J1939 Introduction

The SAE J1939 specification specifies a differential serial communications bus for inter-connecting ECUs on heavy-duty and commercial vehicles. It does this by standardizes a hardware and software protocol for sending messages.

J1939 is replacing J1587, J1708, and J1922.

J1939 Datalink

J1939 is a half duplex, multi-master, bidirectional network with a network speed of 250K bits per second.

J1939 Physical Layer

Physically J1939 is a twisted pair of wires with differital signaling. J1939 networks use a CAN transceiver which generates a passive signal for a bit value of 1 and an active signal for a bit value of 0. Using passive bits allows for non-destructive bus arbitration.

The logical value transmitted onto the bus is the difference in voltage between the two wires. A logical 1 is when the difference between CAN_H and CAN_L is less than 200mV. A 0 is when the difference is more than 200 mV.

J1939 Message Destination

The majority of messages defined by J1939 are broadcasted, has opposed to destination specific. This means that the data is transmitted on the network without a specific destination. This allows all devices to use the data without having to request the data. When a message is transmitted to a specific address, the address is included in the CAN indentifier.

J1939 Messages

For J1939 message definitions on a J1939 network, see J1939-21, J1939-71, and J1939-73.

J1939 Industry Groups

J1939 Specifications

• SAE J1939 Recommended Practice for a Serial Control and Communications Vehicle Network
• SAE J1939-11 Physical Layer 250K bps
• SAE J1939-13 Off-Board Diagnostic Connector
• SAE J1939-15 Reduced Physical Layer, 250K bps
• SAE J1939-21 Data Link Layer
• SAE J1939-31 Network Layer
• SAE J1939-71 Vehicle Application Layer
• SAE J1939-73 Application Layer - Diagnostics
• SAE J1939-81 Network Management Protocol
• SAE J1939-82 Self Compliance