Autzoo.1657
net.periphs
utzoo!henry
Tue May 11 19:26:53 1982
Vadic 3451PA autodialing modem -- a review
Basic rating: not bad.
The 3451PA (this may not be the right part number, actually -- the box
ours came in says 3451AD-P) is an autodialing version of the 3451P
answer/originate triple modem. I strongly suspect, from inspection of
the interior, that the difference is just new firmware. The price
difference is trivial (a hundred bucks or so) and R-V will also upgrade
existing 3451P's for some modest fee.
The thing is a fully-functional 3451P triple modem. Talks 300 baud and
both kinds of 1200, automatically recognizing what you are dialing in at.
(There is a strapping option that puts a speed signal onto one of the
RS232 lines so your system can autobaud if your terminal interface lets
you see a reasonable number of modem lines -- DZs don't). It connects
to the phone line via a standard garden-variety 4-wire modular jack.
In the absence of nonstandard strappings, it supplies the usual modem
control lines -- Ring Indicator means phone's ringing, Carrier Detect
means there's carrier, Data Terminal Ready tells the modem it's ok to
answer the phone (and dropping DTR tells the modem to hang up). NOTE:
the documentation addendum that comes with the autodialing version claims
that some of these lines are tied high to make strange terminals behave;
fortunately, it appears to be lying -- the lines behave in the standard
ways on our modem.
The autodialer does pulse dialing only and is not very smart. There
is no dial-tone detector or busy-signal detector. There is a character
you can put in the number to say "pause 5 seconds here". Once the dialing
is complete, the modem waits 20 seconds or so for carrier and then gives
up. Nevertheless, this is enough (except for some of you Bell types who
must have pulse dialing -- but you shouldn't need brand-X autodialers).
The autodialer features are set up for use from a terminal, but it's
not hard to use them from uucp. To get into autodialer mode, the phone
must be on hook and you hit ESC CR. After that there is a simple ASCII
dialogue, which fortunately does *not* include echoing, and which thus
fits fairly well into uucp's stimulus-response login handling. Our
uucp thinks our link to decvax is a hardwired line with a rather complex
login sequence! Once the call has gone through, the autodialer says
"ON LINE" and then you have a fully transparent data path -- you cannot
accidentally kick the thing back into autodialer command mode. The
thing has a couple of extra features like stored numbers that are not
worth bothering with for computer use. Oh yes, and it has the nicest
approach yet to dialout baud-rate control: it autobauds the ESC CR,
and uses that as the speed for the call.
There are two issues that need to be addressed to bring the thing up on
your system. One is that you need a way to send characters to a line
which has no carrier on it, so you can talk to the autodialer -- the
standard drivers won't let the open() complete until carrier is there.
If you are using the thing for dialout only, just wire the carrier line
high; this is what we did as a temporary measure. The other issue is
that the autodialer command mode can't take a burst of characters at
full speed. I put a very simple change into our uucp conn.c so that
a tilde (~) at the start of a response means "sleep 1 second before
each character". This is crude but it works.
We ordered this thing almost blind, in a desperate hurry to get a PO
out the door before the money vanished, the afternoon before I went
on vacation. We are very pleased with the way it turned out.
Despite a number of interruptions and complications like a cross-wired
modular adapter (existing hardware here, *not* from Racal-Vadic), our
3451PA autodialed decvax less than one working day after it arrived.
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