Re: Extending the range of null modem to 100ft.

[et472@FreeNet.Carleton.CA (Michael Black)]

Andy McFadden (fadden@fadden.com) writes:
> Ed Eastman <noone@nowhere.net> wrote:
>> No, you don't need a live phone line, in fact it is better to not have 
>> one or else you'd need two phone lines so they could REALLY call each 
>> other.
>> 
>> Run a phone cord from one modem strait to the other.  Tap ATD on one 
>> telecom software then ATA on the other and your modems will initiate and 
>> answer (squacks, etc).
> 
> Interesting.
> 
> If you take two standard phones and plug them together, nothing happens.
> There's no power on the line, and you can't get enough out of a piezo-
> electric microphone to drive a phone signal.
> 
But that's because the phones need voltage in order to operate.  The
carbon microphones in the traditional phone does not output a voltage,
it acts like a variable resistor and needs a voltage source to vary into
the receiver at the other end.

> For this to work, one or both modems must be supplying current on the
> phone line.  Some of the telco devices I've worked with have used an
> optical component (can't think of the part name now) to electrically
> isolate the analog and digital components.  As with common household
> phones, these didn't supply power.
> 
Few or no modems will use opto-couplers for the audio signal; they
use transformers.  What you are referring to is the circuitry in the
modem to watch for ringing, and there may be checking to ensure that
the phone line is live.

What people are suggesting is commands to tell the modems to ignore
that stuff, and send (and receive) away, without worrying if there
is a dial tone, or if the phone line rang.

Unlike phones (and recent phones with active circuitry emulate old
style phones, and beside the electronics need power off the phone line
to operate), a modem gets power from the computer (if it's internal)
or from an AC adaptor.  I don't think any modem ever got power from
the phone line.

So once you tell the modem to ignore the control circuitry, the rest
oif the operation is like sending tones over wires.

  Michael

> All of which makes me wonder what percentage of modems do supply power.
> It would have to be on the "answering" side, since it needs to start
> the negotiation.
> 
> You may also need to enable "blind dialing", since modems tend to want
> to hear a dial tone before they start dialing.  A steady carrier tone
> may be enough though -- I don't think it's picky about frequency.
> 
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