party line approach with four conductor cables going in a loop shared by all the remote devices and the control panel. This approach is simple in concept and is economically feasible. However, one major disadvantage is the dependency on a single cable for power and signaling. In cases where reliability is of extreme importance, two or even three cables taking different
routes throughout the system may be connected in parallel. Fig. 3 gives the driver circuitry required to derive an expandable bus. This design takes advantage of recent advances in the single chip microcomputer technology to reduce the interface between
the central station and the local stations.
2. 1 Central control task
A central unit provides a centralized point to monitor and control the system
