1、 TIA TELECOMMUNICATIONS SYSTEMS BULLETIN Application Guidelines for TIA/EIA-485-A TSB-89-A (Revision of TSB-89) January 2006 TELECOMMUNICATIONS INDUSTRY ASSOCIATION The Telecommunications Industry Association represents the communications sector of NOTICE TIA Engineering Standards and Publications a
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19、EOF, AND THESE CONTENTS WOULD NOT BE PUBLISHED BY TIA WITHOUT SUCH LIMITATIONS. TSB-89-A Application Guidelines for TIA/EIA-485-A CONTENTS 1 INTRODUCTION 1 2 GLOSSARY OF TERMS AND DEFINITIONS . 1 3 SYSTEM DEFINITION . 3 3.1 GENERAL DESCRIPTION. 3 3.2 APPLICATIONS 4 3.3 ENVIRONMENT 4 3.4 SYSTEM DIAGR
20、AMS . 5 3.5 INTERFACE DEFINITION 8 4 ELECTRICAL CHARACTERISTICS . 8 4.1 INTERCONNECTING MEDIA 8 4.2 BUS LOADING 13 4.3 NOISE BUDGETING 19 4.4 FAULT CONDITIONS . 20 5 SYSTEM SPECIFICATION REQUIREMENTS 22 6 LIST OF REFERENCES 23 List of Figures FIGURE 1 - GENERAL 485 SYSTEM CONFIGURATION. 5 FIGURE 2 -
21、 485 BUS WITH STUB CABLES OFF A MAIN BACKBONE CABLE 6 FIGURE 3 - POINT-TO-POINT CONNECTION 6 FIGURE 4 - ONE DRIVER AND MULTIPLE RECEIVERS (MULTIDROP). 6 FIGURE 5 - ONE RECEIVER AND MULTIPLE DRIVERS. . 7 FIGURE 6 - STAR CONFIGURATION (NOT RECOMMENDED) 7 FIGURE 7 - 485 BACKPLANE EXAMPLE 7 FIGURE 8. -
22、485 BUS DC CIRCUIT MODEL 9 FIGURE 9 - MAXIMUM LINE LENGTH VERSUS SIGNALING RATE. . 11 FIGURE 10 - SIMPLE DIFFERENTIAL TERMINATION. 12 FIGURE 11 - CAPACITIVE COUPLING OF THE TERMINATION RESISTOR. 12 FIGURE 12 - LOW COMMON-MODE IMPEDANCE TERMINATION 13 FIGURE 13 - MAXIMUM NUMBER OF UNIT LOADS VERSUS C
23、OMMON-MODE VOLTAGE AND REQ14 FIGURE 14 - DETERMINING IF LOAD REPRESENTS A LUMPED CIRCUIT TO THE 485 BUS 16 FIGURE 15. - NOISE MODEL FOR A 485 CIRCUIT 17 FIGURE 16. - IDLE-LINE FAILSAFE BIAS CIRCUIT. 20 FIGURE 17. - OPEN-CIRCUIT FAILSAFE BIAS CIRCUIT . 21 FIGURE 18. - SHORTED-LINE FAILSAFE BIAS CIRCU
24、IT. 21 TSB-89-A FOREWORD This document was prepared as part of the A revision to TIA/EIA-485 to separate applications information that was interspersed in the standard and to embellish it. As the working group proceeded, it became clear the effort would have grown into a book in an attempt to cover
25、all of the application aspects. The results are necessarily a compromise between thoroughness and time spent. It is the hope of this group that this bulletin provides sufficient guidance to apply TIA/EIA-485-A and to add to this document in the future. TSB-89-A 11 Introduction This engineering publi
26、cation provides guidelines for applying circuits complying with TIA/EIA-485-A, referred to as 485 hereafter, to form a balanced multipoint data bus. The versatility of the 485 electrical standard covers a wide variety of data interchange applications all of which this publication cannot cover. The i
27、ntent is to provide basic design guidelines of the physical layer. In applying the drivers and receivers defined in 485, the reader should keep several important considerations in mind. The first consideration is the actual configuration of the system with regard to the number of drivers and receive
28、rs, the operating speed of the system, the method of interconnecting the equipment, and the system margin. The implementer should consider performance capabilities of the equipment in establishing the margin allotments. The referencing standard should specify these requirements. 2 Glossary of terms
29、and definitions Balanced is a description of two circuits that have identical electrical properties such that when driven with signals of equal magnitude but in opposite directions there is no ac common-mode energy in the two circuits (see Differential mode). Common-mode Voltage is one-half of the v
30、ector sum of the voltages between each conductor of a balanced interchange circuit and ground. The common-mode voltage is the sum of ground potential difference, driver common-mode output voltage (generator offset voltage), and longitudinally coupled noise. Common-Mode is the shared signal component
31、 of two or more signals to a common reference point. Mathematically, the common-mode signal is the arithmetic mean of the signal amplitudes. Data Signaling Frequency is12T, where T is the minimum unit interval and uses the units Hz (Hertz). (It is generally the largest magnitude frequency component
32、of the signals Fourier series representation.) Data-Signaling Rate is 1Twhere T is the minimum unit interval and uses in the unit bit/s (bits per second). Its value is also the same as the clock frequency in a single-edge sampled synchronous transmission. Data Transfer Rate is the number of desired
33、bits of data received per unit time. It may be different from the data-signaling rate, which uses the same units. Differential Mode is a signaling mode that uses one of a driven signal pair as the zero potential reference. Distributed-Parameter circuit is a model of an electrical circuit with large
34、physical dimensions with respect to the wavelength of the input signal. Electromagnetic Compatibility (EMC) is the modern term describing the absence of conducted and radiated emissions or susceptibility that exceed specified limits or cause system performance degradation. TSB-89 2Electromagnetic In
35、terference (EMI) replaced RFI in the 1950s and presently describes conducted and radiated emissions that are above specified limits or cause system performance degradation. Eye Pattern is a measure of the data signal transmission path quality using an overlay pattern of random data signal transition
36、s. Generator (See Line Driver) Ground Potential Difference is the difference between the signal ground potential between the active generator and a receiver of an interchange circuit. Longitudinally Coupled Noise Voltage is an unwanted voltage coupled inductively or capacitively between any two poin
37、ts along the balanced interconnecting cable. Hysteresis is the difference between the positive- and negative-going receiver input voltage thresholds. Input Sensitivity is the minimum input signal voltage that a receiver detects. Input Threshold Voltage is the input voltage that causes a receiver to
38、change state. Inter-Symbol Interference is the time displacement of a state transition due to a new wave (subsequent signal) arriving at the receiver site before the previous wave has reached its final value. Jitter is the time variation of a significant time instance of a signal. Line Driver is the
39、 component of an interchange circuit that is a source of the transmitted signal (used interchangeably with generator). Line Receiver is the component of an interchange circuit that provides for the detection of interchange circuit signals and indicates the logical state of the bus to the receiving e
40、quipment. Line Transceiver is a line driver and receiver with driver outputs and receiver inputs connected together at the bus interface. Lumped-Parameter Circuit is a model for an electrical circuit with small physical dimensions with respect to the wavelength of the input signal. Multidrop is a da
41、ta bus structure that has one transmitting and two or more receiving connections. Multipoint is a data bus structure that has two or more transmitting and any number of receiving connections. Node is a point at which conductors from two or more circuit elements join. TSB-89-A 3Off-state Output is an
42、 output of a line driver that does not actively drive the normal operating load to a minimum level for either logical state. (Commonly referred to as a high-impedance state) Party line (See Multipoint) Point-to-Point is a data bus structure that has one line driver and one receiver on it. Propagatio
43、n Delay is the time it takes for the output of a circuit to respond to an input signal. Radiated Emissions (RE) are emissions that originate within equipment or its associated cabling and transmit unguided to the external environment via electromagnetic waves. Radiated Susceptibility (RS) is an unde
44、sirable equipment response that appears on the equipment output because of electromagnetic waves being impressed upon the equipment. Radio Frequency Interference (RFI) (See EMI) Significant Time Instance is the time when a time-varying signal achieves a level or range of levels that signifies a logi
45、c state change. Single-ended Mode is a signaling mode that uses a single driven line. Skew is a time difference between significant instances of different signals or the time difference between significant time instances on the same signal (sometimes referred to as pulse skew). Transmission Line is
46、an electrical model for a distributed parameter interchange circuit. Unbalanced is a signaling method that is not balanced. (Also known as Single-ended Mode) Unit Interval is T, where T is the minimum time interval that can occur between any desired logic state changes in a binary signal (the signal
47、 pulse width). 3 System definition 3.1 General description A 485 bus will normally consist of multiple communication controllers in separate chassis and power domains connected via shielded twisted-pair cabling. There may be one or more signal pairs in the cable each having multiple drivers, receive
48、rs, or transceivers depending upon the application requirements. The signal return path is through the earth ground connection at each chassis or through a wired ground in the cable. It is generally undesirable to have both return paths. Cable routing is generally a daisy chain (direct run from chas
49、sis to chassis) although some systems may allow stub cabling from a main backbone cable. Each signal pair will normally have a resistor, equal to the characteristic impedance between the pair, at the extreme ends of the backbone. TSB-89 43.2 Applications Applications requiring an economical rugged interconnection between two or more computing devices employ 485 drivers, receivers, or transceivers in data transmission circuits. The low-noise coupling of balanced signaling with twisted-pair cabling and the wide common-mode voltage range of 485 allows data exchange at data-signaling ra
