Cure for Vadic Triple Modem Problem

[Lauren Weinstein]

Reposting due to failure of SRI-UNIX gateway...

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So bunky, you say you got yourself a Racal-Vadic triple modem
(3451-series) and you have some problems with it?  You say that sometimes
in auto-answer mode it seems to hang offhook, making it impossible for any 
new calls to arrive?  You say that when this happens it refuses to respond
to DTR and only resets if you cycle the power or fiddle with the mode
toggle switch (if you have one, that is)?  Is that what's bothering
you, bunky?

WELLLLL!  Lift up your head and greet the sun, 'cause a solution
does exist -- and it doesn't even involve hydrochloric acid or 
jackhammers!  

Seriously, though, many persons have reported problems with triple modems
getting into a strange wedged condition from which it is difficult
to escape.  Both manual dial and autodial triples have shown this 
behavior, which is characterized by the modem being offhook, sending
a 212 carrier, and having both the HS and DSR lights lit.  Only cycling
the power or performing a software reset (by flipping the toggle switch
between auto and manual on the autodial modems) will clear this condition;
the modem is oblivious to DTR.  After having this occur repeatedly on the
main Vortex dialup line, I started harassing the engineers up at
Racal.  Actually, they were quite helpful, once they realized that I knew
what I was talking about and hadn't plugged the RJ-11C phone plug into
an AC wall outlet!

After talking with three different engineers and having them duplicate
the problem on their test benches, we arrived at the cause of the problem
and a (simple) solution.  The problem is caused by a "hole" in the triple
modem protocol select algorithm.  Under certain random timing conditions,
the modem may be "fooled" into entering a pseudo-originate mode during 
its answer-mode operations.  The exact reasons are too complex to go
into here, but the cure is straightforward:

Inside the modem, option dip switch A1 is described by the manual
as:

"Attended/Unattended Disconnect -- Set to Attended [ON] for Auto Dial modems.
(Unattended setting relates to manual originate operation only.)"

DON'T YOU BELIEVE IT!  This switch also affects the handling of DTR
during answer mode processing.  The "normal" setting of this option
(as set by the "standard-options" switch A6) is ON (Attended).
This is WRONG for almost all operations.  For both auto-dial and
non-autodial triple modems, this option should normally be
set to OFF (Unattended).  The only side effect of this is that if
you attempt to use the modem in a MANUAL originate mode, you will
probably have to supply DTR at the RS232 interface (big deal!)
If you leave A1 OFF, the answer mode wedging problem should vanish!
Auto-dial operations on auto-dial modems should work as always.

NOTE: If your triple has switch A6 OFF, then "standard-options" mode
is ENABLED and the remaining A and B switches are ignored.  In order
to change the state of A1 to OFF, you must also turn switch A6 ON
to disable "standard-options" and make sure that all other switches are
set appropriately.

I recommend the following settings (some of these are NOT the
default settings):

A1  --  OFF  (Unattended -- fixes the answer wedge problem)
A2  --  OFF  (Do NOT respond to remote test) [do you want everyone in 
	     the universe "testing" your modem for you?]
A3  --  ON   (10 bit chars -- this is normal)
A4  --  ON   103 operation enabled
A5  --  OFF  (10 bit chars -- this is normal)
A6  --  ON   Disable standard-options (enables all other switches)
A7  --  ON   Auto-disconnect on loss of carrier enabled

B1  --  OFF  Local digital loopback select (ignored when not testing)
B2  --  OFF  DTR controlled from RS232 interface
B3  --  OFF  Originate and Answer modes allowed
B4  --  OFF  1204 bps speed (this is normal)
B5  --  ON   Auto-disconnect/Abort timer enabled
B6  --  OFF  Asynchronous operation
B7  --  ON   DSR off in test (ignored when not testing)

In addition, I recommend the following two jumper changes on the
BOTTOM pc board:

Insert jumper "r"  -- enable data rate indicator on RS232 pin 12
Remove jumper "ag" -- do not tie carrier detect high (RS232 pin 8)

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The "wedged" condition mentioned above, being related to a rather
random timing window, is more likely to have been seen on modems
that have a high volume of calls than on low volume incoming lines.
However, it occurs frequently enough that I recommend the option
change for all triple modems being used for incoming calls.

Be sure to let me know if you have any questions about or problems with
this info.  I hope it's of some use, bunky...

--Lauren--