Modeling and Analysis of a Multiplex - Bus

A bus is a collection of one or more wires. A bus is called a multiplex-bus when it is shared by a number of devices. There are many applications of a multiplex-bus. For example, a microprocessor-based measurement system may use a bus (say IEEE-488, VME, etc.) to interconnect a number o f microprocessor-based instruments, e.g., voltmeter, ammeter, data acquisition devices, etc. All the automobile companies are planning to use multiplex-buses (multiplex wiring) in their future vehicles. Advanced electronic features like antilock braking, electronic steering, and electronically controlled transmissions and suspensions can improve performance. But the wiring, components, and electronic control units required for these systems must fight for space in today's small cars, where they may invite diagnostic and repair problems. Multiplexing can reduce wiring complexity and eliminate redundant parts, while simplifying diagnostics. The effectiveness of a multiplexbus has been reported in a number of papers (1-51. As more and more devices are connected to a bus, the speed of the bus reduces due to capacitive loading effect of the devices. Hence, when too many devices are to be connected to a bus, it is necessary to know the maximum rate (e.g., bitskec) at which data transfer can occur. Pulse width modulation (PWM) technique is widely used to transmit digital data in many industrial and consumer electronics applications. We have developed an analytical model for a multiplex-bus system, which will allow us to determine the shape of the pulses of PWM signals at different nodes of the bus. We have also developed a novel algorithm to convert our analytical model into a software package. Currently we have developed software packages for both IBM and Macintosh II personal computers. The software accepts the following parameters as inputs: electrical parameters (e.g., capacitance, resistance, etc.) of the bus and the devices, bit pattern of the PWM signals, and the characteristics of the bits (e.g., pulse period, pulse widths for logic 0 and 1, etc.). The software displays the pulses at different nodes of the bus. By looking at the display one can easily determine whether or not the speed of transmission is too high to detect the bits correctly by the receiving nodes. Our algorithm is very novel and fast. In this paper we will show our analytical model and the novel algorithm in detail. Our model and algorithm will help other people to develop software packages for the analysis of multiplex-bus systems. This software package will help to determine the maximum number of devices which could be connected to a given bus i f a certain minimum transmission speed has to be maintained.