The typical electronic system comprises modules which are designed for mounting in the control room desk or in a cabinet front. Here are standard modules: alarm measuring module, display module, selector module, bridge panel, cabin and mess panel, power distribution unit, central unit for data logging and alarm print-out.
These modules are identical in electrical and mechanical design. The modules are made of sheet steel and are provided with ventilation holes. The internal wiring is provided with a printed circuit backplate, i. e. the "mother board", and ribbon cable. The front can be removed, and the electric boards can be inserted from the front.
Each module has a circuit board for stabilizing the supply voltage to the electronics. The modules are interconnected by ribbon cable — the system bus — used for internal communication in the system. Process signals and the power supply are connected to terminals at the rear of the module on the terminal board.
Alarm and measuring modules. They are designed with space for several analog or on/off channel boards. In front of each channel is a transparent legend plate on which the name of the channel is engraved, and a push-button for visual alarm acknowledgement and manual measuring.
The channel boards, which can be located in these modules according to the customer's requirements, are:
- On/off alarm board, for adapting the on/off transducer signals.
- Analog channel board, for adapting the analog transducer signal.
- Average board, for calculation of mean values.
- Group alarm board, for generation of group alarm signals.
- Blocking board, for adapting the blocking transducer signals of internal blocking signals.
Apart from the electronics for interfacing the display, the module includes circuits for generating start signals to the sirens and the rotating lights. The front is equipped with a push button for audible alarm acknowledgement.
Selector module. It is used for the selection of the engine room status, attended or unattended, and an engineer on duty. The module is equipped with one circuit board per engineer, and switches and indicating lamps in the front. The unit also provides call of the engineers.
Central unit. Systems with alarm print-out or data logging include the central unit. The control unit is connected to the system via the system bus.
Such systems are programmable bus-oriented electronic systems comprising a number of standard units. The functions in the system are controlled from a central processing unit based on a microcomputer.
The functions are determined by a program — the system software — stored on a PROM semi-conductor memory. In addition to the basic functions of the central unit the use of the microcomputer and software make such alarm systems easily expandable to more complex functions.
Typical optional functions available are: presentation of process data on a display, telex transmission of measured values, trend analyses, condition monitoring of the machinery.
The central unit is fitted in a cubicle. The front of the cubicle includes the printer for alarm print-out.
Data Logging
The data logging function is controlled from the control unit. The measured, values are printed out at regular intervals, e. g. every four hours, or by depressing the print-out button.
The central unit addresses the measuring channels with a binary code via the system bus. The address is decoded in each module and one channel is addressed. Signal connections to the system bus, scaling and conversion, are carried out in the same way as during manual measuring.
However, the display is extinguished. The central unit receives the measured value in code and the information on the number of decimals, processes the input signals and controls the typewriter, where the measured values are printed out. The data and time are recorded on each print-out.
