The scope of the CANbus protocol covers the physical and data link layers of the ISO/OSI model. The spec [1] refers to three levels in the CANbus protocol; physical layer, transfer layer and object layer. The physical layer is not defined in the Bosch spec, but is typically shielded or unshielded twisted pair. Idle state is both lines at +2.5 volts. A dominant bit reduces one line, known as CAN_L, to zero, while increasing the other line (CAN_H) to +5 volts while a recessive bit is close to the idle value, with CAN_L slightly above CAN_H, so is “over written” by a dominant bit. A standard for the physical layer of a 500 KBPS vehicle network is defined in SAE J2284-500 [4].
The transfer and object layers between them comprise all the services and functions of the ISO/OSI data link layer. These are discussed in the spec.
Various higher level protocols might be added to CAN itself. Kvaser has some material on this, and Omegas have some links on their website. Of these the Kvaser source is perhaps the more useful. I have also seen an article on higher level protocols that gives an overview of the most important higher layer protocols, especially those used in industrial automation.
The Can in Automation (CiA) trade organisation supports various higher level protocols
2. DeviceNet
3. CAL (CAN application layer)
4. CAN Kingdom
5. SDS (Smart Distributed System)
CiA is an organisation mainly interested in using CAN for industrial automation so it may well be that the protocols listed above are more common in that field than in the automotive field.
In addition to these, Kvaser list J1939 and OSEK. There is an introduction to OSEK in the companion notes, Miscellaneous Notes. J1939 is an SAE standard for a Truck and Bus Control and Communications Network, that uses the CAN protocol, but includes documentation (though not explicit definitions) for each layer in the ISO/OSI stack. There is a brief introduction to J1939 in the companion Other automotive industry protocols. In addition to those listed on the Kvaser site, there is FNOS that appears to be a Ford alternative to OSEK.
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