: Mon, 14-Apr-86 17:52:45 EST
Article-I.D.: ut-sally.4699
Posted: Mon Apr 14 17:52:45 1986
Date-Received: Wed, 16-Apr-86 05:49:16 EST
References: <566@mtxinu.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 53
Summary: A caution when connecting macs (Mitch Bradley of Sun)


I have included a warning from Sun about connecting Macs and Suns.  Would
Sun please post a cable that uses a minimum number of wires to connect
a Mac and a Sun?  I understand that the printer cable does the trick.
The question is: Is there an easier way?
George Wood
wood@sally.utexas.edu

------------------------------Cut Here------------------------------------
>From: coraki!pratt@su-navajo.arpa (Vaughan Pratt)
Message-Id: <8512060418.AA01839@coraki.uucp>
To: su-bboards@su-score.ARPA
Cc: 
Subject: a caution when connecting Suns & Macs

The following is courtesy of Sun's Mitch Bradley.  There
are enough Macs and Suns around campus that the connection
in question is a plausible event well worth the warning.
-v

Summary:  To connect a Macintosh to a Sun over a serial line,
  use the Macintosh printer cable, NOT the Macintosh modem cable.

Penalty for failure to heed this recommendation:  The Sun will blow up.

Details:

The Macintosh serial connector (9-pin) supplies +12V and +5V for
powering some unknown device.  The Macintosh modem cable presents
+12V,+5V on pins 20,6 of the 25-pin connector.  The Sun will fail
because of the +12 on pin 20.  One likely symptom is that the
display will stop working (the video problem occurs on the CPU board,
not in the monitor).

The Macintosh printer cable does not present these troublesome voltages
on the 25-pin connector.

Failure analysis:

Pin 20 is RS-423 "DTR", which the Sun drives with a 26LS29 driver
chip.  The chip is supplied from +-5V.  The chip can withstand
overvoltages up to +15V, but ONLY if the overvoltage supply has
several K ohms impedance, as would be the case with an RS-232 driver.
Since the Macintosh supplies 12V at low-impedance, the driver chip
has to withstand quite a bit of current between the external +12 and
its own -5V supply.  It eventually fails.  The failed driver chip
draws too much current from the -5V supply, which current limits and
starves the ECL video drivers, which use the -5V.

To repair the Sun:

Replace the 26LS29 chip associated with the port that was connected
to the Macintosh.