How sophisticated do you want to get? The simplest way (assuming limited distances) would be to use the 12 V line to drive the coil of a small relay switching a TTL signal (or other appropriate voltage level), then tying each signal to the parallel printer port on your PC. Your software would then mask the word read to determine the status of each pin. Compare this to the last reading to detect a change of state.

If you are talking about significant distances between PC and panels, this may not work too well, in which case you have to get more sophisticated with digital signal transmitters, etc., which are available from the likes of National Instruments at exorbitant prices...

Thanks for the response. I agree that sophistication would mean more price and client would not be eager to cover the bill. I'll register your solution and let you know any development.

Do you just need to know the status of each panel or do you need both status and control? How far apart are the fire panels from each other and the quantity of panels? Also what is the distance to the PC?

Hi! Just need to know the status, no need to control. Fire panel in remote location is about 10 meters from PC and are around 140+ panels disseminated in different geographical areas located far away from host computer. Thanks for response.

Try fiber optic modems. There are models that do exactly what you want. One company I regularly use is OSD. They have a 8-channel relay modem. Not too expensive.

take a look at the manuals for the fire alarm system. many of them make a module that will do what you want, or something that you can modify to your needs. You may also want to look into the local fire codes to make sure that your modifications are "legal". The last thing you want is the inspector to have kittens when he sees what has been done.

one system that I worked with a short time ago had a 485 output jack that you could use for your own interface with a computer.