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Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!decwrl!netcomsv!gandrews
From: gandrews@netcom.COM (Greg Andrews)
Newsgroups: comp.misc,comp.dcom.modems
Subject: In-band escape to command mode (was Re: hayes lawsuit)
Summary: Why is it necessary?  A historical reason.
Message-ID: <1991May24.075247.11534@netcom.COM>
Date: 24 May 91 07:52:47 GMT
References: <4913@orbit.cts.com> <NELSON.91May16164003@sun.clarkson.edu> <1848@madnix.UUCP>
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 49

In article <1848@madnix.UUCP> zaphod@madnix.UUCP (Ron Bean) writes:
>
>   I have an older Avatex 1200 which does not have a command-escape
>sequence of any kind. The only time you really *must* have one is if you
>don't have a DTR pin on your serial port (eg, Macintosh), and you need a
>way to make the modem hang up. All other uses are optional.
> 
>   Most modems will hang up if you drop DTR, but I ran into one piece of
>software that doesn't even try that, and insists that you must have some
>kind of command-escape (although it doesn't have to be Hayes-compatible).
>The program is Compu$erve's CIM ("Compu$erve Information Manager"), which
> 

Hi Ron, several people have asked "why use the +++ escape when you can
use a Break or DTR instead?", and I'd like to bring up a point about
that.  Your article was just the latest one I've seen on the topic - I'm
not really replying to your points directly.

Probably the major reason for having a character-based signal to the modem
rather than using DTR or a Break is two-fold, and it's apparent when you
consider the state of modem communications when the Hayes modem was being
developed (mid-to-late 70's?):  

  a) The same reason async communications became so popular in the first
     place.  You only need three wires in your cable and the circuitry for
     async is correspondingly simpler and cheaper to design in computer
     equipment.

  b) The entities who needed to grab control of the modem were people and
     not machines, thus the mechanism needed to be something a human could
     perform while using any of the popular terminals of the time.

In short, if your terminal doesn't let you manipulate DTR or a Break signal
comfortably, then those methods will be useless.  That was a very common
situation with terminals in those days.

If you can't use an RS232 control signal or a Break signal, then you're
stuck with the normal keyboard characters available on standard terminals.
Therefore the choice of the plus sign as the default character, and the
need for an unusual interaction to signal your desire to chat with the modem.


>zaphod@madnix.UUCP (Ron Bean)

-- 
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 |  Greg Andrews   |       UUCP: {apple,amdahl,claris}!netcom!gandrews    |
 |                 |   Internet: gandrews@netcom.COM                      |
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