Multiplex communications – Channel assignment techniques – Using a separate control line or bus for access control
Patent
1994-04-26
1998-03-31
Chin, Wellington
Multiplex communications
Channel assignment techniques
Using a separate control line or bus for access control
333 32, 333124, H04L 1240, H04L 1244, H04L 2508
Patent
active
057346585
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a data communication system, particularly in motor vehicles, including a plurality of terminal units, which are connected by means of transmission lines for two-way data transfer.
For two-way data transfer between a plurality of user terminal units, BUS-systems, in particular, are known. For use in motor vehicles, which of course, are subject to particularly intense influence from interference, the CAN-BUS-system, for example, is known. In this system, the BUS line is formed by a two-wire line, each of which terminates at both ends almost reflection free by means of a terminating resistor for the useful signals. These terminal units are directly or by way of very short line segments connected to the BUS line. In flexible circuitry with different selections of terminal units, it is not defined which terminal unit is the last one at the end of the bus, i.e., where the terminating resistor must be built in.
SUMMARY OF THE INVENTION
It is therefore the object of the present invention to provide a data communication system of the above-mentioned type which tolerates different constructions in circuitry, particularly in motor vehicles, and also ensures flawless pulse transmission despite its flexibility.
According to the invention, a data communication system including a plurality of terminal units which are connected by means of transmission lines for two-way data transfer, particularly in motor vehicles, has the following characteristics:
the transmission lines form at least one star-shaped network about a passive star node (K) having a load impedance, preferably, a real resistance;
at the end of the transmission lines on the node sides, damping means are arranged which, during high frequency, transforms a portion of the effective resistance in the transmission lines; and
the damping means are chosen such that from each terminal unit in the direction of the network node, the respective transmission line for high frequency appears to terminate at least approximately with its characteristic impedance.
The system according to the invention allows a range of desired configurations and, within certain limits, also a variable number of connected terminal units. It is especially advantageous that by means of the load impedance the direct voltage characteristics of the network may be defined separately from the high-frequency characteristics and especially also, for example, may be matched to the system parameters predetermined by the terminal units. By high frequency is meant in this case, the frequency range which lies significantly above the frequency of the useful signal (occasionally referred to hereinafter simply as the signal) and above the transmission rate in digital communication. During data transmission, interfering transient processes at symbol edges are effectively suppressed and reflections at the network node are prevented, while in the steady state damping means are without effect and the network parameters are essentially only given by the load impedance. In this case, it is assumed that the contacts of non-transmitting terminal units are high impedance in relation to the characteristic impedance of the transmission lines.
The damping means are preferably damping beads of ferrite material. Materials having an initial permeability of approximately 2,000 are preferably used.
In a network configuration having a plurality of star networks in which each has a plurality of transmission lines which are joined at a star node and which are connected to terminal units, the star nodes are connected to one another via connecting transmission lines, preferably in the manner of a tree structure. In a tree structure, there is only one connecting path between any two terminal units, while in a meshed structure, two or more of such connecting paths are present. A tree structure, for example, in a motor vehicle, is of particular advantage if a first group of terminal units, for example, control devices under the hood of the vehicle, must be connected in a system
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Appel Wolfgang
Dorner Juergen
Grimmeisen Sebastion
Haeussler Bernd
Kuehner Thilo
Chin Wellington
Corrielus Jean B.
Daimler - Benz AG
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