Design and development of an emergency fire telephone system for the Cape City Council

Abstract

The project entails designing and installing an emergency fire telephone system (EFTS) for the 23 storey Cape Town Administrative Civic Centre. The original system, with its mostly analogue circuitry, has no documentation available, is difficult to maintain and has become unreliable. After considering alternative systems the most economical option was to expand the original system by adding more extension telephones and to redesign the control section. The new EFTS briefly operates as follows: The status of ninety six extension telephones, installed at the emergency exits on each floor, are displayed on a mimic status display which both operators can monitor. Any emergency call can be identified by a green flashing LED and a distinctive bleep. The LED indicates the exact position and number of the telephone in the building, Each operator has a keypad and a two digit numeric display fitted his telephone. The operator can immediately answer incoming calls by pressing the queue button. Calls queue on a first in first out basis. The number of the extension telephone will be displayed on a numeric display. The operator can also select the extension he wishes to contact, by dialling the extension number on the keypad. The EFTS consists of nine printed circuit boards. A rack mounted Microcomputer board, made up of a Motorola MC6809 microprocessor, six 6821 PlAs, 2 kilobyte RAM, up to 16 kilobyte ROM and a watchdog timer controls the EFTS.Two Telephone Controller boards process voice signals and generate logic control signals for the CMOS voice switching circuitry on the Multiplexer Monitor boards. Six Multiplexer Monitor boards switch the two operators to any of the ninety six extension telephones and continuously monitor the extension telephone lines for faults and handset statuses. Noise and over voltage line protection is provided. The multiplexing of the 192 LEDs on the Mimic Status Display is controlled by the Microcomputer board. An unusual principle used in this design is the combination of low frequency AM and audio to affect communication. Two uninterruptable power supplies provide user independence from mains. Software used for the EFTS is written in 6809 Assembly Language. A Real time interrupt controls the Mimic Status Display. Operator actions are interfaced with the program logic by means of hardware interrupts.