MAXSYS system keypads provide a plain language interface using a large 32 character LCD display with adjustable brightness and contrast. All system programming and user operations can be performed using the LCD keypads. Keypads are connected to PC4010/4020 control panels via a 4-wire system communications bus (Combus) which provides power for the keypads and acts as the system communication channel. Keypads are enrolled onto the system at the time of installation, and are supervised for low voltage and presence by the main panel.