in small offices we use two methods to share modems
and or the pbx connection.
1. Use the old RD-4000 Ring Discriminator that
Command Systems used to make. You can still find used
ones. In the closet, hook the Central Office line to
the input, hook each of the four outputs to a pair to
each modem. Only one of the four modems is allowed to
connect to the co line, any other modem will get a
(non-precise tone) busy signal.
If you are sharing a pbx destined line, run the first
ring sequence (-) to the pbx co line input, instead of
a modem. In this case, three modems and the pbx can
share one line as above. Consider programming the pbx
so this shared co line is not in the "9" autoselect
group. this avoids doing a speeddial, and picking up
the shared line that a modem happens to be using at
the time. you will hear the busy from the rd-4000 and
not interrupt the data call, but it's fustrating to
have to try again.
2. Use modem sharing exclusion modules (available at
fry's etc for about $9 each. You must do this right,
or you will mess up your modem speed. You need one
module for EACH connection (modem, pbx co) You must
maintain pair balance on all your lines to the modem
card or pbx port. Crosswire the Central Office line
to each pair that goes to each modem and pbx co jack.
Now install a module at each modem card and the pbx co
port. Use a standard line cord from the wall jack,
plug this cord into the module (looks like a std two
cord coupler). Plug the 2" jumper from the module
into the modem card. Since the modules are not
balanced devices, you must insert them within inches
of the end device (modem, pbx co port). First device
to go off hook captures the line, all others get
silence.
hope that gives you some ideas. RD-4000 is amazingly
clean to the line. It has double pole relays that only
connect one jack to the line at a time, and the
sensing is over 100kohms and balanced at that. Their
other line sharing devices are not so clean. (esp the
RD2 and RD3 ;-)
-larry
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