396914861238030599415561924AN770_0| Application Note
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AN-770 APPLICATION NOTE
iCoupler Isolation in CAN Bus Applications
by Ronn Kliger and Sean Clark
INTRODUCTION The Controller Area Network (CAN) bus, a robust protocol designed for industrial applications, was originally developed for use in cars. It specifies a maximum cable length of 40 meters and up to 30 nodes. The flexibility and advantages of this specification have resulted in increased use in a wide range of applications. Because the CAN bus system is typically used to connect multiple systems and is often run over very long distances, isolation between the bus and each system connection is critical. Isolation provides protection from overvoltage transients between the CAN bus cable network and the systems connected to it. Isolation also eliminates ground loops in the network, reduces signal distortion and errors, and provides protection from voltage/ground mismatches. The intention of this application note is to give the user a brief overview of the CAN bus protocol, focusing on the system physical layer, as well as an understanding of why isolation is so important to the system. This applicatio
n note also details how to implement isolation in a CAN bus system using Analog Devices' iCoupler products. CAN BUS OVERVIEW The CAN Bus Protocol The CAN bus protocol standard is defined by the International Standardization Organization (ISO) as a serial communications 2-wire bus, with data rates up to 1 Mbps. It uses two layers: a differential signal physical layer, specified as ISO 11898, which provides excellent noise immunity, and a data link layer, which defines how the signals interact and communicate. The Data Frame The CAN bus protocol uses asynchronous data transmission design. The transmitted data is sent in a data frame, which is controlled by start and stop bits at the beginning and end of each transmission. The data frame is composed of an arbitration field, a control field, a data field, a cyclic redundancy check field, and an acknowledge field. The frame begins with a start-of-frame dominant bit, and completes with an end-of-frame field (bit), as shown in Figure 1.
SOF
ARBITRATION CONTROL
DATA
CRC
ACK
EOF
Figure 1. CAN Bus Data Transmission Frame CAN Bus Arbitration The CAN bus protocol also specifies nondestructive bit arbitration, which ensures that no data is lost. It is one of the protocol's most important features. The CAN bus protocol defines the digital logic states on the bus with a logic high as the recessive state and a logic low as the dominant state. It is designed to allow every node to listen and transmit at the same time. All nodes transmit a single dominant start of message (SOM) bit at the beginning of each message. Other nodes will see bus activity and will not attempt to start a transmission until the message packet is complete. After the SOM bit, the arbitration field is transmitted. The arbitration field is 11 or 29 bits long, depending on which variation of the CAN bus protocol is used. The highest priority message has an arbitration field of the highest number of dominant bits; it will transmit a dominant bit first, while the other nodes are transmitting recessive bits. Also known as th
e identifier, the arbitration field prioritizes the messages on the bus. By the time the arbitration field has been sent, all nodes except the highest priority node will have stopped transmitting. If multiple nodes start transmitting at the same time, the node transmitting the highest number of dominant bits always takes control of the bus. All nodes monitor the bus and stop transmitting when a higher priority transmission is recognized. The other nodes attempt to transmit again after the message is completed. In this second attempt, the next highest value arbitration field will take control of the bus, and the arbitration process is repeated. The nondestructive bus arbitration ensures that the highest priority message always gets through.
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