High-speed CAN is the most common CAN transceiver. It is used in nearly every production automobile and is required in model year 2008 and later On-Board-Diagnostic (OBD-II) vehicles sold in the USA.
The high-speed CAN layer derives its name from the fastest form of CAN, which allows use of the full standard speed up to 1 Mbit/s. High-speed CAN networks support baud rates from 40 kbit/s to 1 Mbit/s. The most common rate, 500 kbit/s, is used in automotive environments.
The baud rates affect the total allowable length of the network, which ranges from roughly 1 km at 40 kbit/s to 40 m at 1 Mbit/s. The maximum stub length, or length from the central CAN network that nodes can be placed, is also affected. Generally, CAN network stubs should be less than 1 m.
High-speed CAN is specified or required by the following standards:
- ISO-11898-1, -2, -5
- SAE J1939
- SAE J2284-1, -2, -3,
CAN with Flexible Data-Rate (CAN FD) is the next generation of high-speed CAN communication with evolving standards for higher data rates. NI has enabled speeds up to 8 Mbit/s using the TJA1041 and TJA1043 transceivers through the NI-XNET driver. The overall baud rate for FD networks is the same as high-speed CAN, it is just the data rate that is faster. As transceiver vendors complete qualifications for CAN FD speeds, NI will update our documentation as necessary.
High-Speed/FD CAN Termination
For high-speed/FD CAN, both ends of the pair of signal wires (CAN_H and CAN_L) must be terminated. This is because communication flows both ways on the CAN bus. CAN_L is pin 2 and CAN_H is pin 7 on the standard 9-pin D-SUB connector. The termination resistors on a cable should match the nominal impedance of the cable. ISO 11898 requires a cable with a nominal impedance of 120 Ω; therefore, you should use 120 Ω resistors for termination. If you place multiple devices along the cable, only the devices on the ends of the cable need termination resistors. Figure 1 shows an example of how to terminate a high-speed network.
Figure 1. Terminating a High-Speed CAN Network
High-Speed/FD CAN Cables and Accessories
NI-XNET high-speed/FD and software-selectable/FD CAN interfaces have onboard software-selectable termination. This gives you the flexibility to programmatically add termination to the network whenever needed without changing cables.
Figure 2. NI-XNET Software-Selectable Termination
NI also offers terminated and nonterminated CAN cables for connecting NI high-speed/FD CAN interfaces to CAN devices and networks. Legacy NI CAN devices do not have software-selectable termination, so you may need to purchase terminated cables for use with those interfaces.
In general, when connecting a single CAN device to an NI CAN interface, you can programmatically enable termination or use a terminated cable. In the first option, the NI-XNET device connects a 120 Ω resistor between CAN_H and CAN_L using relays. In the case of the terminated cable, it contains one 120 Ω resistor installed on the CAN_H and CAN_L lines in the connector closest to the NI cable part number label.
When connecting multiple CAN devices that already have termination, NI nonterminated cables are recommended.
The NI CAN breakout box offers a switch to enable and disable 120 Ω and 60 Ω termination on a CAN network for maximum flexibility.
Learn more in the NI-XNET Manual or the NI-CAN Hardware and Software Manual.
Learn more about NI CAN product lines in the NI Embedded Network Products Overview (CAN, LIN, FlexRay).