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Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!lll-winken!csustan!csun!srhqla!tcm
From: tcm@srhqla.SR.COM (Tim Meighan)
Newsgroups: comp.dcom.modems,sci.electronics
Subject: Re: Anyone have a modem dialing out on two lines?
Message-ID: <1166@srhqla.SR.COM>
Date: 4 Oct 89 21:52:29 GMT
References: <282@opel.uu.net>
Reply-To: tcm@srhqla.SR.COM (Tim Meighan)
Followup-To: comp.dcom.modems
Organization: Silent Radio, Los Angeles
Lines: 28

In article <282@opel.uu.net> johnk@opel.UUCP (John Kennedy) writes:

>Got a Telebit Trailblazer+, and two phone lines serving two dialing
>areas.  I'm trying to come up with a scheme that allows me to
>dial out on either of the two lines.

If you aren't using DSR (or other RS-232 handshake line) out of the modem,
and it can be asserted/de-asserted via register settings, just let that
control a DPDT relay that toggles you between the two phone loops.  Then,
you can select which one you want right in your dial sequence.

To interface, you can just have it raise the base lead of an NPN 
transistor, which in turn controls the relay through an external power
source.  You'll want a diode and resistor in series in the control line
to limit current, provide isolation, and keep the -12 volts from appearing
at the transistor.  Don't forget a diode wired "backwards" across the relay
coil leads to keep EMF spikes from destroying the transistor when the relay
turns on and off.

While you could probably find a very low-current relay that would pull-in
directly off the current of the handshake line, it's not really the right
way to go.  These lines are signal lines, have limited current available,
and are not intended to be used as drivers.

Tim Meighan
SilentRadio

"Hit ENTER to exit."