dya@unc-c.UUCP (04/18/84)
The article has long since been trashed ( have only a
160 mb Fujitsu ) but I'll comment on the modems again, any-
way.
Universal Data Systems 9600 A/B dial-through modem is
absolutely WONDERFUL !!!!! You can operate it on dial-up
phone lines without conditioning ( except where there is # 1
ESS and line extenders, which requires type C2 conditioning
); two wire leased line conditioned or not -- up to 100,000
feet w/o load coils with no amplification and who knows with
amplification, or four wire leased service.
The UDS 9600 A/B features automatic digital adaptive EQ
which , if given any signal at all, can compensate for fair-
ly bad envelope delay distortion. Receive carrier levels
can be set at -34 or -43 dBm; transmit can be set at -12 to
0 dBm. An optional high frequency pre-equaliser ( also digi-
tal ) can be manually selected.
In two-wire, half duplex operations, the UDS 9600 A/B
trains on data automatically, 2400 times per second. ( The
manual says 9600 times per second, but the signalling rate
is only 2400 bps ). Delays for turnaround can be set from
15 to 150 milliseconds depending on the system configura-
tion. In four wire operations, the UDS modem can be set for
train on data or signal quality retrain ( usually, the car-
rier is constant in this mode ). It is completely compatible
with CCITT rule v.29 for high speed 9600 baud modems and
their constellation.
The 9600 A/B is fully synchronous, meaning that if you
want to use asynchronous devices, you must purchase an
async-to-sync converter. Cheap async-to-syncs cost about $
300; the one that UDS offers won't do 7200 baud ( but the
modem will ). UDS has a new one, which offers an ECC correc-
tion scheme that supposedly lowers the error rate to one in
several years. The MC 6850 ACIA works fine with these
modems directly ( use the modems tx and rx clocks to drive
the ACIA.)
Fallback to 7200 or 4800 baud is possible with the
back panel switch; you can also select fallback through the
DB-25 connector using a logic level. Also, it is possible to
pulse dial the modem via your own software through the same
connector.
Self-test facilities are extensive; with a piece of
wire jumping DTR to the right level; all you need are two
modems to evaluate the performance of a given phone link. In
four wire operations, digital and analog loopback through
the distant modem can be selected remotely from the local
modem.
Users quickly begin to correlate the front panel
lights with line conditions ( in our application, they are
used for dial up service for medical imaging transmission ).
Experience: We have purchased about 35 of these modems so
far and have found only one place where they would not work
( yet ); suprisingly, it is a local call within the same ex-
change ( 662 ) in Buffalo, NY. If you call long distance
they work; if you call long distance and back in, they work.
But not locally.... This is because the compandor within the
ESS mutilates the training burst sent whenever RTS is as-
serted.
Currently, I have two on my desk which talk to each
other just fine, a second pair is connected between my of-
fice and our PDP-11 which is over 50000 feet away (* in four
wire ). Typically, I will see garbage about once per day on
the screen from line hits. If the lines were conditioned for
audio this probably would never have happened ( my boss had
delusions of grandeur about using RS 422 over this dis-
tance.)
General DataComm 's 4800 bps modem is worthless over
the same link , by the way.
I got these modems to work over 100000 feet incidental-
ly. With conditioning ( and amplification ) they work just
as well in this room as they do from here to Los Angeles...
David ( "last of the Analog" ) Anthony
( decvax,akgua ! mcnc ! urp ! dya )