【文章內(nèi)容簡介】 ly we might have an alarm although there may be a fault with the system) 3 General hardware I :Fig. 2 depicts our design in the simplest of forms. The system uses an open 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 microputer technology to reduce the interface between the central station and the local stations. 3. 1 Central control task A central unit provides a centralized point to monitor and control the system activities. In the system to be described the central control unit serves a fivefold purpose. (i) It receives information from the local stations and operates the alarms and other output devices. (ii) It notifies the operator in case of system malfunction. (iii) It provides an overall system control manual and automatic. (iu) It provides a system test point of local stations and itself. (u) It provides a central point for observation, learning and adaptation. Local stations The local stations can take local decisions regarding recognition of a risk situation, and act independently on local affairs. In this technique we depend on ‘loadtype coordination’, . the lower level units recognize the existence of other decision units on the same level。 the central or the top level provides the lower units with a model of the relationship between its action and the response of the system. It is evident that a powerful machine is required at this stage so that all the required functions can be implemented. The availability of the new generation of microchips makes this architecture a feasible solution. A single chip microputer was chosen over discrete digital and analogue devices to interface to the field devices and to the central microputer. This is the main reason that previously this approach was not feasible. In selecting the microputer for the local stations, the criterion was the requirement for a chip which contains the most integration of the analogue and digital ports required for the interface and the utilization of CMOS technology owing to remoteness of the local stations. The choice was the Motorola 68HC11A4, for the following reasons: (a) It is CMOS technology。 this reduces power consumption. (b) It has a UART on board。 this facilitates serial munication. (e) It has an a/d converter on board。 this eliminates an external A/D. (d) It has 4K of ROM, 256 bytes of RAM, 512 bytes of EERROM with 40 1/0 lines and a 16 bit timer。 this satisfied all our memory and 1/0 requirements at the local station side. 4 System implementation The local station: Fig. 3 is the block diagram of the circuit used to utilize the MC68HCllA4 as a remote fire detecting circuit while Fig. 4 illustrates the same circuit in an expanded form. It can be seen that the single microcontroller can be used to monitor more than one detector, thus reducing system cost. The loop power supply, which is usually between 28 and 26 V, is further regulated by a 5 V 100 mA monolithic low power voltage regulator to supply power to the microcontroller. The onboard oscillator, coupled with an external crystal of MHz, supplies the microcontroller with its timing signal which is divided internally by four to yield a processor frequency of kHz, which is an even multiple of the RS 232 [7] baud rate generator. In this Section the term ‘supervised input or output’ will be used to mean that the function in question is monitored for open and shortcircuit conditions in addition to its other normal functions. More inform