CAN Protocol – An Overview

The acronym CAN stands for 'controller area network.' The ISO 11898 standards define the controller area network as an electronic communication bus. Those standards, among other things, specify how communication occurs, how wiring is built, and how messages are created. A CAN bus is the name given to this system in its entirety.

A Brief Synopsis of the CAN Bus

In simple yet precise terms, The CAN bus is a transmission style bus.  This implies that all broadcasts are "heard" by all nodes. There is no method to transmit a message to a single node; all traffic is picked up by all nodes. The CAN hardware, on the other hand, includes local filtering, allowing each node to respond only to the relevant messages.

The modules in this system are wired-and linked to the bus; if just one node drives the bus to a logical 0, the entire bus is in that state, regardless of how many nodes are broadcasting a logical 1.

The CAN standard distinguishes four types of messages. To manage access to the bus, the messages employ a smart bit-wise arbitration method, and each message is assigned a priority.

The CAN standard also specifies a complex error management and containment method.

CAN Protocol and its Constituents

Following are the different components of the CAN protocol that work in tandem to make a vehicle function. These constituents further have several applications and uses.

·       CAN Messages – Short messages are used in CAN; the maximum utility load is 94 bits. The communications do not have an explicit address; instead, they are contents-addressed, meaning that their contents determine their address implicitly.

·       CAN Physical Layers – Non-Return to Zero (NRZ) with bit-stuffing is used on the CAN bus. There are two signalling states: dominant (logically 0) and recessive (logically 1). (Logically 1). These relate to certain electrical values that differ depending on the several physical layers.

A physical layer specifies the electrical levels and signalling method on the bus, as well as the cable impedance and other parameters.

·       CAN Connectors – CAN connectors are mostly 9, 5 and 6 pin microcontrollers that connect to different parts of the CAN bus.

·       CAN Bit Timing – For timing considerations, each bit on the CAN bus is separated into at least four quanta. The quanta are grouped logically into four groups or segments:

Synchronization Section

ü  Propagation Segment 

ü  Phase Segments 1 and 2

·       CAN Error Handling – Error management is included into the CAN protocol and is critical to the overall functioning of a CAN system. The goal of error handling is to detect mistakes in messages that emerge on the CAN bus so that the transmitter can retransmit an incorrect message.

·       Higher Layer Protocols – The CAN standard defines the hardware ("the physical layer" - there are numerous) and fundamental communication ("the data link layer"). The CAN protocol simply describes how to send tiny data packets from point A to point B via a common communications link. However, a higher layer protocol (HLP) is necessary to handle communication inside a system.

Higher Layer Protocols include industry standards such as J1939, CANopen, CCP/XCP etc.

Conclusion

A Controller Area Network (CAN bus) is a robust and comprehensive vehicle bus standard that allows microcontrollers and devices to connect with each other's applications without the need for a host computer. It is a message-based system that was initially created to conserve copper by multiplexing electrical connections within vehicles, but it may also be used in a variety of other communication scenarios.