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a random amount of time before trying to send again. CAN protocol, however, solves

the collision problem using the principle of arbitration, where only the higheest priority

node is given the right to send its data.

There are basically two types of CAN protocols: 2.0A and 2.0B. CAN 2.0A is the

earlier standard with 11 bits of identifier, while CAN 2.0B is the new extended standard

with 29 bits of identifier. 2.0B controllers are completely backward-compatible with

2.0A controllers and can receive and transmit messages in either format.

There are two types of 2.0A controllers. The first is capable of sending and receiving

2.0A messages only, and reception of a 2.0B message will flag an error. The second

type of 2.0A controller (known as 2.0B passive) sends and receives 2.0A messages but

will also acknowledge receipt of 2.0B messages and then ignore them.

Some of the CAN protocol features are:



CAN bus is multimaster. When the bus is free, any device attached to the bus

can start sending a message.



CAN bus protocol is flexible. The devices connected to the bus have no

addresses, which means messages are not transmitted from one node to another

based on addresses. Instead, all nodes in the system receive every message

transmitted on the bus, and it is up to each node to decide whether the received

message should be kept or discarded. A single message can be destined for a

particular node or for many nodes, depending on how the system is designed.

Another advantage of having no addresses is that when a device is added to or

CAN BUS

NODE

Terminator

Terminato

Figure 9.2: Example CAN bus

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477Advanced PIC18 Projects—CAN Bus Projects

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