Understanding High-Speed ​​Access Unit "ISO11898-2"

Domestic new energy vehicles are developing in full swing. How can we build a more consistent in-vehicle CAN bus network? Let us start learning from international standards.

ISO11898 is the international standard for CAN bus. It was developed by Germany's BOSCH company and has become the standard bus protocol for automotive control systems. With the development of new energy vehicles in China, the double-point system has begun to be widely popularized among traditional car companies. In order to improve the stability of automobiles during the transformation process, the study of international standards has become a top priority.

　　Figure 1 Perspective view of the CAN bus of new energy vehicles

　　Any communication protocol must be built based on physical media, so what characteristics of high-speed (transmission rate 1Mbps) media are worthy of our attention? Today we will use " ISO11898-2 : High-speed Media Access Unit" as a starting point to understand this national standard.

　　1. Characteristics of the medium

　　1. Bus level

　　The bus can be in one of two logical states: receiving or transmitting (see Figure 2).

　　In the receive state, VCAN_L and VCAN_H are fixed at an average level, determined by the bus termination. When Vdiff is below the maximum threshold, the receive state passes during bus idle and receive data bits.

　　Figure 2 CAN bus logic state

　　In terms of physical media, the CAN network harness topology should be as close to the structure of a single wire as possible. The twisted pair used in the cable should be kept at around 50 twists per meter and wrapped with a layer of shielded wire to effectively reduce the impact of interference signals on the differential voltage and avoid harness reflection waves. The distance between d and l needs to reach a certain level.

　　The LS-CAN network segment topology adopts a bus topology, as shown in Figure 3. The specific values ​​are shown in Table 1. The communication rate is 125 k~500 kbit/s. In addition, the following technical requirements must be met:

　　1) In order to reduce standing waves, the distance between each ECU and the trunk line cannot be equal, and the ECUs should not be arranged at equal intervals on the bus, that is, the d value between any two ECUs cannot be equal;

　　2) The distance from the diagnostic interface to the diagnostic equipment must not exceed 5 m.

　　Figure 3 Bus topology

　　Table 1 Network segment topology

　　According to the above characteristics, when designing the vehicle body bus, we need to control the distance from the slave station to the master station and the distance between slave stations to avoid problems such as signal distortion or insufficient amplitude caused by too large or too small distance.

　　2. Conformance Test

　　1 Overview

　　The consistency of the medium access unit should be tested in accordance with the ISO16845 specification, with the impedance, capacitance, and bus transmission delay of CAN_H and CAN_L to ground or between them as the main test items.

　　Internal terminal resistance of CAN_L and CAN_H relative to ground

　　Rin_L and Rin_H are determined by Rtest, and the calculation formula is as follows:

　　Figure 4 Equivalent diagram of the internal terminal resistance test of CAN_L and CAN_H relative to ground

　　Internal differential resistance

　　Rdiff is determined by Rtest during bus idle period:

　　Figure 5 Internal differential resistance test equivalent diagram

　　Internal delay measurement method

　　The internal delay tnode shall be measured at the input of a transmit bit by the CAN bus through the CAN node in idle state with error active. The CAN node shall consider this transmit bit as the start of frame and perform hardware resynchronization. The CAN controller shall detect the stuffing error in the sixth receive bit after the transmit bit and respond with a valid error flag. The time between the external transmit bit and the start of the error flag is tedge to edge.

　　tΔedge=6∙NBT−(toutputRD−toutputDR)

　　tinputRD+toutputRD=tedge_to_edge−7∙NBT+sync

　　The synchronization condition can be eliminated by adjusting the phase to obtain the maximum value of tedge to edge (maximum CAN core sampling error: 1tq).

　　Figure 6 Internal transmission delay measurement

　　3. A quick and reliable method for IOS11898 consistency testing

　　It is extremely inefficient and inaccurate to use a multimeter, Excel data and calculation formulas to complete dozens of test items such as the internal transmission delay, impedance, capacitance, CAN node reception, transmission, output and input, etc., and there are many cases of human calculation errors. Therefore, the one-click test of the CANscope bus analyzer for the above consistency items has greatly improved the efficiency and authority.

　　Figure 7 Four major observation interfaces of consistency test

　　Figure 8 Part of the CANTester physical layer and data link layer consistency test items

　　Figure 9 Equivalent circuit function for input capacitance and resistance testing

　　Figure 10 CANscope bus analyzer

　　CANscope bus analyzer is the world's only solution for CAN bus fault location, interference elimination, and reliability testing. It is compatible with various mainstream CAN bus protocols, which means that CANscope can be compatible and updated in a timely manner for various standards currently implemented or upgraded in the future, making consistency testing more complete, faster, and more reliable.