CiA® 604 series: CAN FD Light - Simplified CAN FD data link layer

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CAN FD Light is intended for use in deeply embedded, price-sensitive sensor/actuator networks. The necessity for a lightweight CAN FD network became evident during the development of systems for modern car lights. The connected LED clusters (responder nodes) are controlled individually by one commander node. Potential further applications may be any control system, where a central host controller manages several simple end-devices with limited resources. Typical examples are heating, ventilation, and air-conditioning (HVAC) systems.

The CAN FD Light specification (CiA 604) standardizes the communication between commander and responder nodes. The communication follows a commander/responder scheme. The responder nodes do not support network arbitration. The responder nodes only transmit CAN FD data frames on request of the CAN FD Light commander. Thus, an accurate clock source at each node is not required. Single monolithic devices without crystal oscillators can be used. This lowers costs and improves the implementation reliability, as crystal oscillators are sensitive to vibration and temperature.

The CAN FD Light responder nodes transmit CAN FD data frames that comply with ISO 11898-1:2015 and provide payloads of up to 64 byte. They support only the base frame format (11-bit identifier). As no bit-rate switching is supported, the same bit rate is used in the arbitration phase and data transmission phase, and the control field bits in the frames are fixed. Currently, the maximum bit rate is 1 Mbit/s, which is fast enough for the intended applications.

Some protocol details

Responder nodes receive and transmit CAN data frames in FBFF (FD base frame format) as specified in ISO 11898-1:2015. They provide acceptance filtering and do not support automatic retransmission of data frames. Additionally, they do not support remote frames (RF), overload frames (OF), and error frames (EF).

Responder nodes send the ACK (acknowledge) bit after error-free reception of a data frame from the commander node. Optionally the sending of the ACK bit can be disabled by the node configuration, which is not compliant with ISO 11898-1:2015. In this case, the CAN FD Light commander node needs to support this configuration.

Error detection and signaling

The responder nodes support some error detection and handling mechanisms as specified in ISO 11898-1:2015. These include stuff-rule check, stuff-count check, and CRC check. Additionally, they check the CAN FD frame format. Accordingly, the supported error types include stuff error, CRC error, and form error. The latter is detected when a recessive IDE (identifier extension) bit, a dominant FDF (FD format) bit, or a recessive BRS (bit rate switch) bit is received. As exception, responder nodes shall accept any value of the ESI (error state indicator) bit, if the CRC value is correct. Responder nodes do not detect bit errors and ACK errors.

The responder nodes do not detect overload conditions, support no overload signaling via overload frames, and do not signal errors via error frames. This means that the responder node does not send EFs and OFs. An error counter is not implemented. In case of error conditions, the responder node triggers the protocol exception event. On this event, the responder node sends recessive bits and enters the bus integration state.

Network wide data consistency is not supported. Data consistency between commander node and responder node is achieved by the commander/responder communication scheme. Error handling is possible only on higher OSI layers, for example detecting that no response is received.

Specification and standardization

The CiA 604 series is going to be enriched by user guidelines and application notes. The development of these documents has already been started. The CiA 604-1 CAN FD Light protocol specification has been submitted to be included in the next edition of ISO 11898-1. A related conformance test plan is under development within CiA.

Title Details
CiA 604-2 version 0.0.1CAN FD Light Part 2: Data link layer and physical coding sub-layer conformance test plan
WD234 KiB2022-04-14Login
CiA 604-3 version 0.0.6CAN FD Light Part 3: System design recommendation
WD638 KiB2023-07-12Login
CiA 604-1 version 1.0.3CAN FD Light Part 1: Data link layer and physical coding sub-layer requirements of responder nodes
WD364 KiB2023-07-21Login