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SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely
voluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefrom, is the sole responsibility of the user.”
SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions.
QUESTIONS REGARDING THIS DOCUMENT: (724) 772-8512 FAX: (724) 776-0243
TO PLACE A DOCUMENT ORDER; (724) 776-4970 FAX: (724) 776-0790
SAE WEB ADDRESS http://www.sae.org
Copyright 1999 Society of Automotive Engineers, Inc.
All rights reserved. Printed in U.S.A.
SURFACE
VEHICLE
400 Commonwealth Drive, Warrendale, PA 15096-0001
RECOMMENDED
PRACTICE
Submitted for recognition as an American National Standard
J1939-21
REV.
JUL1998
Issued 1994-07
Revised 1998-07
Superseding J1939-21 JUL94
(R) Data Link Layer
Foreword—This series of SAE Recommended Practices have been developed by the Truck and Bus Control and
Communications Network Subcommittee of the Truck and Bus Electrical and Electronics Committee. The
objectives of the subcommittee are to develop information reports, recommended practices, and standards
concerned with the requirements, design, and usage of devices which transmit electronic signals and control
information among vehicle components. The usage of these Recommended Practices 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 SAE Recommended Practices are intended as a guide toward standard practice and are subject to change
so as to keep pace with experience and technical advances.
This data link layer is used for all SAE J1939 applications. It is the characteristic that is common across all SAE
J1939 applications.
TABLE OF CONTENTS
1. Scope ....................................................................................................................................................... 3
2. References ............................................................................................................................................... 3
2.1 Applicable Publications ............................................................................................................................ 3
2.1.1 SAE Publication........................................................................................................................................ 3
2.2 Other Publications .................................................................................................................................... 3
3 Definitions................................................................................................................................................. 3
4 Abbreviations............................................................................................................................................ 3
5. Technical Requirements........................................................................................................................... 4
5.1 Message/Frame Format ........................................................................................................................... 4
5.1.1 SAE J1939 Message Frame Format (“CAN 2.0B” Extended Frame Format) .......................................... 4
5.1.2 Parameter Group Number (PGN)............................................................................................................. 7
5.1.3 SAE J1939 Support of “CAN 2.0B” Standard Frame Format Messages.................................................. 8
5.2 Protocol Data Unit (PDU) ......................................................................................................................... 8
5.2.1 Priority (P) ................................................................................................................................................ 9
5.2.2 Reserved Bit (R)....................................................................................................................................... 9
5.2.3 Data Page (DP) ........................................................................................................................................ 9
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SAE J1939-21 Revised JUL1998
-2-
5.2.4 PDU Format (PF)......................................................................................................................................9
5.2.5 PDU Specific (PS) ....................................................................................................................................9
5.2.6 Source Address (SA)..............................................................................................................................11
5.2.7 Data Field ...............................................................................................................................................11
5.3 Protocol Data Unit (PDU) Formats..........................................................................................................12
5.3.1 PDU1 Format.......................................................................................................................................... 13
5.3.2 PDU2 Format.......................................................................................................................................... 13
5.4 Message Types ......................................................................................................................................13
5.4.1 Command ...............................................................................................................................................13
5.4.2 Request .................................................................................................................................................. 14
5.4.3 Broadcast/Response .............................................................................................................................. 15
5.4.4 Acknowledgment .................................................................................................................................... 16
5.4.5 Group Function.......................................................................................................................................17
5.5 Message Priority..................................................................................................................................... 18
5.6 Bus Access.............................................................................................................................................18
5.7 Contention-Based Arbitration.................................................................................................................. 18
5.8 Error Detection........................................................................................................................................18
5.9 Assignment Process for Source Addresses and Parameter Group Numbers........................................ 19
5.9.1 Address Assignment Criteria .................................................................................................................. 19
5.9.2 Parameter Group Assignment Criteria.................................................................................................... 19
5.9.3 Data Field Definition ...............................................................................................................................20
5.10 Transport Protocol Functions..................................................................................................................21
5.10.1 Packetization and Reassembly...............................................................................................................21
5.10.2 Connection Management........................................................................................................................22
5.10.3 Transport Protocol—Connection Management Messages .....................................................................24
5.10.4 Transport Protocol—Data Transfer Message (TP.DT) ........................................................................... 25
5.10.5 Connection Constraints ..........................................................................................................................26
5.11 PDU Processing Requirements ..............................................................................................................27
5.12 Application Notes.................................................................................................................................... 28
5.12.1 High Data Rates ..................................................................................................................................... 28
5.12.2 Request Scheduling................................................................................................................................28
5.12.3 Device Response Time and Time Out Defaults......................................................................................28
5.12.4 Required Responses ..............................................................................................................................28
5.12.5 Transmission of PGNs To Specific or Global Destinations.....................................................................28
5.12.6 CTS Number of Packet Recommendation..............................................................................................28
6. Notes ......................................................................................................................................................29
6.1 Marginal Indicia.......................................................................................................................................29
Appendix A SAE J1939 PDU Processing—Typical Receive Routine ........................................................................30
Appendix B Communication Message Types.............................................................................................................31
Appendix C Transport Protocol Transfer Sequences ................................................................................................. 32
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SAE J1939-21 Revised JUL1998
-3-
1. Scope—As described in the parent document, SAE J1939, there are a minimum of seven documents required
to fully define a SAE J1939 network. This particular document, SAE J1939-21, describes a Data Link Layer
using the CAN protocol with 29 bit Identifiers. For SAE J1939, no alternative data link layers are permitted.
2. References
2.1 Applicable Publications—General information regarding this series of recommended practices is found in
SAE J1939. Unless otherwise indicated, the latest issue of SAE publications shall apply.
2.1.1 SAE PUBLICATIONS—Available from SAE, 400 Commonwealth Drive, Warrendale, PA 15096-0001.
SAEJ1939—Recommended Practice for a Serial Control and Communications Vehicle Network is the
parent document and should be referenced in general.
SAEJ1939-71—Vehicle Application Layer
SAEJ1939-81—Network Management
2.2 Related Publication—The following publication is provided for information purposes only and is not a required
part of this document.
2.2.1 ISO PUBLICATION—Available from SAE, 400 Commonwealth Drive, Warrendale, PA 15096-0001.
ISO11898(Amended)—Road vehicles—Interchange of digital information—Controller area network (CAN)
for high-speed communications—AMENDMENT 1
3. Definitions—Terms and definitions are defined in SAE J1939.
4. Abbreviations
ACK Acknowledgment
BAM Broadcast Announce Message
CAN Controller Area Network
CRC Cyclic Redundancy Check
CTS Clear-To-Send
DA Destination Address
DLC Data Length Code
DP Data Page
EOF End of Frame
ID Identifier
IDE Identifier Extension Bit
LLC Logical Link Control
LSB Least Significant Byte or Least Significant Bit
MAC Medium Access Control
MF Manufacturer
MSB Most Significant Byte or Most Significant Bit
NA Not Allowed
NACK Negative-Acknowledgment
P Priority
PDU Protocol Data Unit
PF PDU Format
PGN Parameter Group Number
PS PDU Specific
GE Group Extension
DA Destination Address
R Reserved
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SAE J1939-21 Revised JUL1998
-4-
RTR Remote Transmission Request
RTS Request-To-Send
SA Source Address
SOF Start of Frame
SRR Substitute Remote Request
TP Transport Protocol
T
h
Hold Time
T
r
Response Time
un Undefined
5. Technical Requirements—The data link layer provides for the reliable transfer of data across the physical
link. This consists of sending the CAN Data Frame with the necessary synchronization, sequence control, error
control, and flow control. The flow control is accomplished by a consistent message frame format.
5.1 Message/Frame Format—Message format will conform to the CAN requirements. The CAN specification
referenced throughout this document is “CAN Specification 2.0 Part B,” September, 1991. It should be noted
that when there are differences between the CAN specification and SAE J1939, then SAE J1939 will be the
guiding document.
The CAN document specifies, in an information routing related discussion, that node addresses are not used.
While this is true for some applications of CAN, it is not true for SAE J1939. The definition of the SAE J1939
network requires that node addressing be used to prevent multiple nodes from using the same CAN Identifier
field (see SAE J1939). Many additional requirements exist in SAE J1939 that are not specified by CAN.
“CAN 2.0B” contains specification of two message formats, Standard Frame and Extended Frame. “CAN
2.0B” compatibility implies that messages of both format can potentially be present on a single network, by
using certain bit codings which allow for the recognition of the different formats. To this point, SAE J1939 also
has accommodations for both CAN data frame formats. But, SAE J1939 will only define a full strategy for
standardized communications using the extended frame format. All standard frame format messages are for
proprietary use following the rules defined in this document.
Therefore, SAE J1939 devices MUST use the Extended Frame format. Standard Frame format messages can
reside on the network, but only as described in this document.
NOTE—Standard frame devices do not respond to network management messages and will not be able to
support the strategy for standardized communications.
Referencing Figure 1, the CAN data frame is parsed into different bit fields. The number and parsing of the bits
in the arbitration and control field differs between the CAN Standard and CAN Extended Frame messages.
CAN Standard Frame messages, shown in “A,” contains 11 Identifier bits in the arbitration field and CAN
Extended Frame messages, shown in “B,” contain 29 Identifier bits in the arbitration field. SAE J1939 has
further defined the Identifier bits in the arbitration field of the CAN Data Frame formats. These definitions can
be seen in Table 1.
5.1.1 SAE J1939 Message Frame Format (“CAN 2.0B” Extended Frame Format)—The CAN extended frame
message, as shown in Figure 1, will encompass a single Protocol Data Unit (PDU). PDUs will consist of 7
predefined fields. These fields will be assimilated from information provided by the application layer. They
are priority, reserved, data page, PDU Format, PDU specific (which can be a destination address, Group
Extension, or proprietary), source address, and data fields. They will then be packaged into one or more
CAN Data Frames and sent over the physical media to other network devices. The layers of the Open
Systems Interconnect (OSI) model that SAE J1939 supports are shown in Figure 2. It should be recognized
that some Parameter Group definitions will require more than one CAN Data Frame to send their
information.
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SAE J1939-21 Revised JUL1998
-5-
FIGURE 1—CAN DATA FRAMES
FIGURE 2—APPLICATION OF OSI MODEL BY SAE J1939
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