On Tue, 27 Aug 2002, Tim McDonough wrote:

>My concern is that there's some evidence in early tests that the internal
>modem can't detect when the line is already in use if there are either
>quite a few phone instruments on the line and especially if there are a
>few extensions that are really "cheap" telephones. Line length may be a
>factor too as you suggest.

This is a well known problem. The fix in our case is external. It involves
wiring a zener diode across a small rectifier bridge and putting it in
series with the line to the 'insensitive' modem. 7V2 worked here before.
You can bypass the bridge with 0.1uF if you are nervous. This is not
really necessary.

You could build this into your circuit (or desensitize the line voltage
sensing opto using a small zener diode or resistors).

Peter

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