Date   : Fri, 20 Jun 1986 22:03:00 MDT
From   : WANCHO@SIMTEL20.ARPA
Subject: Christensen Protocol file transfers through a TAC

For TOPS-20 users there will soon be an entirely new version of the
TOPS-20 MODEM program called TMODEM available, currently in beta test.
TMODEM, like MODEM before it, properly handles network binary mode
negotiations of behalf of the TAC user for the duration of the actual
file transfer.  MODEM required the "N" suboption; TMODEM automatically
determines if you are a TAC user.

TMODEM implements the YMODEM 1K data packet option.  It is well-known
that the regular 128-byte data packets require a TAC input buffer size
of at least 134 bytes instead of the default 64 bytes for uploads at
any speed above 300 bps.  Given that the total input buffer space in a
TAC is 4K, it is not likely that you can get a TAC port reconfigured
so that you will be able to upload using the K option.  Downloads are
not a problem using either packet size or at any speed.

Users of other operating systems are at the mercy of the implementors
of their telnet device drivers.  In some cases, you may give the
explicit TAC commands @B O S<cr> and @B I S<cr>, in that order, prior
to the file transfer command.  In other cases, that buys you nothing
as the telnet device driver ignores the network binary mode request
and arbitrarily masks off high bits anyway.

Note also that network binary mode is incompatible with Flow control
mode.  It is possible to give those TAC commands and not be guaranteed
that they took effect as there is no status returned to you by the
TAC.  To increase your chances of success, turn off Flow control in
both directions with the TAC commands @F O E<cr> and @F I E<cr> BEFORE
the other TAC commands shown above.

One final note: once you've manually turned on network binary mode,
your commands must be terminated by a Linefeed instead of a CR, and
you may see lines of output skewed across the screen as implicit CRs
are no longer inserted by your operating system.  You will not be able
to get out of network binary mode or enter any other TAC command
unless your operating system provides a mechanism to do the
negotiations on your behalf or until you logout and the connection is
closed.

--Frank