johnk@opel.uu.net (John Kennedy) (10/02/89)
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.
I'm assuming that I'll use some sort of solid state relay, or
even a telephone crosspoint chip, but don't know yet how I'll
control it.
So far, the options I'm considering are:
1) A device driver in the host system that will toggle a lead.
This would be actuated just prior to sending the dialing
sequence to the modem, and reset at the end of the call.
2) A UART and controller which listens to the AT sequence being
sent to the modem, and which switches lines upon detection
of a certain AT sequence.
3) A DTMF decoder which listens to an outpulsed sequence from the modem,
then connects one of the two lines in time for the delayed, real
digits.
4) If I replaced the modem with a Telebit T2000, the option exists
to assert the A/A1 leads (second pair) coming out of the modem.
Techs at Telebit confirmed that the TB+ does not have this capability.
These leads could be used to control the switching. Anyone
using these?
Option 1 would require the least hardware, but I have a concern about
leaving the wrong line connected in case of an aborted dialing attempt.
Anyone done anything similar, or have any suggestions?
Thanks in advance,
John
--
John Kennedy johnk@opel.uu.uunet
Second Source, Inc.
Annapolis, MDtcm@srhqla.SR.COM (Tim Meighan) (10/05/89)
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."
wiz@xroads.UUCP (Mike Carter) (10/05/89)
Use a real operating system. If you want to dial out on either line and you need hardware to do it, then there's something wrong with your choice of O.S. I'm running QNX (Que-Nix) on a 286 with two phone lines. I can call in on one, and then tell the computer to dial a number out and connect me through the two. (A complete waste of lines, but I've done it many times.) You need to get a true multiuser, multi-tasking O.S. -Mike -- ============================================================================= = Mike Carter N7GYX, Phoenix AZ| Q: Why did the Chicken cross the road ? = = hplabs!hp-sdd!crash!xroads!wiz| A: To ESCape the Main Menu . = =============================================================================