Im usually working on the computer out in the sunroom at the rear of my little shack and if someone is at the front door theres no way I would know they are there unless I have the tv on the cctv channel. I purchased an inexpensive (Panacom)intercomUnit which included master station Front door station and Room Station.
Of course I wanted to automate it so if someone pressed the Front door button that I could inform Genesis and Homevision and take action ie mute all the Tv,s
Select cctv video channel and camera 1 (front door) etc.
Didnt like the idea of tampering with the front door unit but noticed it had a data Output . I assumed that whenever a button was pressed on any station its id was sent via the data line.
I decided to use a pic micro to monitor the the data line and compare the data stream to data stored in eeprom area of the pic.
I used an opto isolator between pic and intercom but found it loaded the data line and had to install a driver transistor to drive the opto (worked fine after doing that).
Next hurdle was how to capture the front door id. I thought initially I could use an ir led to tx the signal and capture it using my homebrew ir unit. The intercom data stream start signal was not compatible with my ir unit so came up with the following.
I use an input to tell the pic to capture the data and store in eeprom. This input is selected by a removable link. After capturing the data I remove the link and fire up my pic programmer and view the data. I noticed that the first bit wasnt always a consistent value. So in my compare part of my program I just ignored the first data bit and compared the rest.

So now when the pic detects a valid front door data stream it turns on two outputs connected to relays for approx 3 seconds. These relays are connect via dry contacts to Genesis and homevision.

Have a look at the pics