Information Technology Reference
In-Depth Information
errors and permanent failures of nodes. Defective nodes are switched off.
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Unlimited number of connections - any number of units can connect to the bus (without
causing a disruption) and they can also be easily disconnected.
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Guaranteed latency times.
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System-wide data consistency.
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System flexibility - nodes can be added to the CAN network without requiring any
change in the software or hardware of any node and application layer.
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Sleep mode/wake-up - a CAN device may be set into sleep mode without any internal
activity and with disconnected bus drivers. This helps to save power. The sleep mode is
finished with a wake-up by any activity or by internal conditions of the system.
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Acknowledgements - all receivers check the consistency of received messages, acknowl-
edge a consistent message and flag an inconsistent message.
The CAN protocol defines the two layer layers of the OSI seven-layered model, the physical
layer and the data-link layer. It does not contain specifications on higher level protocols,
such as flow control, transportation of larger data packets, node addresses, communication
establishments, and so on. These are implemented by a HLP (higher level protocol), which:
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Standardises the start-up procedures, such as setting the bitrate.
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Creates logical addresses for nodes.
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Formats messages.
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Organises system error handling.
The CAN bus is a truly distributed control system as it does not need a controller to control
the flow of data between nodes.
The CAN bus splits into three main layers, as shown in Figure 21.1, and consists of:
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Object layer - implements part of the data link layer. It involves finding which messages
are to be transmitted, deciding which messages that are received by the transfer layer are
actually to be used and also provides an interface to the application layer.
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Transfer layer - implements the other part of the data link layer. It involves controlling
the framing, performing arbitration, error checking, error signalling and fault confine-
ment. This layer decides whether the bus is free for starting a new transmission or
whether reception is just starting. It also provides other features, such as bit timing.
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Physical layer - the layer involves the definition of the electrical (signal levels and bit
representations) and mechanical aspects (cable/connector type) of the physical connec-
tion.
