Introduction to the Controller Area Network (CAN)

Introduction to the Controller Area Network (CAN)

Trademarks

CANopen is a trademark of CAN in Automation.

DeviceNet is a trademark of Open DeviceNet Vendor Association, Inc.

1 Introduction

The CAN bus was developed by BOSCH ⁽¹⁾ as a multi-master, message broadcast system that specifies a maximum signaling rate of 1 megabit per second (bps). Unlike a traditional network such as USB or Ethernet, CAN does not send large blocks of data point-to-point from node A to node B under the supervision of a central bus master. In a CAN network, many short messages like temperature or RPM are broadcast to the entire network, which provides for data consistency in every node of the system.

Once CAN basics such as message format, message identifiers, and bit-wise arbitration -- a major benefit of the CAN signaling scheme are explained, a CAN bus implementation is examined, typical waveforms presented, and transceiver features examined.

2 The CAN Standard

CAN is an International Standardization Organization (ISO) defined serial communications bus originally developed for the automotive industry to replace the complex wiring harness with a two-wire bus. The specification calls for high immunity to electrical interference and the ability to self-diagnose and repair data errors. These features have led to CAN’s popularity in a variety of industries including building automation, medical, and manufacturing.

The CAN communications protocol, ISO-11898: 2003, describes how information is passed between devices on a network and conforms to the Open Systems Interconnection (OSI) model that is defined in terms of layers. Actual communication between devices connected by the physical medium is defined by the physical layer of the model. The ISO 11898 architecture defines the lowest two layers of the seven layer OSI/ISO model as the data-link layer and physical layer in Figure 1.

Figure 1. The Layered ISO 11898 Standard Architecture

In Figure 1, the application layer establishes the communication link to an upper-level application specific protocol such as the vendor-independent CANopen™ protocol. This protocol is supported by the international users and manufacturers group, CAN in Automation (CiA). Additional CAN information is located at the CiA Web site, can-cia.de. Many protocols are dedicated to particular applications like industrial automation, diesel engines, or aviation. Other examples of industry-standard, CAN-based protocols are KVASER's CAN Kingdom and Rockwell Automation's DeviceNet™.

3 Standard CAN or Extended CAN

The CAN communication protocol is a carrier-sense, multiple-access protocol with collision detection and arbitration on message priority (CSMA/CD+AMP). CSMA means that each node on a bus must wait for a prescribed period of inactivity before attempting to send a message. CD+AMP means that collisions are resolved through a bit-wise arbitration, based on a preprogrammed priority of each message in the identifier field of a message. The higher priority identifier always wins bus access. That is, the last logic-high in the identifier keeps on transmitting because it is the highest priority. Since every node on a bus takes part in writing every bit "as it is being written," an arbitrating node knows if it placed the logic-high bit on the bus.

The ISO-11898:2003 Standard, with the standard 11-bit identifier, provides for signaling rates from 125 kbps to 1 Mbps. The standard was later amended with the “extended” 29-bit identifier. The standard 11-bit identifier field in Figure 2 provides for 2¹¹, or 2048 different message identifiers, whereas the extended 29-bit identifier in Figure 3 provides for 2²⁹, or 537 million identifiers.