CAN Overview
- Updated2023-09-14
- 4 minute(s) read
CAN Overview
CAN FD, ISO Versus Non-ISO
Bosch published several versions of the CAN specification, such as CAN 2.0, published in 1991. This specification has two parts; part A is for the standard format with an 11-bit identifier, and part B is for the extended format with a 29-bit identifier. CAN 2.0 supports frames with payload up to 8 bytes and transmission speed up to 1 Mbaud.
To allow faster transmission rates, in 2012 Bosch released CAN FD 1.0 (CAN with Flexible Data-Rate), supporting a payload length up to 64 bytes and faster baud rates. ISO later standardized CAN FD. ISO CAN FD 11898-1:2015 introduced some changes to the original CAN FD 1.0 protocol from Bosch, which made the CAN FD 1.0 (non-ISO CAN FD) and ISO CAN FD protocols incompatible. These changes are now available under ISO 11898-1:2015. The standards cannot communicate with each other.
NI-XNET supports both ISO CAN FD and non-ISO CAN FD. The default is ISO CAN FD. The NI-XNET API behavior supporting ISO CAN FD mode has been changed slightly to allow new features compared to the Non-ISO FD mode. In Non-ISO CAN FD mode, you must use the Interface:CAN:Transmit I/O Mode session property to switch the CAN I/O mode of transmitted frames. In ISO CAN FD mode, the transmission mode is specified in the database ( CAN:I/O Mode property) or, when the database is not used, in the frame type field of the frame header.
Received data frames in Non-ISO CAN FD mode always have the type CAN Data, while in ISO CAN FD mode the type is more specific, indicating the protocol in which the frame has been transmitted (CAN 2.0, CAN FD, or CAN FD+BRS).
Because an existing CAN FD application developed with NI-XNET 15.0 (which supported non-ISO CAN FD only) might not work with the API changes for ISO CAN FD, NI-XNET 15.5 has introduced a Legacy ISO mode. In this mode, the API behavior is the same as in Non-ISO CAN FD mode, but it communicates on the bus using ISO CAN FD mode.
You define the ISO CAN FD mode when you add an alias for a database supporting CAN FD. In a dialog box (or the XNET Database Add Alias VI ), you define whether the mode default is ISO CAN FD, Non-ISO CAN FD, or Legacy ISO mode. In the session, you still can change the ISO mode with an Interface:CAN:FD ISO Mode property.
Understanding CAN Frame
Timing
When you use an NI-XNET database for CAN, the properties of each CAN frame specify the CAN data transfer timing. To understand how the CAN frame timing properties apply to NI-XNET sessions, refer to CAN Timing Type and Session Mode.
Configuring Frame I/O Stream
Sessions
As described in Database Programming, you typically need to specify database objects when creating an NI-XNET session.
The CAN protocol supports an exception that makes some applications easier to program. In sessions with Frame Input Stream or Frame Output Stream mode, you can read or write arbitrary frames. Because these modes do not use specific frames, only the database cluster properties apply. For CAN, the only required cluster property is the baud rate. If the I/O mode of your cluster is CAN FD or CAN FD+BRS, the FD baud rate also is required.
Although the CAN baud rate applies to all hardware on the bus (cluster), NI-XNET also provides the baud rate properties as interface properties. You can set these interface properties using the session property node.
Configure CAN Frame Input
Stream
If your application uses only Frame I/O Stream sessions, but you want to connect to a CAN FD bus, use the in-memory database :can_fd: or :can_fd_brs: as shown in the following figure. These databases are configured as a CAN cluster with the CAN:I/O Mode set to CAN FD or CAN FD+BRS, as appropriate. If you use either database, you must set the Interface:CAN:64bit FD Baud Rate property.
Configure CAN Frame Input Stream
for a CAN FD Session
