I have long been a vocal critic of the barely readable standards issued by SAE International. And to be fair, SAE is hardly alone—standards bodies, as a species, seem united by a peculiar compulsion: the need to prove their intellectual worth by rendering their documents as impenetrable as possible. Clarity, apparently, is for amateurs. Precision, they argue—while quietly burying it under layers of linguistic fog—is for professionals.
Someone really ought to inform them that it is, in fact, permissible to write documentation that can be understood without a doctorate in cryptography. Technical rigor and readability are not mutually exclusive. In fact, in most industries, they’re considered complementary virtues. But not here. Here, opacity is a feature.
That said, I owe a small debt of gratitude. The collective inability of standards organizations to communicate clearly has been good for business. It helped me sell a fair number of copies of my book on SAE J1939. I’ll freely admit—my writing at the time was not exactly literary brilliance. It was straightforward, sometimes blunt, and yes, perhaps “simple-minded.” But as it turns out, there is a rather large population of equally “simple-minded” engineers who prefer understanding over admiration. They seemed to appreciate a version of the material that didn’t require a translator.
Which brings me to the next act. I’m seriously considering a second edition—this time with a bit of stylistic polish and expanded coverage of SAE J1939-22. A bold choice, given that J1939-22 appears to have taken the already challenging prose of its predecessors and elevated it to an entirely new level of unreadability. It’s almost impressive, in a perverse sort of way.
One begins to suspect that the goal is not to explain the system, but to ensure that only the most persistent—or masochistic—readers make it to the end.
So, with that in mind, let me quote one representative line from the SAE J1939-22 standard under paragraph “Operating Principles/Overview”—an experience that, I assure you, is less about reading and more about survival:
“CEFF data frames, per SAE J1939-21, shall not be transmitted on an SAE J1939-22 network, except for the following two exceptions. The first exception allows the Address Claimed PG to be sent as a single CEFF data frame, per SAE J1939-21. The second exception allows a device to transmit CEFF data frames (per SAE J1939-21) only to determine if the network is an SAE J1939-22 network; CEFF frames shall be ceased once a device determines it is connected to an SAE J1939-22 network.”
I may be wrong—but to me, it makes about as much sense to explain how a car works by dissecting the intricacies of fuel injection as it does to explain a symphony by analyzing the metallurgy of the violin strings.
By the way, “CEFF” is not some mysterious hexadecimal PGN you’re expected to decode through sheer force of will. It simply stands for: C = CAN, EFF = Extended Frame Format (29-bit identifier). That’s it. No hidden meaning, no secret handshake—just a label.
Of course, this being the world of SAE International, we’re treated to the usual fondness for abbreviations—preferably stacked on top of other abbreviations, just to ensure maximum cognitive friction (for instance: C = CAN = Controller Area Network). One sometimes gets the impression that clarity was considered, briefly, and then dismissed as unnecessarily user-friendly.
And here lies the real irony: if your goal is to promote a technology—say, CAN FD within the J1939 ecosystem—then producing documentation that actively discourages comprehension is a curious strategy. Adoption thrives on clarity. Confusion does not inspire confidence; it inspires avoidance.
So, congratulations to SAE for managing to undermine their own message with such consistency and flair. It takes a certain kind of dedication to miss the mark so spectacularly.
In the meantime, I’ve gathered material on J1939-22 from various sources. Unsurprisingly, much of it mirrors the style of the original standard—dense, tangled, and determinedly unhelpful. It will take some time to decode it into something resembling human language. But that’s precisely the plan: to translate the standard into a form that engineers can actually use, rather than merely admire from a safe distance.
Now, all that being said, it’s time for me to do what SAE International apparently chose not to: sit down, wade through the standard with the occasional head shake (and perhaps a quiet sigh), and turn it into something resembling a comprehensible guide to SAE J1939-22.
A Comprehensible Guide to J1939
SAE J1939 is an industry standard widely used in off-highway equipment—construction, material handling, agriculture, and forestry, to name the usual suspects. At its core, it is a communication protocol built on the Controller Area Network, enabling Electronic Control Units (ECUs) in heavy-duty vehicles to exchange data reliably and in real time.
That data can range from the obvious—vehicle speed, torque control, oil and coolant temperatures—to the more nuanced signals that keep modern machines operating efficiently and, ideally, without drama.
The material presented in this book is primarily based on two key documents from the SAE J1939 Standards Collection:
- J1939/21 – Data Link Layer
- J1939/81 – Network Management
Now, here’s where things take a more reader-friendly turn. A Comprehensible Guide to J1939 was written with a very deliberate goal: to explain the J1939 message structure and network management without requiring the reader to develop a sudden appreciation for dense, bureaucratic prose. In other words, it attempts to do what the original documents often do not—communicate clearly.
This book is intended as an accessible and, dare one say, enjoyable reference. It draws on publicly available sources—websites, printed literature—and the practical insights of engineers who have spent more time working with CAN and J1939 than deciphering them. While the content is grounded in the J1939 standard, it does not reproduce any copyrighted material from SAE publications.
To be clear, this book is not a replacement for the full SAE J1939 Standards Collection. Nor does it attempt to compete with the sheer volume of documentation found in standards such as J1939 or J1939/71, which together span roughly 1,600 pages of data definitions alone. Those exhaustive reference tables are not included here—partly out of necessity, and partly out of mercy.
The purpose of this book is far simpler: to provide a guide to the standard that a human being can actually read, understand, and—most importantly—use. More information…
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