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'[OT?] RE: Private telephone network ?'
1999\04\06@103238 by Andres Tarzia

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The obvious answer is to buy and install a small PBX. I can recommend you
some commercial models appropriate for homes or small offices, but I don't
want to pollute the list with that *clearly* OT stuff, so if you want that,
mail me directly and we'll continue from there.

The down side of installing a PBX is that you will probably have to change
your phone wiring from the current "bus" (multidrop?) to "star", with a
separate cable running from each extension to the PBX.

I was toying with an idea to avoid this some time ago. The idea was about
building my own PBX. Instead of needing a separate cable for each extension,
the PBX would "talk" to each phone using the "bus" multidrop cabling present
in many homes. Some kind of conversion device would be needed at each
extension. This device would translate to and from the "cable" protocol to
the normal phone signaling. Each device would have a programmable address,
and that would be extension number. With luck, such a device could fit
within a wall phone adapter. The problem then reduces to what protocol use
for communication between the PBX and the translation devices at each
extension. I figured that if Ethernet can do it, we can do it too.

The advantages are simplified cabling, virtually unlimited extensions (no
need for a line driver for each extension at the PBX), functionality limited
only to what you want to put inside the PBX module, multiple external phone
lines can be managed like any other PBX. The down side is complexity, more
failure prone than a normal PBX (a break in the cable will disable all
extensions downstream, not only one extension), and the fact that some homes
already have "star" cabling.

But it would be a hell of a fun time to design and build that!

The question is: Can it be done with PICs?
The answer is: Probably not.

For each extension, we need to monitor a digital multidrop line, capture
every packet has our address, process it (what may mean ringing the
extension, D/A convert, or whatever), and send some answer back
(acknowledge, A/D convert from the mouthpiece, DTMF signals, and so on), and
that while monitoring a "free" slot on the digital bus line. For
conversations, all of this has to be done *AT LEAST* 8000 times a second,
and that is just for half-duplex.

For the PBX the task is much more difficult, like extension management,
probably bus management, not to mention multi-line, music-on-hold, voice
prompts, tapeless answering machine with multiple mailboxes, Caller-ID
management, dialing memories, special commands (hold, transfer, conference
call, forwarding, and many more), FAX and/or Modem tone recognition, and a
bunch of other things that you expect from a modern digital PBX.

Dammn too much for a PIC! Even for a 17Cxx part...

Any comments, suggestions, criticism?

Regards,
Andres Tarzia
Technology Consultant, SMART S.A.
e-mail: spam_OUTatarziaTakeThisOuTspamsmart.com.ar

{Original Message removed}

1999\04\06@151455 by Lee Jones

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>> My wish is to be able to phone (and why not speak ?) between 2 or 3
>> local telephones without going "out" to the ordinary and public
>> telephone network.

> The obvious answer is to buy and install a small PBX.

In Circuit Cellar Ink magazine a couple(?) years ago there
was a small series of articles on building a PBX.  I think
a PC board was available.

> down side of a PBX is that you will [...] have to change your
> phone wiring [...] to "star", with a separate cable running from
> each extension to the PBX.

If you do this, you can install wires for Ethernet 10/100baseT
and home automation at the same time. :-)

> The idea was about building my own PBX. Instead of needing a
> separate cable for each extension, the PBX would "talk" to each
> phone using the "bus" multidrop cabling present in many homes.
> Some kind of conversion device would be needed at each extension.

Sounds very much like ISDN.  It can support 7 devices at each
terminal unit (though not commonly used that way in the USA).
You'd have 2 B (Bearer) channels, so you could have 2 talk
paths open at once.

ISDN U interface will work over 18 kilofeet of cable.  For an
in-home setup, the much shorter cable runs should let you speed
up the link and add more B channels.  But you could leverage the
existing work on ISDN signaling protocols.

> The question is: Can it be done with PICs?
> The answer is: Probably not.

Central analog PBX, you _might_ be able to manage call setup &
teardown, ring management, off-hook detect, and digit collect
with a high end PIC.  You'd have to support it with lots of
analog stuff (protection circuitry) and a switch matrix.

For the digital, bus-style PBX you'd probably need lots more
horsepower for the central unit than a PIC can muster.  But
the individual terminal units might be well suited to PICs.

                                               Lee Jones

1999\04\06@161239 by Eric Schlaepfer

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Hi,

A PBX (even a small one) may not be needed. A standard 9V battery can be
connected in series with two phones: 9V simulates the 10V talk circuit. Ring
is more tricky as it requires about 90V AC at 20Hz.

I saw a phone line simulator in an Electronics Now (February 1996) called
the Party Line. It simulates six lines, and (wonder of all wonders) it uses
a PIC16C57! Caller ID is also supported.

One might be able to build a simplified version supporting two lines.

Later,

Eric
{Original Message removed}

1999\04\06@163122 by Andres Tarzia

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Eric,

Thank you for answering.

Do you know how can I do to get access to that article?

Regards,
Andres Tarzia
Technology Consultant, SMART S.A.
e-mail: .....atarziaKILLspamspam@spam@smart.com.ar

{Original Message removed}

1999\04\07@171458 by Eric Schlaepfer

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Hi Andres,

You could try the Gernsback web site: http://www.gernsback.com
I don't think they have the article online, but the site does say how to
order back issues.

Later,

Eric

{Original Message removed}

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