SAE J1708-1993 SERIAL DATA COMMUNICATIONS BETWEEN MICROCOMPUTER SYSTEMS IN HEAVY-DUTY VEHICLE APPLICATIONS.pdf
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1、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 therefr
2、om, 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 DOCU
3、MENT ORDER; (724) 776-4970 FAX: (724) 776-0790 SAE WEB ADDRESS http:/www.sae.org Copyright 1993 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
4、National Standard J1708 REV. OCT93 Issued1986-01 Revised1993-10-19 Superseding J1708 OCT90 SERIAL DATA COMMUNICATIONS BETWEEN MICROCOMPUTER SYSTEMS IN HEAVY-DUTY VEHICLE APPLICATIONS ForewordThis Document has also changed to comply with the new SAE Technical Standards Board format. This SAE/TMC Join
5、t Recommended Practice has been developed by the Truck and Bus Electronic Interface Subcommittee of the Truck and Bus Electrical Committee and by the S.1 Study Group of the Maintenance Council. The objectives of the subcommittee are to develop information reports, recommended practices, and standard
6、s concerned with the interface requirements and connecting devices required in the transmission of electronic signals and information among truck and bus components. Objectives: Some of the goals of the subcommittee in developing this document were to: a.Minimize hardware cost and overhead. b.Provid
7、e flexibility for expansion and technology advancements with minimum hardware and software impact on in-place assemblies. c.Utilize widely accepted electronics industry standard hardware and protocol to give designers flexibility in parts selection. d.Provide a high degree of electromagnetic compati
8、bility. e.Provide original equipment manufacturers, suppliers, and aftermarket suppliers the flexibility to customize for product individuality and for proprietary considerations. 1.ScopeThis SAE Recommended Practice defines a recommended practice for implementing a bidirectional, serial communicati
9、on link among modules containing microcomputers. This document defines those parameters of the serial link that relate primarily to hardware and basic software compatibility such as interface requirements, system protocol, and message format. The actual data to be transmitted by particular modules,
10、which is an important aspect of communications compatibility, is not specified in this document. These and other details of communication link implementation and use should be specified in the separate application documents referenced in Section 2. 1.1PurposeThe purpose of this document is to define
11、 a general-purpose serial data communication link that may be utilized in heavy-duty vehicle applications. It is intended to serve as a guide toward standard practice to promote serial communication compatibility among microcomputer-based modules. The primary use of the general-purpose communication
12、s link is expected to be the sharing of data among stand-alone modules to cost effectively enhance their operation. Communication links used to implement functions that require a dedicated communication link between specific modules may deviate from this document. SAE J1708 Revised OCT93 -2- 2.Refer
13、ences 2.1Applicable PublicationsThe following publications form a part of the specification to the extent specified herein. Unless otherwise indicated the latest revision of SAE publications shall apply. It is recommended that a separate applications document be published by the manufacturer for eac
14、h device using the serial link. These documents should define the data format, message I.D.s, message priorities, error detection (and correction), maximum message length, percent bus utilization, and methods of physically adding/removing units to/from the line for the particular application. 2.1.1S
15、AE PUBLICATIONSAvailable from SAE, 400 Commonwealth Drive, Warrendale, PA 15096-0001. SAE J1455Joint SAE/TMC Recommended Environmental Practices for Electronic Equipment Design (Heavy-Duty Trucks) SAE J1587Joint SAE/TMC Recommended Practice for Electronic Data Interchange Between Microcomputer Syste
16、ms in Heavy-Duty Vehicle Applications SAE J1922Powertrain Control Interface for Electronic Controls Used in Medium and Heavy-Duty Diesel on Highway Vehicle Applications 2.1.2EIA PUBLICATIONAvailable from Electronic Industries Association, 12001 Pennsylvania Avenue, NW, Washington, DC 20006. EIA RS-4
17、85Standard for Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems, April 1983 3.Definitions 3.1Access TimeTwo bit times multiplied by the message priority (which ranges from 1 to 8) added to the idle line time. 3.2BaudThe maximum number of analog si
18、gnal transitions per second that can occur on a channel. In this coding system, this is the reciprocal of the bit time. 3.3Bit TimeDuration or period of one unit of information. 3.4Character TimeThe duration of one character. The character must start with a low-logic bit, then 8 bits of data (least
19、significant bit first) followed by a high-logic level stop bit. 3.5ContentionA state of the bus in which two or more transmitters are turned on simultaneously to conflicting logic states. 3.6Differential SignalA two-wire process in which both lines are switches as opposed to a single-ended signal wh
20、erein one line is grounded and the signal line is switched between logic states. 3.7Idle StateThe state that produces a high-logic level on the input of the bus receiver when all transmitters on the network are turned off. 3.8Idle LineThe condition that exists when the bus has remained in a continuo
21、us high-logic state for at least 10 bit times after the end of the last stop bit. SAE J1708 Revised OCT93 -3- NOTEThe idle line serves as the delimiter between messages on the bus. A receiver that cannot distinguish between a stop bit and any other high-logic state may become synchronized with the b
22、us by noting the receipt of 12 consecutive high-logic bits. In the absence of errors, the first low-logic bit (0) following 12 consecutive high-logic bits (1), is the start bit of a message identification character (MID) (that is, the first character of a message). 3.9Message PriorityA measure of me
23、ssage criticality assigned on a scale of 1 to 8 by the appropriate applications document. The most critical message has a priority of one. 3.10 NodeA receiver or transceiver circuit connected to the bus. 3.10.1 START BITInitial element of a character defined as a low-logic level of 1 bit time durati
24、on as viewed at the output of the bus receiver. 3.10.2 STOP BITFinal element of a character defined as a high-logic level of 1 bit time duration as viewed at the output of the bus receiver. 4.Electrical ParametersThe electrical parameters of this serial data link are a modification of the EIA RS-485
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