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Two CAN bus structures are specified in ISO 11898-2 and ISO 11898-3 respectively (there is no description of bus topology in CAN2.0 specification): high-speed CAN bus and low-speed CAN bus.
This post will detail the features and differences between them.
1 High-Speed CAN Bus
ISO 11898-2 defines a high-speed closed-loop CAN communication standard with a communication rate of 125 kbps ~ 1Mbps. When the communication bus length is ≤40 meters, the maximum communication rate CAN reach 1 Mbps.
1.1 Electrical Characteristics
When the high-speed CAN bus is at the dominant level (logic 0), the CAN_H is 3.5V and the CAN_L is 1.5V. At this time, the voltage difference is 2V.
When the high-speed CAN bus is at the recessive level (logic 1), CAN_H and CAN_L are both 2.5V, and the voltage difference at this time is 0V.
1.2 Terminal Resistance
High-speed CAN lines need to add terminal resistance at CAN_H and CAN _ L. The terminal resistance on the cable should match the nominal impedance of the Wire Harness. The terminal matching resistance is generally 120Ω, and each terminal resistance should be able to consume 0.25W power (standard source: ISO 11898-2:2003).
If the high-speed CAN transmission line is not terminated, every signal change on the line will lead to reflection, which may lead to communication failure. Because communication flows in both directions on CAN bus, CAN requires both ends of the Wiring Harness to be terminated. However, this requirement does not mean that every device should have a termination resistor. If multiple devices are placed along the wiring loom, only the devices at the end of the cable should have termination resistance.
1.3 Bus Length
High-speed CAN bus has a maximum bus length of 40m. When the bus length exceeds 40m, the bus speed will be affected. The longest branch length (the distance between the node and the bus) is 0.3m, and the longest branch node distance length is 40m (standard source: ISO 11898-2:2003).
1.4 Wiring Cable
The high-speed CAN bus cable shall meet the physical medium requirements specified in ISO11898.
1.5 Equipment Quantity
The maximum number of devices on the CAN bus depends on the electrical characteristics of the devices on the network. If all devices meet the requirements of ISO11898, at least 30 devices can be connected to the bus.
If the electrical characteristics of the equipment do not degrade the signal quality, meet the ISO11898 signal level specification, and all the equipment on the network meet the specifications of the equipment network, 64 equipment can be connected to the network.
2 Low speed CAN Bus
ISO 11898-3 defines a low-speed open-loop CAN communication standard with a communication rate of 10 ~ 125 kbps. When the transmission rate is 40Kbps, the bus distance can reach 1000 meters.
2.1 Electrical Characteristics
When the low-speed CAN bus is at the dominant level (logic 0), CAN_H is 3.6V, CAN_L is 1.4V, and the voltage difference is 2.2V When the bus is at the recessive level (logic 1), the CAN_H is 0.3V and the CAN_L is 4.7V, and the voltage difference is-4.4V.
2.2 Terminal Resistance
Unlike high-speed CAN, low-speed CAN needs to be terminated on the low-speed CAN transceiver, not on the cable. The terminal needs two resistors: RTH for CAN_H and RTL for CAN _ L. This configuration allows Philips fault-tolerant CAN transceiver to detect and recover from bus failures.
Philips suggests a low-speed CAN network, with an overall RTH and RTL termination resistance of 100ω to 500ω (each). The entire network termination resistance can be determined by the following formula:
Each device on the low-speed CAN network needs a terminal resistor for each CAN data line: RTH is used for CAN_H, RTL is used for CAN_L, and the position of the terminal resistor is as follows:
2.3 Bus Length
In the online topology, more than 20 low-speed CAN nodes can be mounted, and the maximum communication speed can reach 125Kbps when the total network length should not exceed 40 meters.
In star topology, the total length of the network is about 40m, and the distance between nodes is less than 20m.
2.4 Wiring Cable
The low-speed CAN bus cable shall meet the physical medium requirements specified in ISO11898:
2.5 Number of Equipment
The maximum number of devices on the CAN bus depends on the electrical characteristics of the devices on the network. If all the devices meet the requirements of typical low-speed/fault-tolerant CAN, then at most 32 devices can be connected to the bus. If the signal quality of the equipment meets the low-speed/fault-tolerant signal level specification, a higher number of equipment can be connected.
3 Supplementary Knowledge: the relationship between ISO 11898-3 and ISO 11519-2
· ISO 11519 consists of the following parts, collectively referred to as "road vehicles-low-speed serial data communication":
Part 1: overview and definitions
Part 2: low-speed controller area network (CAN)
Part 3: vehicle area network (VAN)
ISO 11519-2: 1994 (low-speed controller area network (CAN)) specifies the data link layer and physical layer of CAN, which is a communication network of up to 125 Kbit/s for road vehicle applications. Low-speed CAN is a serial communication protocol, which supports distributed real-time control and multiplexing. According to ISO 7498, the general architecture of the network is defined in the hierarchy layer defined in ISO-OSI model. This standard allows CAN bus communication to continue when CAN bus connection fails.
ISO 11898-3:2006(Road vehicles - Controller area network (CAN) - Part 3: Low-speed, fault-tolerant, medium-dependent interface, Road vehicles-Controller Area Network (CAN)-Part 3: low-speed, fault-tolerant, media-related interfaces), which specifies the characteristics of establishing digital information exchange between electronic control units of road vehicles equipped with CAN at a transmission rate of 40 Kbit/s to 125 kbit/s.
ISO 11519-2-1994 has been replaced by ISO 11898-3-2006 in 2006, and products conforming to ISO 11898-3 also support products conforming to ISO 11519-2.
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