computer science

SAE J1939

SAE J1939 is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand SAE J1939 rather than just read about it. In short: Society of Automotive Engineers standard SAE J1939 is the vehicle bus recommended practice used for communication and diagnostics among vehicle components. Originating in the car and heavy-duty truck industry in the United States, it is now widely used in other parts of the world.

Key takeaways

SAE J1939 belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SAE J1939 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SAE J1939 from memory before moving on to harder problems.

Reference excerpt

Society of Automotive Engineers standard SAE J1939 is the vehicle bus recommended practice used for communication and diagnostics among vehicle components. Originating in the car and heavy-duty truck industry in the United States, it is now widely used in other parts of the world. SAE J1939 is used in the commercial vehicle area for connection and communication throughout the vehicle, with the physical layer defined in ISO 11898. A different physical layer is used between the tractor and trailer, specified in ISO 11992.

History Originally, Controller Area Network (CAN) was not mentioned in J1939, which covered cars and tractor-trailer rigs, and with some dual and triple use 8-bit addresses assigned by the SAE J1939 board. CAN was not originally free, but its instruction set did fit in the custom instruction format of J1939. This was true as of 2000. Since then, CAN has been included, the chipset for J1939 has been clocked faster, and 16-bit addresses (PGN) have replaced 8-bit addresses. J1939, ISO 11783 and NMEA 2000 all share the same high level protocol. SAE J1939 can be considered the replacement for the older SAE J1708 and SAE J1587 specifications.

Definition SAE J1939 defines five layers in the seven-layer OSI network model, and this includes the Controller Area Network (CAN) ISO 11898 specification (using only the 29-bit/"extended" identifier) for the physical and data-link layers. Under J1939/11 and J1939/15, the data rate is specified as 250 kbit/s, with J1939/14 specifying 500 kbit/s. The session and presentation layers are not part of the specification. The later use of CAN FD is currently discussed. All J1939 packets, except for the request packet, contain eight bytes of data and a standard header which contains an index called Parameter Group Number (PGN), which is embedded in the message's 29-bit identifier. A PGN identifies a message's function and associated data. J1939 attempts to define standard PGNs to encompass a wide range of automotive, agricultural, marine and off-road vehicle purposes. A range of PGNs (00FF0016 through 00FFFF16, inclusive) is reserved for proprietary use. PGNs define the data which is made up of a variable number of Suspect Parameter Number (SPN) elements defined for unique data. For example, there exists a predefined SPN for engine RPM.

Applications SAE J1939 has been adopted widely by diesel engine manufacturers. One driving force behind this is the increasing adoption of the engine Electronic Control Unit (ECU), which provides one method of controlling exhaust gas emissions within United States and European standards. The telematics data is used (via Fleet Telematics Systems) by Fleet Management Systems to let fleet operators keep track of vehicle and driver performance, such as fuel consumption, safety events, and repair time. Because of the demanding standards, engine development is expensive, and truck-based engines are reused in many fields. Consequently, SAE J1939 can now be found in a range of diesel-powered applications: vehicles (on- and off-road and rail), marine propulsion, power generation and industrial pumping.

See also Fleet Management System CANopen CAN FD Flexible Data Rate

References

External links SAE J1939 Standards Collection SAE J1939 Top Level Document J1939 Introduction (Kvaser) Introduction to J1939 (Vector Informatik) A Comprehensible Guide to J1939 (Book) Detailed description of all SPN + FMI combinations. SAE J1939 Protocol Explained - A Simple Intro

Worked examples

Example 1 — a first encounter with SAE J1939

Start with the simplest possible case. Write down what SAE J1939 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SAE J1939 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SAE J1939 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SAE J1939

In research: SAE J1939 appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SAE J1939 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SAE J1939 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Automotive software, Automotive standards, Computer buses, so understanding it makes those chapters shorter.
In everyday life: Look for SAE J1939 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study SAE J1939 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SAE J1939 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SAE J1939 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SAE J1939 in simple terms?

Why does SAE J1939 matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SAE J1939?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SAE J1939.