The Controller Area Network (CAN) bus stands as a pivotal vehicle bus standard, initially conceived to streamline communication among electronic control units (ECUs) within automobiles. Its primary motivation was to significantly reduce the complexity and financial burden associated with the extensive electrical wiring traditionally found in vehicles. Through a process known as multiplexing, CAN bus offered a more efficient solution, allowing multiple devices
to share a single communication channel. While its roots are firmly planted in the automotive industry, the reliability and efficiency of the CAN bus protocol have led to its adoption in diverse fields, including modern applications like 3D printing.
Origins and Core Principles of CAN Bus
Developed with the specific goal of simplifying automotive electrical systems, the CAN bus emerged as a broadcast-based, message-oriented protocol. This design ensures that data integrity is maintained and messages are prioritized effectively. A key mechanism for achieving this is arbitration, a process that allows the device with the highest priority to continue transmitting data even if multiple devices attempt to send information simultaneously. Other devices, recognizing the higher priority, will then defer their transmissions.
This robust communication method is further enhanced by its use of differential signaling. This technique is crucial for mitigating electrical noise, a common challenge in environments like automobiles where various electrical components can generate interference. By transmitting signals differentially, the CAN bus can maintain reliable communication even in noisy conditions, ensuring that critical data reaches its intended destination without corruption. The standard for this protocol, ISO 11898-1:2003, initially provided abstract requirements for the physical layer, focusing on the use of a medium with multiple-access capabilities at the bit level, distinguishing between dominant and recessive states.
The Physical Layer: Electrical and Mechanical Considerations
While the initial CAN bus standard, ISO 11898-1:2003, outlined the link layer protocol, it left the specific electrical aspects of the physical layer to be detailed elsewhere. These electrical specifications, including voltage, current, and the number of conductors, were later formalized in ISO 11898-2:2003, which has since become widely accepted. However, the mechanical aspects of the physical layer, such such as connector types, pin-outs, and cable colors, were not formally specified. This lack of a universal mechanical standard meant that automotive ECUs often featured custom connectors and various cable types, with two lines dedicated to the CAN bus.
Despite the absence of a formal mechanical standard, several de facto standards have emerged. The most common of these is the 9-pin D-sub type male connector, with specific pin assignments for CAN-Low (pin 2), ground (pin 3), CAN-High (pin 7), and CAN V+ (pin 9). This de facto standard allows for a node to have both male and female 9-pin D-sub connectors wired in parallel, with bus power fed to the male connector and drawn from the female connector. This adheres to the electrical engineering convention of terminating power sources at female connectors, helping to avoid the need for custom splitters that can reduce reliability and interchangeability.
Noise Immunity and Signaling Strategy
Noise immunity in the ISO 11898-2:2003 standard is achieved through maintaining a low differential impedance on the bus, typically using 120-ohm resistors at each end. This design, however, means that a dormant CAN bus draws more current compared to other voltage-based signaling buses. The system relies on balanced line operation, where current in one signal line is precisely balanced by an opposing current in the other, providing a stable 0 V reference for receivers. For optimal performance, CAN bus balanced pair signals are best carried in twisted pair wires within a shielded cable, which minimizes RF emissions and reduces susceptibility to interference in the electrically noisy automotive environment.
The electrical implementation described by ISO 11898-2 utilizes a multi-dropped single-ended balanced line configuration with termination resistors. In this setup, a dominant state is asserted when one or more transmitters switch the CAN- line to 0 V and simultaneously switch the CAN+ line to the +5 V bus voltage, creating a current path through the terminating resistors. These resistors are not merely for reflection limitation but are integral to the signaling system. During a recessive state, the signal lines and resistors remain in a high-impedance state, with voltages on both CAN+ and CAN- weakly tending towards a midpoint between the rails. A recessive state is only present when no transmitter is asserting a dominant state. This signaling strategy, which supports only two electrical states conceptually similar to a 'wired AND' network, significantly differs from other balanced line transmission technologies that rely on differential mode voltage crossing a 0 V threshold and often employ three states for multiple access.
