CHAPTER 5
CAN MESSAGING
5.1 CAN ADDRESS CLAIMING
5.1.1 Prerequisite Knowledge
It is assumed that the reader has some level of familiarity with CAN networking and terminology, and the AB product.
This information is not intended to be a tutorial on CAN, nor AB product operation, but rather a resource for information
concerning the requirements and techniques involved in implementing a CAN control interface to the AB product.
5.1.2 CAN Physical Layer
AB CAN physical layer requirements are the same as the SAE J1939 Standard:
• CAN 2.0B devices
• 29 bit CAN message headers
• 250K bits per second
• Proper bus wiring, terminations, etc.
5.1.3 Address Claiming
Rather than using a pre-assigned CAN node address, AB devices use the J1939 method of address claiming, detailed in ‘SAE
J1939-81 Network Management’. Although the integrator must be aware of this, it is not necessary for them to understand or
use this technique. Integrator considerations regarding this are summarized in Appendix A.
5.1.4 General Message Characteristics
To allow AB devices to cooperatively communicate on a vehicle bus shared with other message traffic, all AB messages utilize
the point-to-point and broadcast ranges of message IDs set aside in ‘SAE J1939-71 Vehicle Application Layer’ for vendors to use
to transmit data not otherwise defined in the J1939 specification. The messages are structured as follows:
Header (29 bits) 2 22 11 00 0
8...43...... 65.....87 ......0
[A]
0011101111 [ B ] [ C ]
[A] = 3 bits, Priority (7 = 111 = lowest, 0 = 000 = highest)
2 bits, 00
8 bits, 0xEF
[B] = 8 bits, Target node, or 0xFF for Broadcast
[C] = 8 bits, Source node
Note that the Priority field is used only for bus arbitration. All messages are
parsed based on the low order 26 bits of the header.
Data length – 1 to 8 bytes
Data byte 0 – Message type
Data bytes [1-7] – Data for message type (0 to 7 bytes)
No logical messages are spanned across multiple physical CAN messages.
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