Security system

Plural security units are coupled to plural control units through a communication line including transmission and response lines. Each control unit detects data collision on the transmission line to avoid the data collision. Re-transmission interval is independently assigned to each control unit. If the command to or from a security unit in charge of this control unit is received, the control unit rewrites the conditional data of the security unit in accordance with the detected command. The control (security) unit further includes a transmission (response) line monitoring portion, so that bi-directional communication is provided in addition of the response (transmission) line monitoring portion. Address data is classified to provide all unit communication, a group communication, and independent communications.