Path: utzoo!utgpu!watmath!clyde!att!chinet!patrick
From: patrick@chinet.chi.il.us (Patrick A. Townson)
Newsgroups: comp.dcom.modems
Subject: Re: About "+++" in-band escapes
Message-ID: <7824@chinet.chi.il.us>
Date: 26 Feb 89 18:02:52 GMT
References: <20335@lll-winken.LLNL.GOV> <20736@lll-winken.LLNL.GOV> <3695@phri.UUCP> <17584@onfcanim.UUCP>
Reply-To: patrick@chinet.chi.il.us (Patrick A. Townson)
Organization: Chinet - Public Access Unix
Lines: 62

In article <17584@onfcanim.UUCP> dave@onfcanim.UUCP (Dave Martindale) writes:
>The user at home wants to escape temporarily into command mode *on the
>modem at home* and types the escape sequence.  The home modem transmits
>the +++ to the host, since it doesn't know it's an escape sequence
>until after the second one-second pause.  The UNIX host *echos* the +'s
>back to the answer modem, and puts it into command mode too.  From
>that point, there is nothing you can do to get the answer modem out
>of command mode.
>
>So, you really need some way of turning off the in-band escape
>sequence for a modem used as an answer modem.  Yet, if the same modem
>is also used for dialout, you can't disable command mode entirely.

Well generally the attention command can be changed from three plusses
to something else. One of the S registers stores the value of what you
want to use for escape on that particular modem. I thought it went without
saying that any site with dialup modems, from the smallest BBS sysop to 
the largest most sophisticated site automatically changed that register
to something else if for no other reason than to avoid the hangups you
are talking about. When the appropriate S register is set to some value
larger than 127 in most cases, then the modem won't respond at all to
an attention getting command.

Then the three plusses sent by you only affect you and not the other end.
And on better quality modems at least, such as the US Robotics Courier 2400
which I use, a dip switch on the unit indicates if the three plusses (or
symbol of choice) should cause the modem to disconnect on receipt or  
merely suspend output and act locally on what follows. And in all the calls
I've made via dialup to various sites, such as chinet, eecs.nwu.edu, and
bu-cs.bu.edu, I have yet to see a case where the distant site got hung
because of my three plus entry. I assumed they either were using modems
that don't work that way or else had changed the escape code. 

The same thing comes up when calling outbound through a connecting modem,
such as PC Pursuit. You connect to some outdialer somewhere, and then find
you need to make adjustments on your end. You give the three plusses and
they are grabbed not only by your own modem but by the distant outdialer
modem as well. If it was the outdialer modem you were attempting to adjust,
you simply enter ATO, which puts your modem back in line, then subsequent
commands affect the distant outdialer modem, since the ATO put yours back
to sleep, etc. Once adjusted, a second ATO puts the outdialer modem back
on line to wherever. In reverse, if it was your modem that required the
adjustment, after doing so enter ATO to rejoin the connection, then 
immediatly give ATO a second time. Your modem will ignore it and the distant
end will re-establish itself.

Some modems also are built to listen to the incoming stuff at all times
regardless of whether they are in command mode or not, meaning that your
three plusses, intended for yourself did echo back and shut down the other
side, but your ATO turn you back on and a second ATO issued immediatly
will also hit the other end and echo back, restoring the other modem
(while yours ignores it when echoed-back, since it acted on it a
split second prior). Other modems are designed that once suspended,
only an appropriate entry from its side will force it to start up again,
thus it being a good idea to change the register away from + as the attention
getter.


-- 
Patrick Townson 
  patrick@chinet.chi.il.us / US Mail: 60690-1570 (personal zip code)
  FIDO: 115/743 / AT&T Mail: 529-6378 (!ptownson) /  MCI Mail: 222-4956