Note: This is a practical intro so you will also learn how to decode J data via DBC files, how J logging works, key use cases and practical tips. However, CAN bus only provides a "basis" for communication like a telephone - not a "language" for conversation. In simple terms, J offers a standardized method for communication across ECUs, or in other words:. In contrast, e. Heavy-duty vehicles e.
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Note: This is a practical intro so you will also learn how to decode J data via DBC files, how J logging works, key use cases and practical tips. However, CAN bus only provides a "basis" for communication like a telephone - not a "language" for conversation. In simple terms, J offers a standardized method for communication across ECUs, or in other words:.
In contrast, e. Heavy-duty vehicles e. However, several other key industries leverage SAE J today either directly or via derived standards e. The standardization is a key enabler to data logging use cases across heavy-duty vehicles - more on this further below.
With the rise of heavy-duty telematics , J will increasingly play a role in the market for connected vehicles. In turn, this will increase the need for secure J IoT loggers. The J bit rate is typically K though recently with support for K - and the identifier is extended bit CAN 2. Multibyte variables are sent least significant byte first Intel byte order. PGNs with up to bytes are supported via transport protocol. J is based on CAN, which provides the basic " physical layer " and " data link layer ", the lowest layers in the OSI model.
Here, J serves as a higher layer protocol on top, enabling more complex communication. A higher layer protocol enables communication across the large complex networks of e. Similarly, it specifies how data is to be converted into human-readable data. It does so by providing a family of standards. For example, J is a document detailing the information required to convert a large set of cross-manufacturer standardized J messages into human-readable data more on this below.
Many other CAN based higher layer protocols exist, e. These typically offer some level of standardization within their respective industries - though all of them can be extended by manufacturers. In comparison, the aforementioned passenger cars have unique standards per manufacturer.
In other words, you can use the same J database file to convert e. In simple terms, the PGN serves as a unique frame identifier within the J standard. Here, the PGN starts at bit 9, with length 18 indexed from 1. The resulting PGN is 0F or in decimal Further, the document will have details on the PGN including priority, transmission rate and a list of the associated SPNs - cf.
Engine Speed, RPM , each of which can be looked up in the J documentation for further details. The above is a simplified illustration as the J bit identifier can be broken down further. This is quite educational and also useful if you're interested in J DBC files. Learn more. SPNs are grouped by PGNs and can be described in terms of their bit start position, bit length, scale, offset and unit - information required to extract and scale the SPN data to physical values.
Using these details, it is possible to extract the Engine Speed physical value data e. To do so, note from the PGN info that the relevant data is in bytes 4 and 5, i. Taking the decimal form of the HEX value Intel byte order , we get in decimal. To arrive at the RPM, we conduct a scaling of this value using the offset 0 and the scale 0.
Note how some data bytes in the above are FF or decimal, i. A J DBC file can be used to decode data across most heavy-duty vehicles. Below we illustrate what real J data looks like. The 'raw' J data was recorded from a heavy duty truck using a CANedge2 , while the 'physical values' reflect the output after decoding the raw data via the free asammdf software and the J DBC.
Below is a CSV version of the raw J frames. The sample data also includes a demo J DBC so that you can replicate the conversion steps via asammdf. Once the raw J data is decoded and exported, the result is timeseries data with parameters like oil temperature, engine speed, GPS, fuel rate and speed:. For more on logging J data, see our J data logger and mining telematics articles. The J protocol supports various advanced operations like requests, multi-packet messages and multiplexing:.
Most J messages are broadcast to the CAN bus, but some need to be requested e. It has priority 6, a variable transmit rate and can either be sent as a global or specific address request. The data bytes should contain the requested PGN Intel byte order. Examples of requested J messages include the diagnostic messages DM. As for OBD2, you can use our the transmit list of e. However, two other possible sizes exist: The 3 bytes request message - and variable size messages.
The latter enables communication of data packets beyond the usual 8 bytes limit of the CAN bus format. Such messages are referred to as J multi-frame or multi-packet messages. The J protocol specifies how to deconstruct, transfer and reassemble the packets - a process referred to as the Transport Protocol cf. Two types exist:. It is then followed by up to packets of data. Each of the packets use the first data byte to specify the sequence number 1 up to , followed by 7 bytes of data.
The final packet will contain at least one byte of data, followed by unused bytes set to FF. In the BAM type scenario, the time between messages is ms. Finally, a conversion software can reassemble the multiple entries of 7 data bytes into a single string and handle it according to the multi-packet PGN and SPN specifications.
J data from trucks, buses, tractors etc. By streaming decoded J data to a PC, technicians can analyze and diagnose vehicle issues in real-time. A CAN logger can serve as a 'blackbox' for heavy-duty vehicles, providing data for e. Many of our end users work with J logging in the field - and below we share 6 practical logging tips:.
Standalone J data loggers with SD cards are ideal for logging data from e. A WiFi J logger also enables telematics use cases. This is e. To connect your CAN analyzer to a J asset e. We offer a DB9-J adapter which fits the 9-pin deutsch connector found in many heavy duty vehicles. This method uses induction to record data silently without cutting any CAN wiring. The benefit to this is that you'll have continuous access to the device - unless it is out-of-coverage.
However, in cases where data only needs to be periodically uploaded an alternative can be to upload data via WLAN routers when the vehicles visit e. When logging or streaming J data, software for post processing is key. In particular, the software should support DBC-based J conversion to allow easy conversion to human-readable data.
Note that this is not possible in "silent mode" i. To optimize your J data logging, a number of advanced configurations can be helpful. In particular, the CANedge advanced filters and sampling rate options help optimize the amount of data logged - key for e. Other options include silent mode and cyclical logging, with the latter enabling the logger to always prioritize the latest data useful in e. Since J is standardized, it is critical to encrypt your data 'at rest' e.
Not doing so exposes your data processing to various security risks, incl. For details on securing your J data logging, see our intro to secure CAN logging.
What is J? What does that mean, though? One standard across heavy-duty vehicles In simple terms, J offers a standardized method for communication across ECUs, or in other words: J provides a common language across manufacturers.
J application examples Heavy-duty vehicles e. J future With the rise of heavy-duty telematics , J will increasingly play a role in the market for connected vehicles. The physical value aka scaled engineering value is RPM. CSV file not loaded correctly? Heavy duty fleet telematics J data from trucks, buses, tractors etc. Live stream diagnostics By streaming decoded J data to a PC, technicians can analyze and diagnose vehicle issues in real-time Learn more.
Heavy-duty vehicle blackbox A CAN logger can serve as a 'blackbox' for heavy-duty vehicles, providing data for e. Do you have a J data logging use case? Reach out for free sparring! Contact us.
A Brief Introduction to the SAE J1939 Protocol
SAE J and its companion documents have quickly become the accepted industry standard and the vehicle network of choice for off-highway machines in applications such as construction, material handling, mass transportation, forestry machines, agricultural machinery, maritime and military applications. Note : It is fair to say that the SAE J vehicle protocol is primarily used on diesel engines, which covers all previously mentioned applications. The messages exchanged between these units can be data such as vehicle road speed, torque control message from the transmission to the engine, oil temperature, and many more. Note : Even though being around for many years, the SAE J protocol is still gaining popularity, especially in view of the increased use of fleet management systems, which, unavoidably, will need data from the vehicle network, for instance, to calculate maintenance cycles.
Solutions for your SAE J1939 Networking
SAE J is the standard communications network for sharing control and diagnostic information between electronic control units ECUs which reside on heavy duty and commercial vehicles. Examples of such vehicles are school busses, cement mixers, military vehicles, and semi-tractors. J specifies exactly how information e. J defines the data priority, size, scaling, offset, and transmission rate. The standard goes on to define additional aspects, including how large messages are fragmented and reassembled, message timeouts, network speed, the physical layer, and how applications, not ECUs, acquire network source addresses. J messages are defined and identified by their Parameter Group Number PGN , which is a unique number assigned to every message. The PGN serves to identify the message and its data.
Introduction to the SAE J1939 Protocol
Society of Automotive Engineers standard SAE J 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 J is used in the commercial vehicle area for communication throughout the vehicle, with the physical layer defined in ISO A different physical layer is used between the tractor and trailer, specified in ISO CAN was not originally free, but its instruction set did fit in the custom instruction format of J This was true as of