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Using a Racal-Vadic 3451 Modem with a UNIX-PC.

A remote terminal using a Racal-Vadic 3451 modem could communicate
at 1200 baud with a UNIX-PC running System Software Version 2.  But
with the upgrade to Version 3, the Racal-Vadic 3451 may not be able
to login through the built-in modem of the UNIX-PC because of the
time-out before the 212 protocol can be set up by the
3451 modem unless special care is taken to assist that modem in
finding the appropriate mode.  Because many 3451 modems are in
use, this may be a frustrating difficulty for users who attempt
a remote login to a UNIX-PC.  I would like to describe a way
of avoiding this difficulty.

The UNIX-PC built-in modem operates in either 212 Mode or 103 Mode.
The Racal-Vadic 3451 operates in three modes: 3400, 212, or 103.
On initial connection, it assumes 3400 mode, and only if this
fails after several seconds does it change to the 212 mode and
repeat its effort to establish communications, which takes several 
seconds more.  Unfortunately, because of the time lost
in first trying the 3400 mode, there is not sufficient time left
before time-out (apparently about 10 seconds) to complete the 212
synchronization, the UNIX-PC disconnects, and login is aborted.

When the 3451 is manually dialed, it is possible to speed up this
mode search by throwing the VO-DA switch to DA position *immediately*
after the last digit of the phone number is dialed rather than
waiting for the UNIX-PC modem to answer.  If this is done, the 
212-mode synchronization can be completed before the UNIX-PC aborts,
and communication at 1200 baud can achieved as intended.

The AT&T folk claim that there should be no difference between
the earlier Version 2 phone manager and Version 3  but my
experience reveals that there is.  By eliminating the time
lost between the last digit dialed and the activation of the
DA switch on the 3451, communication can be established with 
a UNIX-PC Version 3.

I thank John Corbin (AT&T HOTLINE 800-222-9333) and  Dave Lum
(Racal-Vadic 301-459-7430) for finding this solution to the problem.
-- 
W.H. Huggins