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9600 baud modems

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Lee Butler

Oct 13, 1986, 12:13:52 PM10/13/86
I am interested in hearing people's experiences with 9600 baud and faster
modems for use with ordinary voice-grade dialup phone lines. Vendor
descriptions and experiences are mostly what I am looking for. Forgive me
if this has been hashed here before, but the need has just arisen and I
am new to the subject.

Lee A. Butler
Space Telescope Science Institute 3700 San Martin Dr. Baltimore MD 21218
Arpanet: or
Usenet: {noao,astrovax,cfa,charm,osiris,nrao1,jhunix}!stsci!butler
Phone: (301) 338-4531


Oct 14, 1986, 1:40:57 AM10/14/86
We have about 25 UDS 9600A/B modems paired with the EC100 error controller.
We've been using them heavily for nearly two years and are reasonably happy.
The two boxes together go for about $2500 quantity one. Since this is a
private error corrected protocol you'll need at least two.

The 9600A/B by itself operates at 9600 baud synchronous half duplex on a
dial line. The EC100 does error correction and retransmission, sync to async
conversion, and half to full duplex simulation. The line turn around can be
set (and used reliably) as low as 30 ms. Unlike some other units we have
tried or heard of, this combination provides quite reasonable interactive
response. I have also used them to run dialup IP connections for both
interactive (telnet) and file transfer (ftp) applications.

The basic unit does not have autodial capability, although if you like to
program such things you can strap it to accept pulse dialing signals on pin
25. Since we were using them for interactive terminals, the lack of autodial
was a small inconvenience compared to the advantage of having 9600 baud

In summary, I would recommend these units if they otherwise meet your needs
(and your bugdet!). We had no trouble getting a couple units from the local
distributor (whose name I can't recall at the moment) to try out for a week
or two.
Richard Johnsson, DEC Western Software Lab, Palo Alto, CA
UUCP: {decvax,ucbvax}!decwrl!johnsson
ARPA: DEC ENet: rhea::johnsson
phone: +1 415 853 6676

Barry Shein

Oct 14, 1986, 7:10:56 PM10/14/86

I have been using a Microcom (AX/9642?) 9600b modem at home now for
about three months over a normal telephone line.

My experience has been that when it is good, it is very very good and
when it is bad it is awful.

Most of the time it just works, good throughput (more or less 9600b to
my Z19, never measured it but subjectively speaking it's MUCH faster
than 1200 or 2400.)

Then it goes into these lapses where it won't hold a line. It uses
a 'reliable' mode above 2400 (also, optionally, at lower speeds.)
When it has lost synch it does something called "retraining" where
the TST light comes on and it tries to re-establish contact. If this
light comes on for more than about 5 seconds I know I am about to
lose the line.

When it goes into these lapses I sometimes can't hold a line through
a UNIX login. I am not at all sure what causes this but my suspicions

1. Some of the chips run too hot. I seem to have improved
things a lot by simply removing the cover.

2. I notice that I have more trouble if the sun is bright,
this is weird but night time and cloudy days I seem to have
little trouble (who knows, it's just an observation.) I don't
think this is caused by the associated room temperature as
that has varied a lot (the sun doesn't shine on the modem either,
could it be causing some sort of noise in the (all above ground)
telephone lines?)

3. Sometimes perhaps the line is just noisy, of course.

I tried kermit from my 7300 with it when I first got it. Throughput
was terrible but I suspect the implementation of kermit (I would
re-tune it for fast output before making a judgement.) It did work
however, for what that's worth. I'd rather spend my time developing a
poor-man's SL/IP for it.

The easy redial helps a lot when it is being uncooperative. I must say
it's nice that they have 10e6 tuning variables some of which make little
sense to me (like PBX1 and PBX0 which they seem to recommend trying if
you are having troubles even if there is no PBX is involved.)

The bottom line is I like it a lot better than a 1200b modem I've been
using. I can always drop back to 1200 or 2400 when things get hopeless
and that's maybe 10-20% of the time at most (sometimes I just go read
a book.)

-Barry Shein, Boston University

Henry Spencer

Oct 20, 1986, 3:01:17 PM10/20/86
> 2. I notice that I have more trouble if the sun is bright,
> this is weird but night time and cloudy days I seem to have
> little trouble...

You say you've ruled out temperature, and the sun doesn't shine directly
on the modem. There is something else to check: are any EPROMs (the
chips with the little glass windows, in case you're not a hardware person)
openly on view, or are the windows covered with something opaque? Some
EPROMs are light-sensitive, in the sense that photovoltaic currents will
foul up their operation without affecting their contents.

Even if the EPROMs are all covered, it's possible that their covers are
not fully opaque. Ordinary paper labels may not be good enough. Try
sticking a bit of aluminum foil over each label if you're in doubt. Or
put, say, a big cardboard box over the modem, so it's in deep shade but
still has some cooling air, and see if that helps.
Henry Spencer @ U of Toronto Zoology

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