Analysis of J1939 Cable Transmission Protocol

Analysis of J1939 Transmission Protocol

The J1939 protocol is based on the CAN 2.0B technical specification, which not only defines the electrical and mechanical characteristics of the physical layer, but also covers detailed specifications of the data link layer, network layer, and application layer.

Physical layer:

Defined the physical characteristics of the cable, such as characteristic impedance (usually 120 Ω), transmission rate (250kbps), terminal resistance (120 Ω), etc.

Using shielded twisted pair cables as the transmission medium, the maximum transmission line length is 40 meters, and up to 30 ECUs can be connected within the same network.

Data link layer:

Responsible for providing reliable data transmission between physical connections, including synchronization, sequential control, fault-tolerant control, and flow control.

The concept of Protocol Data Unit (PDU) has been introduced, which consists of priority, reserved bits, data pages, PDU format, PDU specific fields, source address, and data fields.

Supports multiple types of messages, such as command messages, request messages, broadcast/response messages, confirmation messages, and group function messages.

Network layer:

This mainly describes the bridge function for message transmission between two network parts, which is related to the implementation of J1939 bridge.

Application layer:

Described the actual data content, including parameter group number (PGN), suspicious parameter number (SPN), etc.

The application layer defines the specific meaning and purpose of the message, enabling ECUs to understand and respond to the data sent to each other.

Multi frame transmission mechanism

For messages longer than 8 bytes, the J1939 protocol adopts a multi frame transmission mechanism. This mechanism splits long messages into multiple small packets, each containing a sequence number, so that the receiver can reassemble them into the original information. The multi frame transmission mechanism supports two transmission modes based on connection mode and broadcast mode, ensuring the reliability and flexibility of large data transmission.