There are three main members in the CAN bus family, namely high-speed CAN, fault-tolerant CAN and single-wire CAN, among which fault-tolerant CAN is also called low-speed CAN, what are the differences and similarities between it and the most commonly used high-speed CAN? In this article I would like to share my knowledge about fault-tolerant CAN.

I. Origins of Fault Tolerant CAN

Bosch in 1986 in the SAE (Society of Automotive Engineers) General Assembly put forward the concept of CAN bus, CAN bus is the first in the automotive electronics industry breeding. Subsequently, in 1987, Intel introduced the first CAN controller chip 82526, which lit the spark of CAN bus development. Six years later, the international CAN standard ISO11898/ISO11519 was released, and CAN bus started a prairie fire in the communication field.

Figure 1 CAN Bus Application Industry

ISO11898 is the standard for high-speed CAN and ISO11519 is the standard for low-speed CAN. Initially, both the data link layer and the physical layer of high-speed CAN were specified in ISO 11898, which was later split into ISO 118981 (covering only the data link layer) and ISO 11898-2 (covering only the physical layer). ISO 11519-2-1994 was superseded by ISO 11898-3-2006 in 2006, which means that products compliant with ISO 11898-3 also support products compliant with ISO 11519-2.

Figure 2 CAN Standard Development History

II. Similarities and differences between fault-tolerant CAN and high-speed CAN

As with high-speed CAN, fault-tolerant CAN is also the use of differential twisted-pair transmission, including CAN_H, CAN_L, GND three wires, in strict industrial applications also requires the use of special shielded twisted-pair cable and add the necessary protection circuitry. As shown in Figure 3, the OSI 7-layer communication model as an example, in fact, the CAN bus standard specification of part of the physical layer, the transport layer and all the data link layer rules, while the application layer, the representation layer, the session layer, the network layer did not do any specification. High-speed CAN and fault-tolerant CAN have the same data link layer content, so there is no difference between them in terms of bit transmission timing, bit arbitration, errors, checksums, frame structures, etc.

Figure 3 Fault-tolerant CAN and High-Speed CAN Standardisation Section

Figure 4 illustrates the difference in the physical layer definition between ISO11898 and ISO11519-2 in terms of electrical data comparison. The most significant difference between the two in the physical layer is the CAN_H and CAN_L recessive level value. The maximum communication rate for high-speed CAN is 1Mbps, while for fault-tolerant CAN it is 125Kbps. Additionally, the theoretical number of nodes connected to high-speed CAN is greater than that of fault-tolerant CAN. Fault-tolerant CAN and standard high-speed CAN devices cannot communicate directly with each other. They must be connected through the CANBridge1054 adapter board.

Fig. 4 Comparison of Fault-Tolerant CAN and High-Speed CAN Electrical Signal Data

III. Advantages and disadvantages of fault-tolerant CAN

Despite its low communication rate and limited number of carrier nodes, fault-tolerant CAN communication offers unique advantages. Based on the level signal data presented in Figure 4, we have drawn the signal waveforms for normal operation. Figure 5 shows that the level of CAN_H and CAN_L changes up to 2.25V during explicit and implicit changes, whereas the level change of high-speed CAN is only 1V. This indicates that fault-tolerant CAN has higher anti-interference than high-speed CAN.

Fig. 5 Comparison of Fault-Tolerant CAN and High-Sspeed CAN Signal Waveforms

Furthermore, fault-tolerant CAN ensures uninterrupted communication in the event of a short-circuit or break in CAN_H or CAN_L. The fault-tolerant CAN transceiver automatically identifies the bus state and adjusts the receiver's receiving mode accordingly, which is why it is called 'fault-tolerant'. Figure 6 shows the fault-tolerant CAN transceiver in different states of adjustment.

Fig. 6 Fault-Tolerant CAN Multi-Mode Operating State

Note 1: 75μA Pull Down Current Source Function

Note 2: 75μA Pull-Up Current Source Function

IV. Fault-tolerant CAN Application Circuits

Figure 7 displays the CTM1054T fault-tolerant CAN transceiver module as an example. This module uses a potting process, resulting in low electromagnetic radiation and high immunity to electromagnetic interference. It is fully compliant with the ISO 11898-3 standard, and supports up to 32 nodes in a single network.

Figure 7 Fault-Tolerant CAN Transceiver Module CTM1054T

The design of a fault-tolerant CAN node circuit differs from that of a common high-speed CAN node. It is important to pay attention to the connection method of the termination resistors. Typically, the module is connected to the power supply, the port is connected to the CAN controller and the CAN network bus, and the RTH and RTL are connected to the termination resistors for CANH and CANL respectively, as illustrated in Figure 8.

Fig. 8 Classical Fault-Tolerant CAN Node Circuit Design

Figure 9 shows a typical single CAN-bus network with a maximum communication distance of 1km. To extend the network for more nodes or longer communication distances, devices such as CAN repeaters can be used.

For optimal system performance, fault-tolerant CAN transceivers require a total termination resistance of 100Ω. To determine the number of nodes in the entire network, each transceiver provides a portion of the total 100 Ω termination for the fault-tolerant CAN bus termination resistor configuration. It is not necessary for each transceiver to have the same termination resistor, but the total termination should be 100 Ω. For instance, if there are five fault-tolerant CAN nodes on the bus, the ten resistors connected to the network should have a resistance value of 500 Ω. If there are ten fault-tolerant CAN nodes on the bus, the twenty resistors connected to the network should have a value of 1000 Ω. It is important to maintain consistency in the resistance value of the resistors connected to the network based on the number of fault-tolerant CAN nodes on the bus. The fault-tolerant CAN termination resistor configuration allows for flexible bus topology options, such as star or tree, based on the number of nodes. This feature makes it unique and advantageous compared to other bus configurations.

Fig. 9 Fault-Tolerant CAN Bus Network Topology

In summary, fault-tolerant CAN is well-suited for low-speed, high-reliability industrial applications and can adapt to various complex bus topologies when the number of nodes is fixed. If you require fault-tolerant CAN for your project, please contact us for samples.

When switching to fault-tolerant CAN, use the CAN2 channel of TranSwitch with CANalyst-II (top version pro with fault-tolerant CAN). Debug the fault-tolerant CAN bus directly. Please note that the termination resistor of fault-tolerant CAN is not connected between HL, so it cannot be measured externally with a multimeter.