Pulse or digital communications – Testing
Reexamination Certificate
1999-06-11
2003-02-18
Vo, Don N. (Department: 2631)
Pulse or digital communications
Testing
Reexamination Certificate
active
06522689
ABSTRACT:
TECHNICAL FIELD
The invention relates to a monitoring circuit for a data transmission network, and in particular a Controller Area Network (“CAN”).
BACKGROUND OF THE INVENTION
One form of a data transmission system is a CAN system. The term CAN stands for Controller Area Network. Information about CAN systems in general can be found in the book “Controller Area Network: CAN” by Konrad Etschberger, Carl Hanser Publishing House 1994, ISBN No. 3-446-17596-2. Of interest in the present context are the sections on Protocol Properties on pages 25 and 26 and Data/Frame Format on pages 37 to 43.
CAN systems are employed for example in the field of motor vehicles.
There is a common supply voltage source for the CAN system, e.g., in the form of a motor vehicle battery delivering for instance a battery voltage of 12 V. Furthermore, each network node has an individual operating voltage source associated with or inherent with each network node, which produces from the supply voltage a regulated operating voltage, e.g., in the amount of 5 V, feeding the respective network node. Each operating voltage source delivers an operating potential at a first terminal and a reference potential, for example ground potential or 0 V, at a second terminal.
At least part of the network nodes can act both as a transmitter and as a receiver. For this purpose such network nodes have a transmitting part and a receiving part.
The transmitting part of such a network node has two resistors and two controllable electronic switches connected to the two lines of the double-line bus. One of these lines is connected via a first one of these resistors to the operating potential (5 V) and via a first one of these switches to the reference potential (0 V). The other line is connected via the second resistor to the reference potential (0 V) and via the second switch to the operating potential (5 V). For transmitting digital communications, the two switches are controlled synchronously either to a conducting state or to a nonconducting state. When the switches are controlled to the non-conducting state, the operating potential is present on one line and the reference potential is present on the other line. This switch state, for example, has the logic value “1” associated therewith. When the switches are controlled to the conducting state, the reference potential is present on one line and the operating potential is present on the other line. This switch state then has the logic value “0” associated therewith.
As the transmitting parts of all network nodes capable of transmission are connected in parallel with respect to the two lines, the potential ratio on the two lines, which is associated with logic value “0”, can be produced by closing the two switches of each of the transmissive network nodes. On the other hand, the non-conducting state of the two switches of each network node can be covered up by the conducting state of the two switches of another network node. For this reason, the logic value associated with a closed switch pair (logic value “0”) is referred to as dominant and the logic value associated with a non-conducting switch pair (logic value “1”) is referred to as recessive.
The receiving part of each network node capable of reception comprises a comparator comparing the respective potentials on the two lines with each other. Upon reception of a recessive bit (logic value “1”), for example, a positive potential is created at the output of the comparator, which has the logic value “1” associated therewith. Upon reception of a dominant bit (logic value “0”), a potential corresponding to the reference potential is present at the output of the comparator, which then has the logic value “0” associated therewith. The comparator thus constitutes a decoder for the potential relationships corresponding to the respective transmitted bit on both lines.
For reasons of redundance, the two lines are used in addition to system ground. The message information corresponding to the potential value of the respective bit transmitted is thus transferred both via the one line and via the other line. In case of failure of one of the lines, the further transmission operation can be restricted to the non-failed line. For detecting line failures, two additional comparators can be provided, one thereof comparing the potential of one line and the other one thereof the potential of the other line with a mean potential that is between the operating potential and the reference potential.
There can occur different line failures or line faults or errors, for instance, in the form of short-circuits between the two lines, short-circuits towards system ground, short-circuits towards the operating potential source, short-circuits towards the supply voltage source or in the form of open lines. There are line errors that do not hinder secure decoding of the communications transmitted. There are other line errors against which specific measures need to be taken in order to still render possible correct decoding. More details in this respect can be found in DE 195 23 031 A1.
In a CAN network, the messages or communications are transferred in the form of pulse sequences or frames spaced apart in time. The usual CAN protocol provides that a minimum distance in time is present between the individual frames, that within one frame there must be no more than a predetermined number of successive recessive or dominant bits, and that all receptive network nodes confirm reception of the respective pulse sequence by sending a confirmation pulse during a predetermined time slot (in the following referred to as confirmation time slot), which is the same for all network nodes, at the end of the respective pulse sequence. Issuing of the conformation pulse takes place by controlling the second switch in each confirming network node to the conducting state.
When the line connected via its resistor to the reference potential has a short-circuit towards system ground, the network-node-inherent operating potential sources (5 V) of all confirming network nodes are shorted via the respectively associated second switch and are shorted via this short-circuit to system ground. As a result thereof, a high current pulse flows across the shorted line during such a confirmation time slot. When the CAN network has, for example, 40 receptive network nodes and when the network-node-inherent operating voltage source of each of these 40 network nodes delivers a current of 200 mA to the shorted line, a total current pulse of 8 A arises on this shorted line during the confirmation time slot.
Such high current pulses not only constitute a burden for the supply voltage source, but can also cause disturbances in the data transmission network. During the high shorting current, inductive energy is stored in the line inductance of the shorted line, which upon opening of the second switches of the confirming network nodes is discharged in the form of a voltage pulse on the shorted line. This voltage pulse can affect the non-shorted other line by cross-talk and may cause an interference pulse there. This interference pulse is erroneously interpreted as a communication bit, and the last frame transmitted is rated as having not been transmitted correctly, which causes repeated transmission of this frame. When the line short-circuit is still present during the confirmation time slot for this repeatedly sent frame, a high current pulse again results on the short-circuit line, and as a consequence thereof a new interference bit is created on the non-shorted line and renewed repeated transmission of the already repeated frame is caused. This continues on and on, and the data transmission network remains captive in this loop.
SUMMARY OF THE INVENTION
The invention provides a monitoring circuit for a data transmission network, comprising a plurality of transmissive and receptive network nodes and a double-line bus connecting the network nodes and serving for redundant double transmission of digital communications and having a first line and a second line via which communication pulses transferred
Jorgenson Lisa K.
Seed IP Law Group PLLC
STMicroelectronics GmbH
Tarleton E. Russell
Vo Don N.
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