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									           Vehicle Area Networks(VANs)

                           Mark Grady

CSE 237 Spring 2004
                      What is a VAN?
   • Vehicle Area Networks provide communications
     standards for microprocessors embedded into vehicle
   • VANs were initially associated with heavy trucking,
     then transit buses, but have become standard in
     consumer vehicles as well
   • The predominant standard in the U.S., SAE-J1708
     was adopted in 1986.
   • The predominant standard in Europe, SAE-J1939 was
     adopted in 1994.

CSE 237 Spring 2004
           Why VANs? (or Why Trucks?)

   • Vehicle value and lifetime.
   • Demand for frequent, rapid, and accurate diagnostics
     and maintenance (think truckstop).
   • Heavy trucks and buses frequently utilize “after-
     market” off-the-shelf components (eg radar collision
   • Accident analysis (black boxes)

CSE 237 Spring 2004
   • J1708 defines the hardware
         –   Modification of RS-485
         –   Terminated balanced-line transmission
         –   Defines 10 bit symbols (1 start, 8 data, 1 stop)
         –   Defines 21 byte message (1 MID, 19 data, 1 CS)
         –   Collision detection and arbitration based on priority
         –   Allows 20 device connections
   • J1587 defines the communications protocol
         – Defines specific messages passed over J1708
         – PID (Parameter ID) assigned to each message
         – Provides connection management (acks, flow control)

CSE 237 Spring 2004
   • Designed to provide drive train synchronization.
   • These were NOT built upon the OSI model.
   • Messages are broadcast and devices listen for specific
   • Because of collisions, network does not operate at
     peak efficiency.

CSE 237 Spring 2004
   • CAN = Controller Area Network
         – Proprietary protocol developed by Bosch
         – Not a well-defined standard
         – Primarily concerned with Data Link Layer
   • J1939 is SAE standard based in part on CAN 2.0B
         – Multipart standard based on OSI model (SAE1939/XY
           where X=OSI layer)
         – Provides all of the functionality of J1708/J1587
         – Faster than previous standards for real time data
         – Allows for generic messaging, incl. proprietary msgs.
         – Non-destructive bit-wise message arbitration

CSE 237 Spring 2004
                      OSI Mapping

CSE 237 Spring 2004
    J1708                           J1939
   •   9600 bps                     • 250 kbit (1 Mbit for CAN)
   •   Predefined messages          • Generic messaging
   •   Broadcast messaging          • Node/group/global addressing
   •   20 devices                   • 30 devices
   •   Crash and wait arbitration   • Bitwise arbitration
   •   Used in U.S. for             • Used in U.S. for DriveTrain
       Information Layer              Layer
                                    • Used in Europe for all VAN
                                    • High speed/Fault tolerant

CSE 237 Spring 2004
             “Every bus delivered to U.S. transit
             agencies today uses SAE data
             networks for drivetrain integration.”

                           JOHN J. SCHIAVONE
                           “Understanding and Applying Advanced On-
                           Board Bus Electronics” TCRP Report 43

CSE 237 Spring 2004

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