[Gllug] Caller ID on a POTS line

Christopher Hunter cehunter at gb-x.org
Wed Jan 19 01:09:00 UTC 2011

On Wed, 2011-01-19 at 00:14 +0000, Benjamin Donnachie wrote:
> On 18 January 2011 09:01, general_email at technicalbloke.com
> <general_email at technicalbloke.com> wrote:
> > I want to get caller ID info from my POTS line into my computer.
> If I remember correctly, the standard calls for a reverse in line
> polarity and then the caller's number is sent using standard CCITT V23
> 1200bps modem tones.  

That's pretty much it, though some exchanges just momentarily drop the
"battery" voltage rather than reverse it.  The commercial CLID boxes I
worked on used an optocoupler to detect a battery zero (rather than a
reversal), and triggered a monostable which was used to enable the tone
decoder IC and the display driver.

As I recall, one of my colleagues developed a PIC solution for decoding
and display of CLID data.  I'll see if I can find the assembler code for


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