CAN Wiring - A Practical Reference Guide
CAN Bus Wiring: A Practical Reference Guide Introduction
A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. Its reliability, however, is critically dependent on the physical wiring of the network. This guide covers the essential principles of CAN bus wiring, focusing on the two most critical aspects: network termination and drop cable (dropper) length. Following these rules is essential for a stable and error-free network. Core Network Components • Backbone: The main communication line of the network. It's a single, continuous cable that forms the primary data path. • Terminating Resistor: A 120-Ohm (Ω) resistor placed at each of the two physical ends of the backbone to prevent signal reflection. • Drop Cable (or Dropper/Stub): A shorter cable that connects an individual device (like a controller or sensor) to the main backbone. • T-Connector: A three-way connector used to attach a drop cable to the backbone without cutting or splicing the main line.
Principle 1: Network Termination is Mandatory The single most important rule in CAN bus wiring is termination. Why is it necessary?
A CAN bus transmits high-frequency electronic signals. When these signals reach the end of a wire, they can "bounce" or reflect back down the cable, much like a wave hitting a seawall. This reflected signal interferes with new, incoming signals, corrupting the data and causing communication errors. A terminating resistor absorbs the signal energy at the end of the line, preventing these reflections. The Rule of Two • A CAN bus network must have exactly two 120Ω terminating resistors. • One resistor must be placed at each physical end of the backbone. • Having zero, one, or more than two resistors will cause network failure. Correct Termination: [120Ω]----(Device)----(Device)----(Device)----[120Ω]
Incorrect Termination Examples: WRONG -> No Terminators: (Device)----(Device)----(Device)
WRONG -> One Terminator: [120Ω]----(Device)----(Device)----(Device)
WRONG -> In the Middle (Not at the ends): (Device)----[120Ω]----(Device)----(Device)----(Device)
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