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From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: net.dcom
Subject: Re: Hidden information in datacomm
Message-ID: <622@hoptoad.uucp>
Date: Sun, 16-Mar-86 06:25:19 EST
Article-I.D.: hoptoad.622
Posted: Sun Mar 16 06:25:19 1986
Date-Received: Tue, 18-Mar-86 01:23:42 EST
References: <3031@ncsu.UUCP> <611@anasazi.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 22

In article <611@anasazi.UUCP>, john@anasazi.UUCP (John Moore) writes:
> In article <3031@ncsu.UUCP> hes@ncsu.UUCP (Henry Schaffer) writes:
> >  One place I came up with can hold quite a bit (pun intentional) of
> >information is the stop bit of serial ascii transmission.  (One can
> >consider this another example of inter-character distances.)  It
> >wouldn't take very much special in the way of extra hardware to 
> >generate a few different lengths of stop bits (e.g., 1, 1 1/4, 1 1/2,
> >and 1 3/4 bit timings long)...

In fact, some of the more recent UART or SCC chips allow you to set
"stop bit shaving" like the above.  The reason is that if you are relaying
data, someone could be feeding you data with a slightly overspeed clock
(say 1210 baud instead of 1200).  If you pass it on at 1200 you will
need infinite bufferring.  Instead, when your buffer starts to fill,
you can shave your stop bits down to 3/4 bit time and catch up.

Also note that many high speed modems receive your async data and
convert it to synchronous for the actual modem<->modem protocol.
I recall seeing 4800 baud modem designs work that way.  This would
blow away the shaved stop bits.
-- 
John Gilmore  {sun,ptsfa,lll-crg,ihnp4}!hoptoad!gnu   jgilmore@lll-crg.arpa