A programmable and expandable building automation and control system 10. A system platform supports interchangeable smart card 12, security card 22, power supply card 24, telephone / voice / modem card 26, HVAC relay control 62, auxiliary relay control 66, power 80, telephone interface 30, sensor analog inputs 32, smoke detector interface 54, siren / strobe output 82, tamper loop 52, protected peripheral power supply 72, switched peripheral power supply 74, PLC communication protocol interface 18, RS 232 communication interface 14, RS 485 communication interface 16, touchscreen user interface 28, and “smart” key interface 34 via “smart” key 104. In addition to touchscreen and smart key interface, user-interface with system 10 is accommodated via telephone, personal computer or personal digital assistant, or through infrared or radio frequency transmission.