Can Identifiers And Message Priority - National Instruments NI-CAN User Manual

Chapter 1 Introduction

CAN Identifiers and Message Priority

NI-CAN User Manual
industrial device networking protocols, including DeviceNet, CANopen,
and Smart Distributed System (SDS).
With its growing popularity in automotive and industrial applications, CAN
has been increasingly used in a wide variety of diverse applications. Usage
in systems such as agricultural equipment, nautical machinery, medical
apparatus, semiconductor manufacturing equipment, and machine tools
testify to the incredible versatility of CAN.
When a CAN device transmits data onto the network, an identifier that is
unique throughout the network precedes the data. The identifier defines not
only the content of the data, but also the priority. A CAN identifier, along
with its associated data, is often referred to as a CAN Object.
When a device transmits a message onto the CAN network, all other
devices on the network receive that message. Each receiving device
performs an acceptance test on the identifier to determine if the message is
relevant to it. If the received identifier is not relevant to the device (such as
RPM received by an air conditioning controller), the device ignores the
message.
When more than one CAN device transmits a message simultaneously, the
identifier is used as a priority to determine which device gains access to the
network. The lower the numerical value of the identifier, the higher its
priority.
Figure 1-1 shows two CAN devices attempting to transmit messages, one
using identifier 647 hex, and the other using identifier 6FF hex. As each
device transmits the 11 bits of its identifier, it examines the network to
determine if a higher-priority identifier is being transmitted simultaneously.
If an identifier collision is detected, the losing device(s) immediately cease
transmission, and wait for the higher-priority message to complete before
automatically retrying. Because the highest priority identifier continues its
transmission without interruption, this scheme is referred to as
nondestructive bitwise arbitration, and CAN's identifier is often referred to
as an arbitration ID. This ability to resolve collisions and continue with
high-priority transmissions is one feature that makes CAN ideal for
real-time applications.
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