TELECOM Digest OnLine - Sorted: Re: Physically Protecting The Local Loop Metwork?

Re: Physically Protecting The Local Loop Metwork?

Gordon Burditt (
Tue, 20 Dec 2005 00:27:10 -0000

> I think I remember once seeing a little circuit board that did nothing
> except guarantee Part 68 compliance. (It even had its own Ringer
> Equivalence Number, a whole 0.0B.) It was designed for people who
> wanted to attach their own homebrew projects to the phone line but not
> worry about causing problems. I don't remember where I saw it, sorry,
> but you could probably find something like it wherever you buy other
> bare electronic circuit thingies.

A long time ago, when they first started allowing other people to
connect modems to a phone line, but NOT directly, there was the DAA
("Data Access Arrangement", I think). I worked with these in the late
1970's. You rented it from the phone company. It had a defined
interface so you could pass voice through it, take the phone off the
hook, pulse dial, detect ringing, etc. For tone dialing you'd just
take the phone off hook and send tones. For pulse dialing you'd do
the equivalent of rapidly jiggling switch-hook. Most of it was
providing isolation between the phone line and your side so if
lightning hit your gadget, it wouldn't get through to the phone line
(much). It was also supposed to protect the other way. Typically
there was transformer isolation for the voice signal and maybe relay
or optical isolation for the ringing signal and switch-hook.

Eventually they built these into modems, but I can still see a use for
these as interfaces to one-off projects that aren't worth going
through FCC certification for.

Gordon L. Burditt

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