4
Medium, Implementation and Physical Layers in CAN
The preceding chapters have hopefully brought us up to date with the nature and the operation, considered in its strictest sense, of the CAN (controller area network) protocol – part 1 of ISO 11898.
As I have mentioned several times, the original specification describing the protocol was drawn up to meet the requirements of multimaster communication applications, for real-time operation with multiplexed wiring. If your memory serves you well, you will recall that the designer's official specifications cover the whole data link layer and the bit specification of the OSI/ISO (International Standardization Organization/Open Systems Interconnect) communication model, while leaving the user free to choose the network topology and the type of transport medium to be used.
Rather evasively, therefore, I have avoided giving any electrical values for the signals in the diagrams in the preceding chapters, simply using the terms ‘recessive’ and ‘dominant’ for the values of the bits.
This model does not cover the actual structure of the physical layer, and, to avoid having a plethora of different solutions, many companies and organizations have worked on the problem of specifying the electrical properties required in a node at the interface with a transmission medium. After all, regardless of all other considerations, the system has to work and has to be constructed in the real world from real elements.
The aim of this chapter is to describe ...
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