J1939 References...
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Tech Brief: J1939 Network Testing
The goal of the Federal Motor Carrier
Safety Administration (FMCSA) is to reduce the large truck fatality rate
by 41 percent from 1996 to 2008. This reduction translates into a rate
of 1.65 fatalities in truck crashes per 100 million miles of truck
travel. The Office of Bus and Truck Standards and Operations, which
produced this Tech Brief, develops and promotes national motor carrier
safety program goals, priorities, and initiatives. It provides technical
expertise and advice in the development and deployment of motor carrier
safety programs, including the development of regulations. The Office
determines national motor carrier safety operational program
requirements, standards, and procedures for vehicle and roadside
operations, driver and carrier operations, and physical qualifications
of truck and bus drivers.
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J1939 C Library for CAN-Enabled PICmicro® Microcontrollers
J1939 is a series of SAE recommended practices that have been developed to provide a standard architecture by which multiple electronic systems on a vehicle can communicate. It was developed by the Truck and Bus Control and Communications Network Subcommittee of the Truck and Bus Electrical and Electronics Committee, but its use is not limited to truck and bus applications. J1939 has been implemented in a broad range of vehicles and transportation systems. J1939 provides a communication protocol over a CAN network. The CAN network is comprised of two or more interconnected Electronic Control Units (ECUs). As per the SAE J1939-11 specification. the ECUs are connected using linear shielded twisted pair wiring, with a data rate of 250 Kbits/second.
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AUTOMOTIVE CAN BUS
The cost, weight and complexity of a
vehicle's wiring has increased dramatically. The traditional method of
providing a separate wire for each function was adequate when electrical
systems were relatively simple. But the introduction of electronic
engine management, active safety, navigation aids and luxury equipment
have demanded a more sophisticated approach. A computer-controlled
network for vehicles is now being implemented, in principle like the
network between PCs in an office, but vastly more secure, responsive to
real-time events and immune to external electromagnetic interference.
The Society of Automotive Engineers Inc. defines recommended practices
for vehicles. The purpose of their set of seven J1939 documents is to
provide an open interconnect system for electronic systems that allows
devices to communicate with each other by providing a standard
architecture. The J1939 specification is an implementation of a
Controller Area Network (CAN bus) which permits any device to transmit a
message when the bus is idle. Every message includes an identifier
defining the message priority, who sent it, and what data is contained
within it. Collisions are avoided due to the arbitration process that
occurs while the identifier is transmitted (using a non-destructive
arbitration scheme). This permits high priority messages to get through
with low latency (delay) times because there is equal access on the
network for any device. In particular the J1939 specification includes a
high level protocol, network speed, types of connector etc. The bus
length can be up to 40 metres (unless repeated) with one metre stubs. It
runs at 250k bits per second and up to 30 nodes may be connected.
Introduction to J1939
J1939 is used in the commercial vehicle
area for communication in the engine compartment and between the tractor
and trailer. In this application note, the properties of J1939 should be
described in brief. Parameter groups combine similar or associated
signals. In the specification SAE J1939-71 the parameter groups are
defined with the signals they contain. In addition, some
manufacturer-specific parameter groups can be used. Parameter groups
with up to 8 data bytes are transmitted in a CAN message. With more than
8 bytes, a transport protocol is used.
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J1939, CANopen and the future for CAN
In February 1986, Robert Bosch introduced
the CAN serial bus system at the SAE congress in Detroit. It was
designed to handle short messages, support multi-master access, and
offer a high degree of reliability. In mid-1987, Intel delivered the
first CAN chip, and today more than 20 chip manufacturers produce
devices that offer CAN interfaces. CAN quickly gained a dominant
position among bus protocols. In 1999 close to 60 million CAN
controllers made their way into applications; more than 100 million CAN
devices were sold in the year 2000. A big advantage of the CAN bus
compared with other network solutions is the price/performance ratio.
Price-wise, CAN is the most affordable network next to a regular serial
channel.
CiA
establishes J1939 Interest Group
CAN in Automation (CiA) members have
installed a technical working group for J1939-based specifications. The
work-plan 2006 of the IG includes the publication of CiA 501
(J1939-based communication profile for recreational vehicles) and CiA
502 (J1939-based application profile for recreational vehicles)
specifications. The IG chaired by Edward Heck (HED) is going to define
the content of the CANopen PGNs (parameter group number). These CAM11
and CAM21 (CANopen application messages) will be used to remotely
control CANopen device in a sub-layered network that is accessible via
gateway devices. This tunneling mechanism can even be used if the
CANopen application is trailer-mounted. In this case the CANopen service
request is transmitted via the J1939-based power-train network and the
ISO 11992-compatible truck/trailer point-to-point network, which is also
CAN-based.
