10.2.1.1.1 CAN Termination

The ISO11898 standard specifies the interconnect to be a twisted-pair cable (shielded or unshielded) with 120-Ω characteristic impedance (Z_O). Resistors equal to the characteristic impedance of the line must be used to terminate both ends of the cable to prevent signal reflections. Unterminated drop lines (stubs) connecting nodes to the bus must be kept as short as possible to minimize signal reflections. The termination may be on the cable or in a node, but if nodes may be removed from the bus, the termination must be carefully placed so that it is not removed from the bus.

Figure 18. Typical CAN Bus

Termination may be a single 120-Ω resistor at the end of the bus either on the cable or in a terminating node. If filtering and stabilization of the common mode voltage of the bus is desired, then split termination may be used (see Figure 19). Split termination improves the electromagnetic emissions behavior of the network by eliminating fluctuations in the bus common-mode voltages at the start and end of message transmissions.

Figure 19. CAN Bus Termination Concepts

10.2.1.2.1 Example: Functional Safety Using the SN65HVD257 in a Redundant Physical Layer CAN Network Topology

CAN is a standard linear bus topology using 120-Ω twisted-pair cabling. The SN65HVD257 CAN device includes several features to use the CAN physical layer in nonstandard topologies with only one CAN link layer controller (μP) interface. This allows much greater flexibility in the physical topology of the bus while reducing the digital controller and software costs. The combination of RXD DTO and the FAULT output allows great flexibility, control, and monitoring of these applications.

A simple example of this flexibility is to use two SN65HVD257 devices in parallel with an AND gate to achieve redundancy (parallel) of the physical layer (cabling and PHYs) in a CAN network.

For the CAN bit-wise arbitration to work, the RXD outputs of the transceivers must connect through AND gate logic so that a dominant bit (low) from any of the branches is received by the link layer logic (μP) and appears to the link layer and above as a single physical network. The RXD DTO feature prevents a bus stuck dominant fault in a single branch from taking down the entire network by forcing the RXD pin for the transceivers on the branch with the fault back to the recessive after the t_{RXD_DTO} time. The remaining branch of the network continues to function. The FAULT pin of the transceivers on the branch with the fault indicates this through the FAULT output to their host processors, which diagnose the failure condition. The S pin (silent mode pin) may be used to put a branch in silent mode to check each branch for other faults. Therefore, it is possible to implement a robust and redundant CAN network topology in a very simple and low-cost manner.

These concepts can be expanded into more complicated and flexible CAN network topologies to solve various system-level challenges with a networked infrastructure.

A. CAN nodes with termination are PHY A, PHY B, PHY An and PHY Bn.

B. RXD DTO prevents a single branch-stuck-dominant condition from blocking the redundant branch through the AND logic on RXD. The transceivers signal a received bus stuck dominant fault through the FAULT pin. The system detects which branch is stuck dominant and issues a system warning. Other network faults on a single branch that appear as recessive (not blocking the redundant network) may be detected through diagnostic routines and using the Silent Mode of the PHYs to use only one branch at a time for transmission during diagnostic mode. This combination allows robust fault detection and recovery within single branches so that they may be repaired and again provide redundancy of the physical layer.

Figure 20. Typical Redundant Physical Layer Topology Using the SN65HVD257 Device