Diagnostics as a key application for customer satisfaction cannot function without diagnostic communication – and today this is predominantly implemented via bus systems. The communication mechanisms are generally specified by diagnostic protocols and implemented on the respective buses. However, this is handled differently throughout the vehicle lifecycle. After all, diagnostics initially takes place at the ECU level, with the ECUs then being integrated step by step until the complete vehicle is formed.

The buses used for diagnostics, however, remain the same. In most cases, three different types are used:

• K-line
• CAN and CAN FD
• (Automotive) Ethernet

The K-line (alongside the SAE J1850 derivatives used in the United States) is the original bus system. For a long time, it was also mandated for OBD (on-board diagnostics, in this case the legally prescribed diagnostics according to SAE J1979). It is a serial interface with byte-by-byte bidirectional transmission between the tester and the ECU. Functionally, the K-line is similar to the RS232 interface used in computer systems, but it operates at the voltage levels commonly found in vehicles: 12 V (passenger cars) and 24 V (trucks). A logical “0” is encoded with 0 V, while a logical “1” is encoded at battery level (12 V/24 V). The K-line is still used in numerous vehicles today, mainly in vehicles produced in small volumes or based on proven vehicle architectures.

CAN is a serial bus developed in the 1980s for networking multiple ECUs within a vehicle. Messages on the CAN bus are identified by a CAN identifier (CAN ID) and have a length of up to eight bytes. The CAN ID can be 11 or 29 bits long; today, 29 bits are commonly used. Since multiple nodes communicate on the bus simultaneously, collision handling is required (CSMA/CR in this case). With CAN, the message with the higher priority is transmitted successfully – i.e. the one with the lower CAN identifier. Transmission is differential over a two-wire line. At the recessive level, both lines are at 2.5 V; at the dominant level, one is at 1.5 V and the other at 3.5 V. CAN is generally mandated for OBD today.

CAN FD follows the same basic structure as CAN, but the data phase can use a higher transmission rate, allowing up to 64 bytes instead of 8 bytes to be transmitted in the same amount of time. CAN and CAN FD currently dominate applications in the automotive environment.

Automotive Ethernet defines full-duplex transmission over a two-wire line, similar to transmission methods familiar from building networks. Transmission is also differential, with the differential voltages used depending on the transmission rate. Ethernet is used with TCP/IP as the data link layer, resulting in relatively high overhead. For ZEVs (Zero Emission Vehicles), Ethernet can be used instead of CAN for OBD.

When comparing CAN and Ethernet for automotive applications, each has various advantages and disadvantages. CAN is a widely used protocol and is generally very resistant to interference. Networks of ECUs can be implemented with relatively little hardware effort, which is why CAN is the most commonly used bus system for this application. For flash programming of conventional ECUs, it achieves good payload data rates. In modern vehicle architectures with domain controllers and/or HPCs (High Performance Computers) and large software packages, however, its performance is often no longer sufficient. In these cases, Ethernet is essential both for the backbone – i.e. the connection between the HPCs – and for flash programming.

Other bus systems used in vehicles are generally not used directly for diagnostics. Instead, diagnostics is usually performed via a gateway that already carries out important functions within the respective bus system. One example is the LIN bus, where the LIN master controls communication and also serves as the diagnostic access point.