Proof of concept by Daimler Buses

The Daimler Truck business unit Daimler Buses has substituted in a normal articulated bus one CAN FD network by a CAN XL network. The German bus maker showed this in a test drive with special guests.

The sun was just breaking through the clouds, when I got on the bus at the front door. The driver, Wolfgang Griebel, leader of the Daimler Truck research project, asked for the ticket. Yes, you needed a ticket for the test drive. After he invalidated the ticket, Griebel said: “Please, sit down or hold on.” Then, he started the test drive. A few circles around the test garage building proofed that the CAN XL communication functioned without any problem. Griebel, the first driver using a CAN XL network, asked Dr. Jan Taube from Vector to increase the network load by means of additionally introduced CAN XL data frames. The normal load was around 35 percent plus 15 percent needed the add-on camera mounted in the front of the bus. Taube monitored his connected CAN XL tool: “75 percent, 90 percent, 97,5 percent, and now 99 percent.” There was still a proper communication without any problems. The bus curved smoothly on the test course, braked when demanded, and the video stream was operating without interruptions. Of course, the CAN XL data frames with the video data had a lower priority as the frames containing control data.

Half a year ago, in summer 2022, Daimler Truck has started the CAN XL proof-of-concept project. In co-operation with Bosch, NXP, Rohde & Schwarz, and Vector, the CAN XL network was designed. It connected four legacy CAN FD connectable ECUs (electronic control unit). The CAN FD interfaces were linked to CAN XL bridge devices. The 3-bit priority of the legacy messages were mapped to the priority ID field enhanced by some other bits. The priority bits were selected in a way, that they meet the recommendations for a 1-Mbit/s arbitration bit rate. The test network was running at 500 kbit/s in the arbitration phase. In the data phase, the bit rate was increased to around 14,5 Mbit/s on a larger than 60-m network topology. “We will go for lower bit rates, in serial buses,” explained Griebel, who leaded the research project. The research wanted to evaluate the limits.

The message header information was mapped into the 32-bit acceptance field of the CAN XL data frame. The up to 64-byte CAN FD data field containing application parameters were put one-to-one into the CAN XL data field. This means, the possible length of up to 2048 was not used. The bridge function was implemented on CAN XL evaluation boards from C&S group hooked up with NXP's CAN XL prototype transceiver (Albi 2.0).

The CAN XL network comprised four nodes: the braking ECU, the central gateway device, the steering wheel sensor, and the air-treatment ECU. The daisy-chain network was terminated at the central gateway device close to the driver cabin and the braking ECU at the back axle. Additionally, there were linked to the CAN XL network two Ethernet bridges for the camera and the screen. This CAN XL communication mapping Ethernet frames used data field length of 1500 byte. There was also a CAN XL connected Vector tool to monitor the network traffic and to increase the network load. An oscilloscope with a CAN XL interpreter by Rohde & Schwarz was attached, too.

Before we were taking the bus ride, we convened in a meeting room listening to some introduction into CAN XL and the proof-of-concept project. Dr. Arthur Mutter and Florian Hartwich from Bosch explained shortly the benefits of CAN XL and the joint development within the nonprofit CiA association. Matthias Muth from NXP added some information about the CAN XL physical layer. Wolfgang Griebel and Christina Hafer from Daimler Truck provided details about the proof-of-concept network design. The overall length of the CAN XL network was around 60 m. The not terminated stubs had only a length of some 10 cm. “We reused the CAN FD wiring harness,” explained Christina Hafer. For the bridge mapping Ethernet frames to CAN XL frames and vice versa, the SDT specification (CiA 611-1) was applied. It was released in October 2022. The CAN XL/FD bridge was using on the CAN FD side a data bit rate of 5 Mbit/s. The arbitration bit rate was identical (500 kbit/s). The CAN XL data bit rate was 14,5 Mbit/s. A funny side story: Two bridge devices based on the mentioned evaluation boards were mounted below the vehicle and packed into Tupperware-like boxes to protect the electronics against dust and humidity.

When I left the bus, I thanked Wolfgang Griebel and his team for the save ride. He smiled and responded: “This was not my first time of proofing network technologies.” It was the first real usage of CAN XL in a vehicle with passengers. These passengers were special guests: all of them had a deep knowledge on CAN technology. Some of them are involved in the development of CAN XL and others were part of the proof-of-concept project – some even belonged to both groups. Therefore, it is not surprising that nobody was afraid. I enjoyed to practice the CAN XL technology, which the CiA community has developed in the last years. There are more things to do: the CANsec protocol is knocking on the door, the CAN community expects some CAN XL design guidelines, and the conformance test plans need to be finalized.

hz