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5、10 Automatic Baud Rate Detection Process RATIONALE This is the first version of the document. The purpose of this document is to establish a common method of automatically detecting the baud rate of an SAE J1939 network. This is needed so manufacturers that provide devices capable of operating on SA
6、E J1939 networks with different baud rates can reliably detect the baud rate of that network segment without interrupting network communications. FORWARD This series of SAE Recommended Practices has been developed by the Truck and Bus Control and Communications Network Committee of the Truck and Bus
7、 Electrical and Electronics Steering Committee. The objectives of the committee are to develop information reports, recommended practices, and standards concerned with the requirements design and usage of devices that transmit electronic signals and control information among vehicle components. The
8、usage of these documents is not limited to truck and bus applications; other applications may be accommodated with immediate support being provided for construction and agricultural equipment, and stationary power systems. These documents are intended as a guide toward standard practice and are subj
9、ect to change so as to keep pace with experience and technical advances. TABLE OF CONTENTS 1. SCOPE 2 2. REFERENCES 2 2.1 Applicable Publications . 2 2.1.1 SAE Publications . 2 2.1.2 ISO Publications 2 3. DEFINITIONS AND ABBREVIATIONS . 2 3.1 Definitions . 2 3.2 Abbreviations 3 4. TECHNICAL REQUIREM
10、ENTS 4 4.1 Overview . 4 4.2 Baud Rate Detection ECU Classifications 4 4.2.1 Permanently Installed ECU (PI-ECU) Classifications . 4 4.2.2 Temporarily Installed ECU (TI-ECU) Classifications 5 4.3 SAE J1939 Network Architecture for Baud Rate Detection 5 4.4 Baud Rate Detection Procedure General Constra
11、ints 7 4.5 PI-ECU Baud Rate Detection Procedure 7 4.6 TI-ECU Baud Rate Detection Procedure While In Silent Baud Discovery Mode 9 4.7 TI-ECU Baud Rate Detection Procedure While In Active Baud Discovery Mode . 10 SAE INTERNATIONAL J1939-16 OCT2015 Page 2 of 16 5. NOTES 13 5.1 Revision Indicator 13 App
12、endix A Baud Rate detection Use Cases . 14 Figure 1 SAE J1939 Network Architecture for Baud Rate Detection 6 Figure 2 PI-ECU Baud Rate Detection Behavior Model . 8 Figure 3 TI-ECU Silent Mode Baud Rate Detection Behavior Model 10 Figure 4 TI-ECU Active Baud Discovery mode Behavior Model . 12 Table 1
13、 Standard 11 Bit Message TI-ECUs use to detect the Baud Rate . 11 1. SCOPE Define a process enabling devices to detect the baud rate of a network automatically with the intent of minimizing or eliminating communication disruption between existing devices. 2. REFERENCES 2.1 Applicable Documents The f
14、ollowing publications form a part of this specification to the extent specified herein. Unless otherwise indicated, the latest issue of SAE publications shall apply. 2.1.1 SAE Publications Available from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel: 877-606-7323 (inside
15、USA and Canada) or +1 724-776-4970 (outside USA), www.sae.org. SAE J1939-11 Physical Layer, 250 Kbps, Twisted Shielded Pair SAE J1939-13 Off-Board Diagnostic Connector SAE J1939-14 Physical Layer, 500 Kbps, Twisted Shielded Pair SAE J1939-15 Physical Layer, 250 Kbps, Un-Shielded Twisted Pair 2.1.2 I
16、SO Publications Copies of these documents are available online at http:/webstore.ansi.org/ ISO 11898-1 Road Vehicles Controller area network (CAN) - Part 1: Data link layer and physical signaling ISO 15765-4 Road Vehicles Diagnostic communication over Controller Area Network (DoCAN) Part 4: Requirem
17、ents for emission related systems 3. DEFINITIONS AND ABBREVIATIONS 3.1 DEFINITIONS 3.1.1 ACK Error A CAN protocol error that occurs when the transmitter fails to detect a dominant level asserted bit in the Acknowledgement (ACK) bit of a CAN frame. 3.1.2 Active Baud Discovery Mode This is where the E
18、CU is operating in Baud Discovery Mode (see 3.1.4) while in Active Mode (see 3.1.3). SAE INTERNATIONAL J1939-16 OCT2015 Page 3 of 16 3.1.3 Active Mode The CAN device operating in this mode is an active participant of the network. It asserts a dominant value in the ACK bit when it successfully receiv
19、es a CAN frame. 3.1.4 Baud Discovery Mode Operational mode of an ECU when it can detect the baud rate of the CAN network based on the procedures listed in this document. 3.1.5 Bit-time Measurement The method of determining the CAN network baud rate by measuring the time duration of a single bit. 3.1
20、.6 Permanently Installed ECU The classification of an ECU which is required for normal operation of the system, including but not limited to ECM, TCM, instrument cluster, body controller, etc. 3.1.7 Silent Baud Discovery Mode This is where the ECU is operating in Baud Discovery Mode (see 3.1.4) whil
21、e in Silent Mode (see 3.1.8). 3.1.8 Silent Mode The CAN device operating in this mode will not transmit messages, nor assert the ACK bit for any received CAN frame. Some CAN hardware specification documents refer to a “Listen Only Mode”, which might not be the same as Silent Mode. Regardless of the
22、terminology used in the CAN hardware specification, it is important that the mode chosen for Baud Discovery Mode be one that does not participate in the ACK or error processes. The CAN device operating in Silent Mode is prohibited from transmitting dominant bits but is capable of receiving messages.
23、 3.1.9 Temporarily Installed ECU The classification of an ECU which is not required for the normal operation of the system, including but not limited to service tools and datalink adapters for tools. 3.2 ABBREVIATIONS ACK Acknowledge CAN Controller Area Network ECU Electronic Control Unit ECM Engine
24、 Control Module PI-ECU Permanently Installed ECU TCM Transmission Control Module TI-ECU Temporarily Installed ECU UML Unified Modeling Language SAE INTERNATIONAL J1939-16 OCT2015 Page 4 of 16 4. TECHNICAL REQUIREMENTS 4.1 Overview This document describes the processes used to detect the baud rate of
25、 the SAE J1939 network segment by ECUs with the ability to adjust their CAN baud rate while in use. The specified approach provides a reliable method to detect the baud rate of that network segment without interrupting network communications. This document describes the Silent Baud Discovery Mode fo
26、r baud rate detection whereby a baud rate adjustable ECU puts its CAN device into Silent Mode and cycles through each supported baud rate until it successfully receives a 29 bit extended frame message. Successfully received 11 bit standard frame messages shall not be used for baud rate detection bec
27、ause some devices may transmit 11 bit standard frame messages in attempt to detect the baud rate. The general baud rate detection behaviors are the same for all baud-rate adjustable ECUs; however, there are some minor behavior differences depending upon whether the ECU is intended as permanently ins
28、talled or temporarily installed on the network. This document also describes the Active Baud Discovery Mode for baud rate detection whereby a temporarily installed baud rate adjustable ECU can transmit a special 11 bit standard frame test message so that an ECU can determine the baud rate of this ne
29、twork segment. There may be some networks where no periodic messages are transmitted by any of the fixed baud rate ECUs but the temporarily installed baud rate adjustable ECU still needs to determine the network segment baud rate. In such a scenario, a temporarily installed baud rate adjustable ECU
30、will never detect the network baud rate using the Silent Baud Discovery Mode detection method. In the active baud rate detection method, a baud rate adjustable ECU puts its CAN device into Active Baud Discovery Mode and cycles through each supported baud rate as it transmits the special 11 bit test
31、message until another ECU on the network asserts the Acknowledge bit. This only works when there is one baud rate adjustable ECU that is in Active Baud Discovery Mode at a time. Multiple baud rate adjustable ECUs performing active baud rate detection could result in the wrong baud rate detection by
32、those baud rate adjustable ECUs if both are sending 11 bit test messages at the same baud rate and the CAN device in each asserts the Acknowledge bit of the others test message. The processes in this document are intended to be performed after power is applied to the ECU and before it transmits any
33、messages. Although these processes are not intended to be performed during the same key cycle after an ECU has determined the baud rate of the SAE J1939 network segment, nothing prevents an ECU from entering Baud Discovery Mode after a baud rate has been determined. However, this document does not s
34、pecifically address those situations. Bit-time measurement is an alternate method of baud rate detection. However, it is not considered a robust baud rate detection method and is discouraged for baud rate detection on SAE J1939 networks. 4.2 Baud Rate Detection ECU Classifications There are six diff
35、erent baud rate detection ECU classifications. The classifications are based upon the permanence of network installation and the ability for run time baud rate adjustment. The ECU classifications are used to describe the minimum requirement on SAE J1939 network architecture for baud rate detection a
36、nd describe and associate specific baud rate detection behaviors for baud rate adjustable ECUs. Three of the ECU classifications identify fixed baud rate ECUs and three of the ECU classifications identify baud rate adjustable ECUs. 4.2.1 Permanently Installed ECU (PI-ECU) Classifications “Permanentl
37、y Installed ECU” is an ECU which is required for normal operation of the system and, therefore, is expected to be connected to the J1939 network segment at all times. PI-ECUs are typically ECUs which are dedicated to a single vehicle or application, and a PI-ECU rarely, if ever, is connected to anot
38、her different vehicle during its useful life. Some examples of permanently installed ECUs are an engine ECU, transmission ECU, instrument cluster, and body controller. 4.2.1.1 Adjustable Baud Rate PI-ECU This classification describes a permanently installed ECU that will adjust its CAN baud rate to
39、the SAE J1939 network segment baud rate, provided that rate is supported by the ECU. This class of ECU is recommended to store the last detected baud rate in some form of persistent memory over ECU power cycles to expedite the baud rate discovery time on the next ECU power up. SAE INTERNATIONAL J193
40、9-16 OCT2015 Page 5 of 16 4.2.1.2 Fixed Baud Rate PI-ECU with Periodic Message Transmits This classification describes a permanently installed ECU that communicates on the SAE J1939 network segment using one designated baud rate. This class of ECU automatically transmits one or more periodic message
41、s onto the SAE J1939 network. The periodic messaging by this class of ECU is the cornerstone for baud rate detection by baud rate adjustable ECUs. 4.2.1.3 Fixed Baud Rate PI-ECU with No Periodic Message Transmits This classification describes a permanently installed ECU that communicates on the SAE
42、J1939 network segment using one designated baud rate. This class of ECU does not automatically transmit any periodic messages onto the SAE J1939 network. 4.2.2 Temporarily Installed ECU (TI-ECU) Classifications “Temporarily Installed ECU” describes an ECU which is not required for normal operation o
43、f the system, but may be connected to the SAE J1939 network segment randomly and for short periods of time as required. TI-ECUs are typically considered to be ECUs that are not dedicated to any single vehicle or application; rather, the nature of the TI-ECU involves being connected to any number of
44、different vehicles over its useful life. 4.2.2.1 Adjustable Baud Rate Only TI-ECU with Silent Baud Discovery Mode This classification describes a temporarily installed ECU that will adjust its CAN baud rate to the SAE J1939 network segment baud rate, provided that rate is supported by the ECU. This
45、class of ECU will only use Silent Baud Discovery Mode for detecting the SAE J1939 network segment baud rate. 4.2.2.2 Adjustable Baud Rate Only TI-ECU with Active Baud Discovery Mode This classification describes a temporarily installed ECU that will adjust its CAN baud rate to the SAE J1939 network
46、segment baud rate, provided that rate is supported by the ECU. This class of ECU will first use Silent Baud Discovery Mode for detecting the SAE J1939 network segment baud rate. If the baud rate is not detected through the Silent Baud Discovery Mode, then this class of ECU will go into Active Baud D
47、iscovery Mode and potentially transmit CAN frames in order to detect the SAE J1939 network segment baud rate. This class of ECU should be used sparingly since the Active Baud Discovery Mode operation has the potential to interrupt network communications. 4.2.2.3 Fixed Baud Rate TI-ECU This classific
48、ation describes a temporarily installed ECU that communicates on the SAE J1939 network segment using one designated baud rate. This class of ECU should only be installed on SAE J1939 networks designated with the same baud rate to avoid interrupting network communications due to baud rate differences
49、. 4.3 SAE J1939 Network Architecture for Baud Rate Detection SAE J1939 network architecture must conform to several constraints regarding the number and types of connected ECUs in order for ECUs to reliably detect the network segment baud rate. The SAE J1939 network architecture constraints are shown in the UML diagram in Figure 1. The constraints are described in this section. a. The SAE J1939 network segment is recommended to have at least two active fixed baud rate PI-ECUs (see 4.2.1.2 and 4.2.1.3), and at least one of these fixed baud rate
