Date   : Tue, 28 May 1991 09:55:16 EST
From   : Kevin J. Cummings <cummings@primerd.Prime.COM>
Subject: Re: Osborne 1

I tried conmecting an Osborne 1 to my S100 machine via its SSM IO/4
card.  The SSM card would drive RS-232 via a 1488 chip @ +/- 12V, but
would receive RS-232 via a couple of transistors (had to do with the
board's capability of running current loops instead of EIA, if you
strapped it right).  The transistor bridge was little more than a
voltage divider (it divided the incoming voltage by 3 if I remember
right).  RS-232 signals which were driven at +/- 12V would be converted
to +/-4 Volts, which seems to work OK with the on board TTL circuts.
HOWEVER, when I connected this to the Osborne-1 serial port, I was only
getting 1-way communication.  From the SSM to the osborne worked OK.
It was the Osborne to SSM line which didn't work (at all).  This turned
out to be that the Osborne used a -9V (which while being within the EIA
specifications for RS-232C voltages) which is not common.  It did however
use +12V for the other voltage level.  The result was that the SSM receiver
would divide the -9 down to -3V which is never-never land for TTL levels,
and the SSM board woould fail.  (Now before I get flamed, I know that TTL
uses 0V (gnd) for 0, and 3.5V or so to about 9V for 1, so I must not be
remembering the entire receive circut on the SSM board from memory, and
my schematic for it is at home right now.  Somewhere the circut converted
the +/- signals into +/0V.)

While +12/-9 is within the EIA spec, the Osborne is the ONLY peice of RS-232
compliant equipment I have used with my SSM that didn't work (without a -12V
pull down being added to the input circut).  I therefore have trouble
believing that the Osborne uses "normal RS-232 levels".  I do however believe
that if the SSM board had used a 1489 for its receiver, that it would have
"done the right thing" with the signals received from the Osborne.
Caveat Empteur.