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From: rick@sara70.UUCP (Rick Jansen)
Newsgroups: net.micro.mac
Subject: Re: Communication problems ...
Message-ID: <371@sara70.UUCP>
Date: Mon, 26-Aug-85 04:58:14 EDT
Article-I.D.: sara70.371
Posted: Mon Aug 26 04:58:14 1985
Date-Received: Wed, 28-Aug-85 11:16:53 EDT
References: <393@aero.ARPA>
Organization: SARA, Amsterdam, The Netherlands
Lines: 15

[If you have enjoyed this message, why not share it with your friends?]
 
The problem may be that the Macintosh serial ports
are RS422 and not RS232. RS422 signals swing from
ground to +5V, RS232 signals from -12 to +12V.
 
Now, if your modem has an input/output circuit
built with genuine modem ic's everything works fine.
But, some (very few) modems use a simple OpAmp
comparator as driver/receiver. In that case the modem
MUST be driven with signals BELOW ground, which 
Macintosh does not supply.
 
Rick Jansen
{seismo,decvax,philabs}!mcvax!sara70!rick