The classification of the two layers of controls is based upon layers of decision making, with respect to the facts that
(a) When the decision time comes, the making and implementation of a decision cannot be postponed
(b) The decisions have uncertainty
(c) It will isolate local decisions (e.g. locally 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 Block diagram of remote station
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.
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.
Fig 4 Remote station circuit diagram
(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.
3.2 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 ‘load-type coordination’, e.g. 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 microcomputer was chosen over discrete digital and analogue devices to interface to the field devices and to the central microcomputer. This is the main reason that previously this approach was not feasible.
In selecting the microcomputer 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 communication. (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.
5 Main loop
Fig 5 Main loop flow chart
6 Conclusion
This paper describes the development of a large scale fire detection and alarm system using multi-single chip microcomputers. The architecture used is a two-level hierarchy of decision making. This architecture is made possible by the new CMOS microcontrollers which represent a high packing density at a low power consumption yet are powerful in data processing and thus in decision making. Each local station could make an autonomous decision if the higher level of hierarchy allows it to do so. It has been tried to keep the system design in general format so it can be adapted to varying situations. A prototype of the described system has been built and tested . The control part of the central station is implemented with a development card based on
MC 68000 microprocessor (MEX 68KECB, by Motorola), which has a built-in monitor called Tutor. The application programs were developed using the features provided by this monitor. The local stations’ controllers were designed using the MC 68705R3, single-chip microcontroller.