Notes On J1939 At 500K (J1939-14) And Auto Baud Rate Detection

SAE has approved the definition of the J1939 protocol running at 500k, and it is in the J1939-14 document. The idea was that this new protocol speed would be completely invisible to the application and the end-user. To make this transition easier, VDAs will be doing automatic baud rate detection between 250k/500k (and other common CAN speeds 125, 666, 1Mb). Also, service tools (RP1210/SAE J2534, hand-held devices, etc) will be using special cables with Deutsch 9-pin connectors in the color lime green to denote this the cable is meant for a J1939 Type 2 connector (see J1939-13, or RP 1202) and can handle the 500k baud rate. The Type 2 connector will also fit into the Type 1 Deutsch 9-pin socket to which we are accustomed (typically black); however, a Type 1 (assumed to be 250k) will not fit onto a Type 2.

An adapter that supports automatic baud rate detection should try to auto-baud without causing error frames on the vehicle network due to baud rate mismatch (i.e., connecting in "passive mode," not setting the CAN ACK bit). It was determined in the SAE J1939 committee that "no errors" may not be possible, as the VDA may have to send a message to provoke a response from other controllers.

To denote that an adapter can do automatic baud detection for a protocol (we will not limit automatic speed detection to just one protocol), the VDA vendor will place the string "Auto" in the ProtocolSpeed  parameter in the [ProtocolInformation]  section of all protocols that the vendor can detect automatically. If the application wants to know what speed they did connect at, they can issue an RP1210_SendCommand ()  using the RP1210_Get_Protocol_Connection_Speedrnnemoric  . Automatic baud detection for CAN-based protocols is mandatory for RP 1210-compliance. Implementation of this feature for protocols other than CAN is optional.