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 15096-0001STANDARDSubmitted for recognition as an American National StandardJ2056-3REAF.FEB2000Issued 1991-06Reaffirmed 2000-02Superseding J2056-3 JUN1
4、991Selection 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 engineers ha
5、ve 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 employme
6、nt 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) Considerati
7、ons. 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 . 53.1.4 Drive
8、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 73.3.1 Compute
9、r Simulation of EMI Levels . 7SAE 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 Inherent Features of Shielded/Coaxial Cable Networks .154.2 Network Architecture Options .154.3 Key
10、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 Architecture Options .185.2.1 Active Star 195.2.2 Passive Star205.2.3 Single Ring .215.2.4 Double R
11、ing 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 Division Multiplex (Multistrand Fiber Cable) 265.4 Communication Protocols.265.4.1 Bit Wise Cont
12、ention 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 Data Rates275.5.4 Failure Modes .275.5.4.1 Consequences of Failure286. Summary & Conclusions 28A
13、PPENDIX 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 limitations of using a twisted pairas the transmission medium.The choice of transmission medium is a large
14、 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 Networks:a. Class ALow-Speed Body Wiring and Control Functions, i.e., Control of Exterior Lampsb. C
15、lass BData Communications, i.e., Sharing of Vehicle Parametric Datac. Class CHigh-Speed Real-Time Control, i.e., High-Speed Link for Distributed ProcessingSAE 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 Cl
16、ass A Network. That is, the Class B Bus must be capable of communications that would perform all of thefunctions of 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 mann
17、er, the Class C Bus isintended as a functional superset of the Class B Bus.1.3 Electromagnetic Susceptibility (EMS) 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
18、B Networks that use twisted pair operating at data rates below 50 kbps and methods have beenfound to overcome the communication 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 co
19、nnectordesign is used at the higher data rates. Also, if data communication requirements dictate transmission ratesabove 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
20、 Electromagnetic Interference (EMI) ConsiderationsA key concern is the generation of EMI when theClass C Vehicle Multiplexing 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 eng
21、ineerscommonly accept that either a fiber optic or a shielded cable will be required for the transmission medium atdata 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 s
22、ystems, will drive future development. These factors andother, as yet undefined, needs for Class C communication will eventually drive the implementation ofautomotive fiber optic systems for higher data transfer rates.2. References2.1 Applicable PublicationsThe following publications form a part of
23、this specification to the extent specifiedherein. Unless otherwise indicated, the latest issue of SAE publications 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
24、Henry W. Ott, Bell Laboratories, Noise Reduction Techniques in Electronic Systems, A Wiley-IntersciencePublications. Second Edition, 1988.2.2.2 CISPR/D/WG2 (Secretariat) 19, September 1989, International Electrotechnical Commission, InternationalSpecial Committee on Radio Interference (CISPR), Subco
25、mmittee D: Interference Relating to MotorVehicles and Internal Combustion Engines, Working Group 2, Test Limits and 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, SP
26、IE Vol. 468 Fibre Optics 84, pp. 174-185 (1984).2.2.4 W. A. Rogers, D. R. Kimberlin, and R. A. Meade, Soc. Automotive 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.
27、 Conf. Automotive Electronics IEE Vol. 229, pp. 160-164, 1983.SAE 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
28、.2.2.9 K. Sasai, Sitev Conf., 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. Stee
29、le, A Fiber Optic Connection 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
30、, Mitsubishi Rayon, Private 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
31、 each other is to reduce the 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
32、.3.1 Inherent Advantages/Disadvantages 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 TWIST
33、ED PAIR NETWORKSThe desire 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 d
34、eveloped. The failure modes 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 devic
35、ethat also contains the receiver 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
36、 known. The connector industryis 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 f
37、actors such as the maintenance 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 developme
38、nt is an extension of familiar 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 impeda
39、nce mismatching at the connector is a concern. Perhapsdevelopments in ribbon cabling techniques and insulation displacement connectors could improve thisimpedance matching situation.SAE J2056-3 Reaffirmed FEB2000-5-3.1.2 RADIATED LINE LOSSESThe biggest problem to overcome is the fact that for data r
40、ates 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 risetime was responsible for most of the EMI. The present automotive
41、 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 capacitance loading,impedance mismatching at the connector, main
42、tenance 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 pair without incurringexcessive losses or spectral distortion
43、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 longitudinal noise). At Class C data rates it is much more diffic
44、ultto 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 wavelength in length. Switching noise and spikes are broadband and
45、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. This wire harness resonance is too close to the filter cutof
46、f 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 for data rates above 100 kbps. Thefiltering techniques for r
47、eceiver 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 of twisted pair is a very strongadvantage. It can accommodate
48、 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 s
49、ignificant 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 transitions and is bit synchronizedso that invalid bit testing can be effective. Invalid bit testing has proven to play a large role in providing dataintegrity in a high EMI environment. PWM, for example, has two transitions per bit with 1/3 bit times betweentransitions. NRZ has a maximum of one transition per bit but without the added overhead of synchronizingtra
