Overview of CAN protocols and standard specifications

CAN protocol and standard specification 1. CAN protocol standardized by ISO

The CAN protocol has been standardized by ISO and has two versions, such as ISO11898 and ISO11519-2, which are no different at the data link layer, but differ in the physical layer.

(1) About ISO11898: This standard is used for high-speed CAN communication. In the beginning, the data link layer and the physical layer were specified in the standard ISO11898, and later split into ISO11898-1 (data link layer only) and ISO11898-2 (physical layer only)

(2) About ISO11519: This standard is used for low speed (up to 125kbps) CAN communication

2. The difference between ISO11898 and ISO11519-2

Figure 6 shows the specified range of the CAN specification. Sublayers of three physical layers: PLS (Physucal Signaling Sublayer) sublayer, PMA (Physical Medium Attachment) sublayer, MDI (Medium Dependent Interface) sublayer, PMA and MDI sublayer The definition is not the same.

CAN bus basics (2)

Table 3 lists the differences in physical layers between ISO11898 and ISO11519-2. Figure 7 shows the relationship between communication speed and bus length.

Communication speed and bus length need to be set by the user according to system requirements.

CAN bus basics (2)

Bus topology:

The CAN bus usually consists of two lines (CAN_High and CAN_Low). The CAN controller is connected to the bus through a transceiver. The level of the bus is determined by the potential difference between CAN_High and CAN_Low. The bus has two levels: dominant and Implicit, the bus is always at one of these 2 levels at any given time. For a logical "wire-to-wire" bus, the dominant and recessive levels are treated as logic 0 and logic 1, and a transmitting unit can send a message to the receiving unit by changing the bus level.

The physical layer termination impedance, dominant level, and recessive level differential voltage specified by ISO11898 and ISO11519-2 are different.

Figure 8 shows the characteristics of the ISO11898 and ISO11519-2 physical layers. Note that ISO11898 and ISO11519-2 require a transceiver to meet the corresponding standards. Table 4 lists the main transceiver ICs that meet ISO11898 and ISO11519-2.

CAN bus basics (2)

3, CAN and standard specifications

In addition to ISO, CAN specifications are standardized by industry standards organizations such as SAE, and are standardized by a number of private research institutions and companies.

Table 5 except for some basic standard specifications, Figure 9 shows the communication protocol for cars classified according to communication speed

CAN bus basics (2)

SAE: on behalf of the Society of Automotive Engineers (Society of AutomoTIve Engineers)

NMEA: On behalf of the National Association of Marine Educators (NaTIonalEducators AssociaTIon)

SDS: Represents SmartDistributed System

Class: Category name of SAE

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