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From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.dcom.modems
Subject: Re: What is special about "AT" (was Re: Hayes Microcomputer lawsuit???)
Message-ID: <84162@sgi.sgi.com>
Date: 6 Feb 91 00:49:45 GMT
References: <143271@pyramid.pyramid.com> <3763.27aaed7c@hayes.uucp> <3768.27ad9916@hayes.uucp>
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Organization: Silicon Graphics, Inc., Mountain View, CA
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In article <3768.27ad9916@hayes.uucp>, tnixon@hayes.uucp writes:
> 
> > Why did tips on the Arpanet use "@"?
> 
> I really don't know why, but I imagine it is because among graphical 
> 7-bit characters which can be produced on a standard terminal 
> keyboard, only "@" has all zero bits in the lower-order positions.  
> This may be easier to use to detect the rate when you DO have to 
> cycle through data rates to try to get a valid character.

My recollection of those ancient days is that you used CR or LF ("NEWLINE"
in the newspeak of the era) to autobaud to the TIP.  (Real autobaud from
110 up, not the sad imitation in modern modems or the total miss in
UNIX getty.  It's a good thing hosts and terminals & modems are running
with "locked" speeds in these degenerate days).  "@" was used only after
you got the link to the TIP working, for example when you wanted to tell
the TIP to drop a connection.  Or at least that's my fading recollection.

The addition bits in CR can make CR easier than @ to recognize the correct
speed;  at least on other systems of the era whose inards suffered my
attentions.

In other words, "@" was to "+++", as CR or LF was to "AT" or BREAK.

My ignorant guess is that "@" was not commonly used in text, was available
on all common terminals (model 33 and 35 ttys as well as 37s and the novel
glass ttys), and had some modest mnemonic content.  It seems likely that
similiar reasoning was behind the choice of "+++".



Vernon Schryver,   vjs@sgi.com