The CAN in Automation (CiA) association has set up a Special Interest Group to develop a primary/secondary data link layer protocol based on CAN FD for sensor/actuator and LED networks .
The CAN FD Light group is chaired by Fred Rennig from ST Microelectronics and brings together 20 experts from different companies.
The group aims to develop a “primary/secondary” data link layer based on the CAN FD protocol as specified in ISO 11898-1:2015. The primary node implements the CAN FD protocol and sends just 11-bit identifier FD frames. The secondary nodes transmit data frames in FBFF (FD base frame format) only on request of the “handler” node. This avoids the implementation of high-performance and costly oscillators.
The protocol engine of the secondary nodes is simplified compared to a CAN FD controller as error and overload frames are not supported. Nevertheless, data fields of up to 64 byte are possible. The bit-rate is not switched and limited to one fixed value, for example 1Mbit/s., which is deemed fast enough for the target applications of LED lighting networks.
As well as embedded networks such as hundreds of LED devices in passenger cars, the primary/secondary approach is also suitable for door control subsystems and air-conditionings with a lot of sensors.
This follows the development of the third generation of CAN networking protocol. CAN XL provides more than 2 048 byte of payload including a 1-byte protocol type field indicating the content of the 4-byte address field and the 2 048 byte data field. One of the features is the separation of frame priority and address information. CAN XL also adds a cascaded CRC (cyclic redundancy check) featuring a Hamming distance of 6, meaning five randomly distributed bit-errors are detected under all conditions.
The CAN XL protocol embeds some OSI management information such as the virtual CAN (VCAN) field. This 1-byte field enables to run up to 256 channels on a single network segment. In order to filter in hardware some of the CAN XL protocol fields, it is recommended to implement a 64-bit acceptance filter covering the 4-byte address field, the protocol type field, etc.
In addition, an embedded data link layer security protocol is under development. This CADsec protocol features a node-to-node protection comprising a 4-byte header with cipher-control information, the secure channel ID, and the freshness value as well as the 128-bit authentication tag. The payload can be encrypted optionally.