This wiring switch is not so loopy
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THE CONVENTIONAL wiring loom of a vehicle electrical system can be replaced with a single loop of cable, according to Robert Bosch Ltd, PO Box 166, Rhodes Way, Watford.
The Bosch Multiplex control system resembles a ring-main system for domestic electrical installation and consists of a single loop of cable over which all the operational and monitoring functions can be directed from a single control station.
This loop forms a continuous ring to which each item of equipment (or group of items) is connected through a switch. The switch operates on receiving a " code word' consisting of a series of electrical pulses sent out by a central control unit.
Each item of equipment responds to its own individual "code word" totally ignoring any other command sent along the cable.
Connect
The controls used by the driver for operating the various items of electrical equipment are connected to the central multiplex unit, which selects and sends out the appropriate code word to switch on lights, horn, flasher, etc.
The relevant switch connects its associated unit to the power supply cable which may be the normal live wire of a conventional system or a separate insulated power supply wire in the multiplex cable itself.
In the full Multiplex system, there are three separate wires in the ring cable—the command wire which carries the code words, the power supply wire and a third wire which can feed back information.
So far as the latter function is concerned, a typical operation would be where the headlights have been switched on, the unit can send back a signal on this third wire to switch on a pilot lamp on the instrument panel. This circuit can also carry information on engine speed, fuel level, water temperature, etc.
Co-ordinate
The system is simplified by connecting groups of suitable items to "sub-stations," each sub-station serving up to eight pieces of equipment.
While the group is conventionally wired, each item has its separate code and access to the ring cable via its own substation. With up to 16 substations on the ring, each serving eight conventionally wired switched units, a total of 128 functions can be controlled or monitored. Under development, again by Bosch, is a Central Electronic System to co-ordinate all the separate systems under a single master control. As well as providing a continuous monitoring of information on the vehicle it also co-ordinates the operation of any electric function such as electronic ignition on a petrol engine or anti-skid control.
It is also possible to monitor brake lining thickness and tyre pressures or even to detect a broken filament anywhere in the lighting system with all the lights switched off.
Trouble
Fault warning signals can be displayed on the instrument panel and even classified according to urgency. Bosch reckons that systematic monitoring like this can show up trouble before it develops with a consequent drop in downtime and repair costs.
Most of the information coming into the system arrives in analogue form—ic as a continuously changing signal like the rise and fall of a liquid level or the up and down movement of a brake pedal.
This information is then converted into digital form—ie into groups of electrical pulses—as it is easier to store and process and it allows the functions of the system to be programmed to suit any particular vehicle and operating condition.
Servicing
The whole concept of the Bosch Central Electronic system was made possible by developments in the transistor field whereby the size of components could b e reduced dramatically. One important part of the system made up from approximately 3,000 transistors takes up only 3sqmm of space.
On the servicing side,. Bosch claims that the new systems give easier servicing and fault diagnosis than could be achieved by any other means.