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 entirelyvoluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefro
2、m, 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.TO PLACE A DOCUMENT ORDER; (724) 776-4970 FAX: (724) 776-0790SAE WEB ADDRESS http:/www.s
3、ae.orgCopyright 2000 Society of Automotive Engineers, Inc.All rights reserved. Printed in U.S.A.SURFACEVEHICLE400 Commonwealth Drive, Warrendale, PA 15096-0001INFORMATIONREPORTSubmitted for recognition as an American National StandardJ2056-3REAF.FEB2000Issued 1991-06Reaffirmed 2000-02Superseding J20
4、56-3 JUN1991Selection of Transmission MediaForewordIt has been commonly accepted by most automotive RF engineers that a Class C Network at atransmission rate above 100 kilobits per second (kbps) will require either a fiber optic or a shielded cable for thetransmission medium. Some communications eng
5、ineers have proposed that transformer coupling to a twisted pairmay be an acceptable alternative to a fiber optic or a shielded cable.It has also been generally recognized that the EMI levels available in a vehicle to corrupt data transmission arevery high and cannot be filtered out of the data. The
6、 employment of a fiber optic or a shielded cable for thetransmission medium would also solve this EMI problem.TABLE OF CONTENTS1. Scope . 21.1 Background 21.2 Interrelationship of Classes A, B, and C. 31.3 Electromagnetci Susceptibility (EMS) Considerations 31.4 Electromagnetic Interference (EMI) Co
7、nsiderations. 32. References . 32.1 Applicable Publications 32.1.1 SAE Publications 32.2 Other Publications 33. Twisted Pair 43.1 Inherent Advantages/Disadvantages of Twisted Pair Networks. 43.1.1 Familiarity of Twisted Pair Networks 43.1.2 Radiated Line Losses. 53.1.3 Receiver Susceptibility . 53.1
8、.4 Drive Problems and Line Losses 53.2 Network Architecture Options. 53.2.1 Data Encoding Communication Protocols 53.2.2 MFM Encoding Applied to Vehicle Multiplexing 63.2.3 Siefried Encoding . 63.2.4 Arcnet Encoding . 73.2.5 I/O Hardware Configuration 73.3 Key Concerns of Twisted Pair Networks 73.3.
9、1 Computer Simulation of EMI Levels . 7COPYRIGHT Society of Automotive Engineers, Inc.Licensed by Information Handling ServicesSAE J2056-3 Reaffirmed FEB2000-2-3.3.2 Four Media Driving Techniques Considered.93.3.3 Medium Driving and Encoding Techniques Conclusions104 Shielded/Coaxial Cable 154.1 Inh
10、erent Features of Shielded/Coaxial Cable Networks .154.2 Network Architecture Options .154.3 Key Concerns of Shielded/Coaxial Cable Networks.165. Fiber Optic 165.1 Inherent Features of Fiber Optic Systems 165.1.1 Principal Advantages 175.1.2 General Advantages .175.1.3 Disadvantages 185.2 Network Ar
11、chitecture Options .185.2.1 Active Star 195.2.2 Passive Star205.2.3 Single Ring .215.2.4 Double Ring 225.2.5 Linear Tapped Bus .235.2.6 Network Architecture Conclusions255.3 I/O Hardware Configuration 255.3.1 Time Division Multiplex (TDM)255.3.2 Frequency (Wavelength) Division Multiplex255.3.3 Space
12、 Division Multiplex (Multistrand Fiber Cable) 265.4 Communication Protocols.265.4.1 Bit Wise Contention Resolution Based Protocols .265.4.2 Non-Contention Based Protocols .265.5 Key Concerns of Fiber Optic Systems265.5.1 New Culture/Education for Automotive Environment275.5.2 Length of Link .275.5.3
13、 Data Rates275.5.4 Failure Modes .275.5.4.1 Consequences of Failure286. Summary & Conclusions 28APPENDIX A301. ScopeThis SAE Information Report studies the present transmission media axioms and takes a fresh look atthe Class C transmission medium requirements and also the possibilities and limitatio
14、ns of using a twisted pairas the transmission medium.The choice of transmission medium is a large determining factor in choosing a Class C scheme.1.1 BackgroundThe Vehicle Network for Multiplexing and Data Communications (Multiplex) Committee hasdefined three classes of vehicle data communication Ne
15、tworks:a. Class ALow-Speed Body Wiring and Control Functions, i.e., Control of Exterior Lampsb. Class BData Communications, i.e., Sharing of Vehicle Parametric Datac. Class CHigh-Speed Real-Time Control, i.e., High-Speed Link for Distributed ProcessingCOPYRIGHT Society of Automotive Engineers, Inc.L
16、icensed by Information Handling ServicesSAE J2056-3 Reaffirmed FEB2000-3-1.2 Interrelationship of Classes A, B, and CThe Class B Network is intended to be a functional superset ofthe Class A Network. That is, the Class B Bus must be capable of communications that would perform all of thefunctions of
17、 a Class A Bus. This feature protects the use of the same bus for all Class A and Class B functionsor an alternate configuration of both buses with a “gateway” device. In a similar manner, the Class C Bus isintended as a functional superset of the Class B Bus.1.3 Electromagnetic Susceptibility (EMS)
18、 ConsiderationsInherent with the high data rates of a Class C Busis a higher probability of electromagnetic interference (EMI) corrupting data. There has been a lot of researchon Class B Networks that use twisted pair operating at data rates below 50 kbps and methods have beenfound to overcome the c
19、ommunication problems (SAE J1850). But, it is commonly agreed that the corruptionof serial data by EMI will be an issue if a twisted pair or any other kind of conventional wiring and connectordesign is used at the higher data rates. Also, if data communication requirements dictate transmission rates
20、above 50kbps, another technique may be required because 50 kbps is the practical upper limit of these ClassB Networks (SAE J1850) that use twisted-pairs and conventional bus drivers.1.4 Electromagnetic Interference (EMI) ConsiderationsA key concern is the generation of EMI when theClass C Vehicle Mu
21、ltiplexing Network is utilizing twisted pair for the transmission medium operating at datatransmission rates above 50 kbps. It is because of this EMI concern that most automotive RF engineerscommonly accept that either a fiber optic or a shielded cable will be required for the transmission medium at
22、data rates above 100 kbps.It is expected that the growth of data communications on vehicles, the issue of shielding cost requirements,and electromagnetic compatibility of copper-based systems, will drive future development. These factors andother, as yet undefined, needs for Class C communication wi
23、ll eventually drive the implementation ofautomotive fiber optic systems for higher data transfer rates.2. References2.1 Applicable PublicationsThe following publications form a part of this specification to the extent specifiedherein. Unless otherwise indicated, the latest issue of SAE publications
24、shall apply.2.1.1 SAE PUBLICATIONAvailable from SAE, 400 Commonwealth Drive, Warrendale, PA 15096-0001.SAE J1850Class B Data Communications Network Interface2.2 Other Publications2.2.1 Henry W. Ott, Bell Laboratories, Noise Reduction Techniques in Electronic Systems, A Wiley-IntersciencePublications
25、. Second Edition, 1988.2.2.2 CISPR/D/WG2 (Secretariat) 19, September 1989, International Electrotechnical Commission, InternationalSpecial Committee on Radio Interference (CISPR), Subcommittee D: Interference Relating to MotorVehicles and Internal Combustion Engines, Working Group 2, Test Limits and
26、 Methods of Measurement ofRadio Disturbance from Vehicle Components and Modules: Conducted Emissions, 150 kHz to 108 MHz andRadiated Emissions, 150 kHz to 1000 MHz2.2.3 A. L. Harmer, SPIE Vol. 468 Fibre Optics 84, pp. 174-185 (1984).2.2.4 W. A. Rogers, D. R. Kimberlin, and R. A. Meade, Soc. Automoti
27、ve Eng. 88, pp. 50-56 (1980).2.2.5 M. W. Lowndes and E. V. Phillips, 4th Int. Conf. Automotive Electronics IEE Vol. 229, pp. 154-159, 1983.2.2.6 P. G. Duesbury and R. S. Chana, 4th Int. Conf. Automotive Electronics IEE Vol. 229, pp. 160-164, 1983.COPYRIGHT Society of Automotive Engineers, Inc.Licens
28、ed by Information Handling ServicesSAE J2056-3 Reaffirmed FEB2000-4-2.2.7 K. Sekiguchi, Int. Fibre Optics and Commun. 3, pp. 56-60, 1982.2.2.8 T. Sasayama, Hirayama, S. Oho, T. Shibata, A. Hasegawa, and Y. Minai, 4th Int. Conf. AutomotiveElectronics IEE Vol. 229, Nov. 1983.2.2.9 K. Sasai, Sitev Conf
29、., pp. i-ii, May 1983.2.2.10 R. E. Steele and H. J. Schmitt, SPIE Vol. 840, Fiber Optic Systems for Mobile Platforms 87.2.2.11 G. D. Miller, SPIE Vol. 989, Fiber Optic Systems for Mobile Platforms II 88, pp. 124-132 (1988).2.2.12 D. A. Messuri, G. D. Miller, and R. E. Steele, A Fiber Optic Connectio
30、n System Designed for AutomotiveApplications, Soc. Automotive Eng. #890202, Feb. 89.2.2.13 T. W. Whitehead, Du Pont Electronics Private Communications (1989).2.2.14 T. Sasayama and A. Hideki, SPIE Vol. 989, Fiber Optic Systems for Mobile Platforms 11, 88.2.2.15 M. Kitazawa, Mitsubishi Rayon, Private
31、 Communications (1989).3. Twisted PaIrA Twisted Pair is defined to be a transmission line consisting of two similar conductors that areinsulated from each other and are twisted around each other to form a communication channel. The purposefor twisting the conductors around each other is to reduce th
32、e electric and magnetic field interaction with otherconductors. In recent years there has been a lot of research on Class B Networks that use twisted pairoperating at data rates below 50 kbps. At Class C data rates (100 kbps) many new problems needdevelopment and attention.3.1 Inherent Advantages/Di
33、sadvantages of Twisted Pair NetworksAs the result of widespread Class Bnetwork development, a lot of research has been completed on the use of copper-based twisted pair for atransmission media. Class C development is an extension of that activity.3.1.1 FAMILIARITY OF TWISTED PAIR NETWORKSThe desire
34、to use twisted pair for the transmission medium of aClass C Network by the automotive industry is universal. This desire is twisted pairs biggest advantage. Atlower data rates, the automotive wiring requirements for twisted pair and connector techniques are wellknown and developed. The failure modes
35、 such as shorts to ground and battery have been extensivelystudied. The use of proper techniques for termination have been developed. An effective I/O can be easilyachieved by integrating the transmission hardware, used for driving the twisted pair, into an interface devicethat also contains the rec
36、eiver and some external discrete filter components for EMI rejection. Bidirectionaldata transfer is easily obtainable using the same twisted pair for both reception and transmission. Statisticalstudies have provided data so that the reliability of a twisted pair network is known. The connector indus
37、tryis currently developing insulation displacement type connectors so that in the future automated machinescan be programmed to place bus connector drops as required, further reducing the cost of the wiringharness. Of course, at Class C data rates many of these and other factors such as the maintena
38、nce of twistuniformity and the harness interconnection requirements are likely to change. A large investment inresearch and development must be completed in order to demonstrate feasibility. The magnitude of the taskcould easily be underestimated even though this development is an extension of famil
39、iar work.In most communications systems, the length of line is a large factor in determining the upper limit of datarates. However, line length in automotive networking is relatively small and does not play a major role, butthe number of connectors and losses due to impedance mismatching at the conn
40、ector is a concern. Perhapsdevelopments in ribbon cabling techniques and insulation displacement connectors could improve thisimpedance matching situation.COPYRIGHT Society of Automotive Engineers, Inc.Licensed by Information Handling ServicesSAE J2056-3 Reaffirmed FEB2000-5-3.1.2 RADIATED LINE LOSS
41、ESThe biggest problem to overcome is the fact that for data rates above 100 kbps, theradiated line losses are very high (2.2.1). These radiated line losses cause transmitter line driver problemsand generate large amounts of EMI. The work at Class B data rates demonstrated that the transition risetim
42、e was responsible for most of the EMI. The present automotive quality of a twisted pair network mediumdoes not exhibit good transmission line characteristics. Also the capacitance load to the output driver fromthe twisted pair was measured to be approximately 2000 pfd. At Class C data rates this cap
43、acitance loading,impedance mismatching at the connector, maintenance of twist uniformity, and drive symmetry matchrequirements between bus outputs make it very difficult, if not impossible, to design an output driver. Thechallenge will be to achieve a low enough output impedance to drive a twisted p
44、air without incurringexcessive losses or spectral distortion of the transitions especially for data rates above 1 Megabit per second(Mbps).3.1.3 RECEIVER SUSCEPTIBILITYThe receiver is very susceptible to coupled (capacitive/inductive) and longitudinalnoise interference (see 3.3.2 for details on long
45、itudinal noise). At Class C data rates it is much more difficultto devise a filter that could eliminate the coupled line noise. The severity of this problem can be understoodby realizing that the vehicle wiring harness appears to be resonant around 25 to 30 MHz which isapproximately a quarter wavele
46、ngth in length. Switching noise and spikes are broadband and excite thewiring harness to resonate at high levels. At Class B data rates this broadband noise is coupled into thecircuit but is effectively eliminated by the filter. For Class C multiplexing the data rates required may be at 1to 10 Mbps.
47、 This wire harness resonance is too close to the filter cutoff frequency for traditional filteringtechniques to be very effective.3.1.4 DRIVE PROBLEMS AND LINE LOSSESThe transmitter drive problem and line losses cause many experts toconclude that twisted pair and shielded twisted pair are not usable
48、 for data rates above 100 kbps. Thefiltering techniques for receiver susceptibility would also leave the network highly susceptible to datacorruption and thus require very sophisticated error detection or reconstruction techniques.3.2 Network Architecture OptionsThe suitable topology configurations
49、of twisted pair is a very strongadvantage. It can accommodate any configuration from a Star, Tee, Bus, Ring, Daisy Chain, or variousHybrids. Many data encoding techniques have been employed with twisted pair as the transmission mediumwith a variety of I/O hardware configurations.3.2.1 DATA ENCODING OF COMMUNICATION PROTOCOLSThe data encoding technique has a significant effect onthe radiated EMI. To achieve the highest possible data rate it is important to choose a data encoding methodthat has the fewest transitions per bit with the maximum of time between trans
