Re: Modem to Modem again

From: Floyd Davidson (
Date: 08/01/03

Date: 01 Aug 2003 01:52:03 -0800

Tomi Holger Engdahl <> wrote:
>Floyd Davidson <> writes:
>> Bob Hauck <bob@this-is.invalid> wrote:
>> >On 30 Jul 2003 16:42:34 -0700, John Culleton <> wrote:
>> >
>> >> Now I have the two machines connected physically modem to modem via
>> >> the usual telephone modular cord.
>> >
>> >Most inexpensive consumer modems won't supply talk battery or ring and
>> Niether are needed.
>Neither of those are needed technically for the data communications.
>Because normal telephone line supplies the line current, many
>modern modems rely on the existence of those to operate correctly.

I've run across exactly one in the past 10 years or so.
Interestingly enough, that modem was intended for the European
market. Perhaps that is a characteristic common there, but it
seems to be rare here in the US.

As I pointed out to the OP in my first response, if it does
require loop current 9 volt "transistor battery" in series with
the line will provide what is needed. He need not worry though,
as it is a rare modem indeed that needs it.

>> >so won't work in this configuration. Hopefully you have the manuals for
>> Why wouldn't it work?
>Practically all modern modems use line interfacing circuity that
>has active electronics on the both side of the line isolation transformer.
>The electronics on the line side of the isolation transformer is
>generally powered through the telephone line (it uses the normal
>telephone loop current as it's power). This kind of modem does
>not generally work without the telephone line loop current being present.

I don't think there are many in the US that require loop current
to operate.

>Some modems I have examined seem to have circuits in them to sense
>the presense of telephone loop current, os they can notify the
>user if the line is not connected to modem and user tried to
>go on-line.
>A circuit like this can be used to supplu the needed line power
>to modems that need it.
>Get two phone jacks and a 680 ohm, 1/2 W resistor.
>Connect the components to make the circuit below:
>reen 680 1/2W + 12V - Red
>To Phone To Phone
>Red Green

Skip the resistor, it isn't needed. A common 9 vdc battery,
with no resistor at all, will work just fine.

>Please note that with so called 56k modems you can't get
>the full line speed with this kind of circuit, because
>two consumer 56k modems just can't communicate with each other
>at 56k speed in any conditions. 56k modem can only communicate
>at the promise speed in ideal conditions with the digital modem
>(modem DSP card connected to digital telephone ciecuit) is
>dial-in server your ISP has. In practical life the speed is
>less than 56 kilobits per second.

Two subscriber v.90 modems will use v.34 protocols when
connected back to back.

>I think you can expects speed in order of 30 kilobits or so
>with this kind of modem-to-modem arrangement.


Floyd L. Davidson           <>
Ukpeagvik (Barrow, Alaska)