But what is a J SPN? For a given entry of this message ID, we also logged 8 bytes of raw data - now, how do we interpret and convert this? In that case we see from the PGN info that relevant data is in byte 4 and 5, i. Taking the decimal form of 0x Intel byte order , we get decimal. To arrive at the RPM, we conduct a scaling of this value using the offset 0 and the scale 0. The result is RPM.
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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. Fleet management is also tightly associated with the Internet of Things IoT , and transportation is considered one of the fastest growing markets for IoT. The CAN bus is primarily used in embedded systems, and as its name implies, is a network technology that provides fast communication among microcontrollers up to real-time requirements, eliminating the need for the much more expensive and complex technology of a Dual-Ported RAM.
CAN is a two-wire, half duplex, high-speed network system, that is far superior to conventional serial technologies such as RS in regards to functionality and reliability and yet CAN implementations are more cost effective.
CAN networks can be used as an embedded communication system for microcontrollers as well as an open communication system for intelligent devices. Some users, for example in the field of medical engineering, opted for CAN because they have to meet particularly stringent safety requirements. Similar requirements had to be considered by manufacturers of other equipment with very high safety or reliability requirements e.
The greatest advantage of Controller Area Network lies in the reduced amount of wiring combined with an ingenious prevention of message collision meaning no data will be lost during message transmission. Without CAN Vs. Controller Area Network Is a serial networking technology for embedded solutions. Operates at data rates of up to 1 Megabit per second. Supports a maximum of 8 bytes per message frame. Does not support node IDs, only message IDs.
One application can support multiple message IDs. Supports message priority, i. Supports two message ID lengths, bit standard and bit extended. Does not experience message collisions as they can occur under other serial technologies. Is not demanding in terms of cable requirements. Twisted-pair wiring is sufficient. The SAE J Higher Layer Protocol Even though extremely effective in automobiles and small, embedded applications, CAN alone is not suitable for projects that require a minimum of network management and messages with more than eight data bytes.
In fact, J provides a far better data bandwidth than any of these automation protocols. J attempts to define standard PGNs to encompass a wide range of automotive, agricultural, marine and off-road vehicle purposes.
The actual data in the data field is described by the SPNs. You may find some of them by browsing the Internet, but be aware that there is no complete online reference. A description of all PGNs is out of the scope of this book. Rather than relying on a myriad of protocol functions, SAE J uses predefined parameter tables, which keeps the actual protocol on a comprehensible level. Parameters groups are, for instance, engine temperature, which includes coolant temperature, fuel temperature, oil temperature, etc.
In addition, it is possible to use manufacturer-specific parameter groups. Parameter Groups Parameter Groups contain information on parameter assignments within the 8-byte CAN data field of each message as well as repetition rate and priority.
A Brief Introduction to the SAE J1939 Protocol