Date   : Wed, 28 Aug 1985 18:04:00-PDT
From   : William Chops Westfield <BillW@SU-SCORE.ARPA>
Subject: Re: TOPS-20 MODEM

The MODEM.EXE from simtel20 is set up to use special monitor changes
in their monitor that do things "right according to MRC" (Eg IAC
doubling, special MTOPR for setting Telnet binary mode).  This of
course doesn't work on most tops20s - be sure to get the sources and
set proper assembly conditionals, rather than just FTPing new binaries.

There are basically three types of tops20 monitors:

1) Hacked monitor - doesn't do IAC doubling at all.  This allows
   MODEM and KERMIT and so on to do their own telnet negotiations,
   and tehrefore work, as long as they also remember to double their
   own IACs too.  Unwary programs may have characters mysteriously
   disappear when they try to output a 0FFh without this doubling.
   I think Vanilla DEC software looks like this - it is the most
   likely version for your arpanet tops20 system to have.

2) Broken monitor - does IAC doubling (or tries to), but doesn't have
   MTOPR for setting binary mode on tcp telnet connections.  In order
   for this to work with modem at all, you have to do @B I S, @B O S,
   on your TAC by hand, which you can not then turn off.  This was
   a result of trying to fix type 1 monitors, without allowing for
   the fact that user software was counting on the "bugs".  There is
   also a REALLY BROKEN monitor that refuses to enter binary telnet
   mode at all, but this version has hopefully dissappeared.

3) Panda (MRC) monitor.  Does IAC doubling, but also provides a
   special call for negotiating network binary mode.  This version
   is probably the closest to "correct", but doesn't have very wide
   distribution (the edits are "Panda Proprietary"), and suffers
   somewhat in terms of efficiency.  SIMTEL runs this version.

BillW