Some time ago, I subscribed to the Caller Id service to know who
is calling me without being required to pick-up the phone. Unfortunately,
my basic caller id display could not memorize enough numbers, and I had
to write them manually in a computer to keep them. That's why I decided
to find a way to log the calling numbers directly to my computer.
After studying how caller id worked, I tried to buy a caller id decoding chip,
but they all seemed to be discontinued. I bought another caller id module, expecting
to find a suitable chip inside, but it was a single IC which did everything (Display,
Finally, someone gave me an old caller id module, and when I opened it, I discovered
that it used a chip very was similar to the ones I wanted to buy :). The chip
was different in that it required many external components to work, so
I decided to leave it in the module, and to add the components I needed instead
of building a complicated pcb. This has the side effect that the module still works
as it always did, but with an additional feature: Serial data out.
The caller id system in canada works like this:
Between the first and seconds ring, the telco sends the modulated caller
id data, at a speed of 1200 Bauds, 8N1. Once the data is demodulated, the
caller id module interprets the data and displays the phone number and name.
There exists 2 standars to encode the data. SDMF (Simple Data Message Format) and MDMF
(Multi Data Message Format). SDMF was the first standard used, and only supported
the caller phone number and time. MDMF supports the phone number, name, time, locality(long distance
My caller id module uses the MT8843 demodulator. The
demodulated serial data comes out of the pin number 17(data), at a rate
of 1200 bauds, if the chip is in mode 0. The mode 0 is selected by connecting
pin 9(mode) to ground. Fortunately, the caller id module worked this way. To
send the data to the computer, all I had to do was to make sure the voltage
levels are compatible. The TTL data coming out from the chip has to be converted
to rs232 levels (+-12Volts). Also, it is important to isolate the computer from
the phone line. I used an opto coupler to do this. Here's my schematic:
The signal coming from the MT8843 controls a NPN transistor (a 2n3904 on the
schematic, but a 2n2222 would work too) which drives the opto coupler's led. This way,
when the MT8843 signal is high, the opto led is on, which result in -12 volts on the
RD signal. If the MT8843 signal is low, the opto led is off, and there is +12 volts
on the RD signal.
As you may have noticed, the rs232 +12 and -12 voltages are coming from the computer. The idle
state of the TD pin is -12 (the computer does not send anything so it stays at -12), and the
DTR signal is kept at +12 volts by software.
I cannot be held responsible for any damages that could occur to you
or your equipment while following the procedures present on this page.
Also, I GIVE ABSOLUTELY NO WARRANTY on the correctness and usability
of the informations on this page. Please note, however, that the procedures
above have worked in my case without any damages or problems.