Automatic CAN Bus baud rate detection comes in handy when you need to connect to a CAN network without knowing its actual transmission speed. Rather than playing with CAN baud rate settings and testing if you are able to see data, the automatic detection will deliver the right result within seconds, however, depending on the busload. A heavy busload will deliver quicker results than a bus with occasional data frames.
When it comes to SAE J1939, a CAN-based higher-layer protocol for diesel engines, the automatic baud rate detection proves its efficiency. It can seamlessly connect a device to a vehicle network operating at either 250k or 500k baud, eliminating the need for a configuration system tailored for each engine.
However, testing or connecting a CAN node (ECU) with automatic baud rate detection can be frustrating without looking into more background information.
Here are the rules when testing or connecting such a device:
- The CAN Bus node (ECU) can only detect the CAN Bus transmission rate when it is connected (and starts up) to a running network.
- You cannot test the ECU’s detection capabilities by connecting it to a single ECU. The other ECU’s CAN controller will detect errors on the CAN Bus and cease sending data frames.
- You need at least two other ECUs (i.e., a running network) who communicate with each other before you can connect or test the ECU with automatic baud rate detection.
How does automatic baud rate detection work?
The easiest way to detect the CAN baud rate is to switch to Silent Mode (to avoid error frames; see information in the next paragraph) and set up a loop to initialize standard baud rates (e.g., 100k, 125k, 250k, 500k, 1000k). After each initialization, check whether you received a valid data frame or an error. In case of an error, switch to the next baud rate. If you receive valid messages, switch off the Silent Mode and continue operation with the baud rate as detected.
What is Silent Mode?
The Silent mode allows a node to be set to a state, in which it is absolutely passive to the bus, meaning it is “invisible” to the rest of the network and it does not influence the bus in any shape or form. The node does not even provide an ACK frame when a message is placed on the CAN bus correctly. Thus, the Silent mode provides a perfect Listen-Only mode for diagnostic features such as the baud rate detection.
Automatic baud rate detection algorithm used by Copperhill jCOM.J1939 devices
As I mentioned previously, the automatic baud rate detection under SAE J1939 can be used to connect a device to a vehicle network that runs at either 250k or 500k baud without having to use a configuration system that is individually designed for each engine.
In accordance with the SAE J1939 Standard, all of our jCOM.J1939 devices with automatic baud rate detection will test the bus connection until a valid connection was found. That also means, the device will stay idle forever in case it cannot connect to the bus.
Once a connection is detected, the process will finish within shortest time, however, depending on busload. A heavy busload will deliver quicker results than a bus with occasional data frames.
A Comprehensible Guide to J1939 is the first work on J1939 besides the SAE J1939 standards collection. It provides profound information on the J1939 message format and network management combined with a high level of readability.
The information in this book is based on two documents of the SAE J1939 Standards Collection:
- J1939/21 – Data Link Layer
- J1939/81 – Network Management
This book is an attempt to create an enjoyable and readable J1939 reference for everybody. The information provided in this book is, besides the SAE J1939 Standards Collection, based on publicly available information such as, but not limited to web sites and printed literature as well as contributions by engineers familiar with Controller Area Network and the J1939 protocol. The information in this book, while based on the J1939 standard, is not a reproduction of any copyrighted SAE document. More Information…
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