AVR-Chat

At 11:53 AM 2/1/2005, Franco wrote:

>Hi!
>To sense the logic line is very simple with a MCu or a couple of Smith 
>Trigger buffers.

VBG.. It seems simple at first.
I have seen this done, in systems that were not isolated from the line.
a 10M divider and zener works acceptably well, once you find out that 
zeners have bad knees, and which ones have better knees.


>You only need know first the Vcc values for each state of the line, 48Vdc 
>is ready to use less than this is in use 15-4Vcc, and more than 48V is ringing.

Unless you're on a 24V or even a 12V line, and yes they do exist.
More than 48V is ringing, unless you're on a short line to a hot CO with 
52-58V.
If the instrument draws minimal current (20mA) then you may see as high as 
18V on a line in use.

Sorry, phone systems really don't work with voltage in a reliable fashion.



That being said, you can get a clear indication of ring, by using a bridge 
rectifier, small filter cap rated for 250V+ and a 75V zener.
The ringing signal is 90VAC at 20Hz, added in series to the -48V nominal on 
the line.
When you see this high voltage on the output of the bridge, you can use a 
few mA to light an optoisolator, and take the output of that into your circuit.

>You can sense too if you are directly connected to a POTS line from you 
>service provider, becose if you are
>using a phone central the VCC and some logic of the line is descarted from 
>the Telco central.

I can't really parse that..


>You can use the tone inside of the line to sense using a Voice detector 
>(Vox), if the tone of the line is
>ringing, ocuped or not available, this is the way the mos modem use to 
>know the state of each call.

There are "precise call progress" chips that will do this for you, 
otherwise it's a big PITA without DSP.