In the fast-evolving world of vehicle electronics—whether commercial road vehicles, off-highway machines, or off-road construction equipment—ensuring functional safety has never been more critical. To meet these growing demands, SAE has introduced two new standards designed specifically for functionally safe communication over CAN networks: SAE J1939-76 for Classical CAN (CAN CC) and SAE J1939-77 for CAN FD (Flexible Data Rate).
The document Functionally-Safe J1939 Communication by Travis Breitkreutz of Caterpillar, recently featured in CAN Newsletter 2/2024, provides a deep dive into these standards, explaining their purpose, structure, and the practical challenges they address.
Purpose of the Document
The main goal of Breitkreutz’s article is to:
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Explain how the SAE J1939-76 and SAE J1939-77 standards enhance the reliability of safety-critical communication.
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Clarify the types of communication errors that must be detected and mitigated to achieve functional safety according to IEC 61784-3.
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Illustrate how the structure of messages and assurance data provides defenses against these errors.
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Describe the evolution of the specifications, noting shortcomings in earlier versions and how ongoing revisions address them.
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Highlight the different “profiles” under J1939-77 to match various system needs.
In short, it’s about making sure that when machine parts talk to each other, they do so safely—even if something goes wrong along the way.
Key Content and Insights
1. Communication Errors Defined by IEC 61784-3
Before jumping into J1939-specific solutions, the document explains the different types of communication errors that can jeopardize safety:
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Corruption (data altered in transmission)
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Unintended repetition (sending the same message again accidentally)
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Incorrect sequence (messages arriving out of order)
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Loss (messages disappearing)
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Unacceptable delay (messages arriving too late)
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Insertion (unexpected messages received)
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Masquerade (messages treated as safe when they aren’t)
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Addressing errors (messages sent to the wrong recipient)
Countermeasures such as sequence numbers, CRCs (Cyclic Redundancy Checks), and connection authentication are essential to detect and correct these problems.
2. J1939-76: Safety Over Classical CAN
The SAE J1939-76 standard uses a Safety Data Group (SDG) method consisting of two parts:
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Safety Header Message (SHM): Contains assurance data like CRC, sequence numbers, and addressing.
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Safety Data Message (SDM): Contains the actual safety-critical application data.
There are two versions:
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SHM1 (2020 version): First iteration but had CRC coverage and sequence number limitations.
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SHM2 (under development): Improved CRC coverage and expanded sequence number fields for better safety assurance.
While robust, both versions double bandwidth usage and are not router-friendly, limiting some applications.
3. J1939-77: Safety Over CAN FD
The SAE J1939-77 standard is more advanced, leveraging CAN FD capabilities and offering three distinct profiles depending on application needs:
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Profile 1: Minimal overhead, fixed payload size (8 bytes), not ideal for routing.
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Profile 2: Variable payload size (0–19 bytes) with system-specific identifiers (DataID) for routing.
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Profile 3: Supports very large payloads (up to 65,526 bytes), uses 64-bit CRCs for ultra-high integrity, and is router-friendly.
Each profile uses an assurance trailer containing CRC, sequence number, and identification data, inserted into the message structure.
4. Strengths and Limitations
The updated standards offer:
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Stronger error detection.
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Support for larger payloads (especially via CAN FD).
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More flexibility across systems, including those that route messages through complex networks.
However, challenges remain:
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Increased bandwidth demands (especially with J1939-76).
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Higher computational complexity (especially with 64-bit CRC in Profile 3).
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The need for system-specific identifiers (no universal DataID registry).
Conclusion: A Step Forward in Functional Safety
Breitkreutz’s analysis highlights that functional safety in automotive and industrial communication systems is no longer a luxury—it’s a necessity. The new SAE J1939-76 and J1939-77 standards represent a robust and thoughtful approach to managing communication faults in safety-critical applications, aligning closely with international standards like IEC 61784-3.
As vehicles and machines become increasingly connected, intelligent, and autonomous, these developments ensure that they also become safer and more reliable, especially in harsh real-world environments.
Reference:
This blog post is based on “Functionally-Safe J1939 Communication,” authored by Travis Breitkreutz (Caterpillar), presented at the 18th International CAN Conference (iCC), and published in CAN Newsletter 2/2024.
Full source: CAN Newsletter 2/2024, CAN in Automation (CiA)
SAE J1939 Starter Kit and Network Simulator
Our JCOM.J1939 Starter Kit and Network Simulator is designed to allow the experienced engineer and the beginner to experiment with SAE J1939 data communication without the need to connect to a real-world J1939 network, i.e., a diesel engine. It may sound obvious, but you need at least two nodes to establish a network. That fact applies especially to CAN/J1939, where the CAN controller shuts down after transmitting data without receiving a response. Therefore, our jCOM.J1939 Starter Kit and Network Simulator consists of two J1939 nodes, namely our jCOM.J1939.USB, an SAE J1939 ECU Simulator Board with USB Port.
The jCOM.J1939.USB gateway board is a high-performance, low-latency vehicle network adapter for SAE J1939 applications. The board supports the full SAE J1939 protocol according to J1939/81 Network Management (Address Claiming) and J1939/21 Transport Protocol (TP).
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