CAN Newsletter magazine
CAN XL is more than just a data link layer plus a physical medium access sub-layer. CAN XL comprises also higher-layer protocol specifications and add-on services.
The complete article is published in the June issue of the CAN Newsletter magazine 2021. This is just an excerpt.
Originally, in-vehicle network experts from Volkswagen initiated the CAN XL development. In the beginning, the focus was on the CAN XL data link layer featuring a data field ranging from 1 byte up to 2 048 byte. In the CAN XL protocol, the priority indication and the frame acceptance are separated. In Classical CAN and in CAN FD, the CAN- ID field provides both functions: bus access priority and frame filtering. In CAN XL, there is the 11-bit priority field and the 32-bit acceptance field containing address or frame content information.
The CAN XL protocol also embeds OSI (open system interconnections) layer management information. This includes the Service Data Unit Type (SDT) field and the Virtual CAN Identifier (VCID) field. Higher layers provide this information to indicate to the receiving nodes the used next higher OSI layer respectively to run several communication applications in parallel on the same cable. The SDT field is similar to the Ethertype function.
OSI layer management information is nothing new. A typical example is the setting of bit-timing parameters. The software driver of the host controller can do this statically, when it initiates the CAN controller. Another option is a separate configuration interface, e.g. DIP switches or USB or second CAN interface. Alternatively, you can use the same CAN interface running a dedicated protocol, such as specified in CiA 305 for CANopen applications. The CAN XL protocol controller can be connected to any CAN transceiver with an attachment unit interface (AUI) as specified in ISO 11898-1. Additionally, it features a PWM (pulse width modulation) coding and decoding to be connectable to CAN XL SIC (signal improvement capability) transceivers.
The CiA (CAN in Automation) special interest group (SIG) CAN XL coordinates all these specification activities. The physical layer is developed within a task force (TF) reporting to the SIG. There is also the TF higher-layers specifying the SDU types and the CAN XL frame fragmentation, which can be used to improve the real-time capability of the CAN XL communication in case of transmitting frequently blocks of long frames.
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