J1939

The network protocol SAE J1939 describes the communication on a CAN bus in utility vehicles for the transmission of diagnostic data (eg engine speed, temperature) and control information.

Area of ​​application

The protocol has been defined by the International Society of Automotive Engineers (SAE ) and works on the Physical Layer with CAN High Speed ​​according to ISO 11898. The application focus is in the range of powertrain and chassis of commercial vehicles. To use the protocol is used in heavy vehicles for both on-street and off-road driving (construction machinery). Related to J1939 are the ISOBUS to ISO 11783 for farm machinery, NMEA 2000 for maritime use, and the Truck & Trailer Interface according to ISO 11992nd Similarly, the FMS standard is based on J1939 communication.

Plug pin

J1939 pinout

Protocol Description

J1939 is a multi-master system with decentralized network management without channel-based communication. It supports up to 254 logical nodes and 30 physical control devices per segment. The information is described as parameters (signals ) and four memory pages (Data Page ) in parameter groups (PGs ) are combined. Each parameter group can be identified by a unique number, the Parameter Group Number (PGN ), are identified. Regardless of each signal a unique SPN ( Suspect Parameter Number) is assigned.

Data transmission

The vast majority of communication occurs cyclically and can be received by all control devices without the explicit request for data (broadcast). Moreover, the parameter sets are optimized to have a length of 8 data bytes. This allows a very efficient use of the CAN protocol. Certain information such as Configuration data or diagnostic data can be exchanged only between the two control devices ( peer to peer). The definition of communication, broadcast or peer-to -peer, is a property of the parameter set used. This depends on the parameter group in addition to the definition of which parameters are transmitted, and the transmission from.

Transport protocol

If large amounts of data need to be transmitted, transport protocols (TP ) for use:

  • BAM ( Broadcast Announce Message) and
  • CMDT ( Connection Mode Data Transfer).

With BAM TP to transfer the data via broadcast takes place at all nodes. Between the transmitter and receiver, there is no data flow control ( handshake). When CMDT TP data between exactly two control units to be replaced. The case held the control data flow in case of error allows a restarting of communication without a complete repetition of the data transmission. In addition, the CMDT TP allows a confirmation of receipt of the data by the recipient.

Distributed Network Management

Thus, on a CAN network that peer- to-peer communication is possible, each control unit must be assigned a unique address in the range 0-253. To avoid that, during operation incorrectly occur two control devices with the same address, it requires a sophisticated strategy - the Network Management (NM ). J1939 NM is decentralized. That each controller must implement a minimum functionality of the NM. The tasks of the NM are:

  • Resolving address conflicts ( minimum requirement)
  • Ongoing testing whether control device addresses are assigned twice in a network ( minimum requirement)
  • Change of the control device addresses at runtime
  • Unique identification of a controller using a globally unique name
  • This name is also used to detect which function has a control device on the network

The standard is divided (documents ) in the following layers:

Fleet Management System J1708 Broadcasting (networking)
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