Glenn Johansson wrote: > > I've seen a California-designed house alarm with a built-in > > modem/communicator. On closer inspection (opening the damn thing) > > it turned out to be a 16MHz 16C74 doing to whole lot. Quite > > impressive. > > But did it emulate an actual modem or just a DTMF-sender? An alarm > just needs to send the touch-tone frequencies as far as I can > understand, plus perhaps a digit or two to inform about the alarm > type. Glenn: Only the very cheapest alarm panels communicate via DTMF tones... All the "real" ones use various (incompatible) FSK schemes to communicate with the central station. Most central stations don't use Bell 103 (the 300 baud modem standard), so very few panels perform the communication using off-the-shelf modem chips... In nearly all cases, the (bidirectional) communication is handled in software, usually using a resistor ladder to do the outgoing D/A conversion. FSK reception is generally handled completely in software by timing zero-crossings of the incoming signal. > The day someone writes PIC code to emulate a 300 bps or even an 1200 > bps bidirectonal modem, and distribute the code public domain, THEN > I will be impressed. ;-) It's not that hard... In a previous life, I was employed by an alarm company that did a VERY comprehensive communicator using a 6805 running at 4 MHz. I think it was first released in, like, 1982. Of course, that communicator was half-duplex and didn't need to understand the Hayes "AT" modem command set... Adding "AT" compatibility and making it full-duplex might be a little difficult, especially if you wanted to use one of the smaller PICs like the 16C54. -Andy === Meet other PICLIST members at the Embedded Systems Conference: === 6:30 pm on Wednesday, 1 October, at Bytecraft Limited's booth. === === For more information on the Embedded Systems Conference, === see: http://www.embedsyscon.com/ === Andrew Warren - fastfwd@ix.netcom.com === Fast Forward Engineering - Vista, California === http://www.geocities.com/SiliconValley/2499