机械电子专业毕业设计英文翻译.doc

Low-Power, Slew-Rate-Limited RS-485/RS-422 TransceiversGeneral DescriptionThe MAX481, MAX483, MAX485, MAX487MAX491, andMAX1487 are low-power transceivers for RS-485 and RS-422 communication. Each part contains one driver and one receiver . The MAX483, MAX487, MAX488, and MAX489 feature reduced slew-rate drivers that minimize EMI and reduce reflections caused by improperly terminated cables ,thus allowing error-free data transmission up to 250kbps. The driver slew rates of the MAX481, MAX485, MAX490,MAX491, and MAX1487 are not limited, allowing them to transmit up to 2.5Mbps.These transceivers draw between 120A and 500A of supply current when unloaded or fully loaded with disabled drivers . Additionally, the MAX481, MAX483, and MAX487have a low-current shutdown mode in which they consume only 0.1A. All parts operate from a single 5V supply .Drivers are short-circuit current limited and are protected against excessive power dissipation by thermal shutdown circuitry that places the driver outputs into a high-impedance state. The receiver input has a fail-safe feature that guarantees a logic-high output if the input is open circuit .The MAX487 and MAX1487 feature quarter-unit-load receiver input impedance, allowing up to 128 MAX487/MAX1487 transceivers on the bus. Full-duplex communications are obtained using the MAX488MAX491, while the MAX481, MAX483, MAX485, MAX487, and are designed for half-duplex applications.ApplicationsLow-Power RS-485 TransceiversLow-Power RS-422 TransceiversLevel TranslatorsTransceivers for EMI-Sensitive ApplicationsIndustrial-Control Local Area NetworksFeaturesIn MAX Package: Smallest 8-Pin SOSlew-Rate Limited for Error-Free DataTransmission (MAX483/487/488/489)0.1A Low-Current Shutdown Mode(MAX481/483/487)Low Quiescent Current:120A (MAX483/487/488/489)、230A (MAX1487)、300A (MAX481/485/490/491)-7V to +12V Common-Mode Input Voltage RangeThree-State Outputs30ns Propagation Delays, 5ns Skew (MAX481/485/490/491/1487)Full-Duplex and Half-Duplex Versions AvailableOperate from a Single 5V SupplyAllows up to 128 Transceivers on the Bus(MAX487/MAX1487)Current-Limiting and Thermal Shutdown forDriver Overload ProtectionThe MAX481/MAX483/MAX485/MAX487MAX491 and MAX1487 are low-power transceivers for RS-485 and RS-422 communications. The MAX481, MAX485, MAX490,MAX491, and MAX1487 can transmit and receive at data rates up to 2.5Mbps, while the MAX483, MAX487,MAX488, and MAX489 are specified for data rates up to 250kbps. The MAX488MAX491are full-duplex transceivers while the MAX481, MAX483,MAX485,MAX487,and MAX1487 are half-duplex. In addition, Driver Enable pins are included on the (DE) and Receiver Enable (RE) MAX481, MAX483, MAX485, MAX487, MAX489,MAX491, and MAX1487. When disabled, the driver and receiver outputs are high impedance. 128 Transceivers on the Bus。The 48k, 1/4-unit-load receiver input impedance of the MAX487 and MAX1487 allows up to 128 transceivers on a bus, compared to the 1-unit load (12k input impedance) of standard RS-485 drivers (32 transceivers maximum). Any combination of MAX487/MAX1487 and other RS-485 transceivers with a total of 32 unit loads or less can be put on the bus. TheMAX481/MAX483/MAX485 and MAX488MAX491 have standard 12k Receiver Input impedance.Test Circuits Driver DC Test Load Receiver Timing Test Load Driver/Receiver Timing Test Circuit Driver Timing Test LoadMAX483/MAX487/MAX488/MAX489:Reduced EMI and ReflectionsThe MAX483 and MAX487MAX489 are slew-rate limited, minimizing EMI and reducing reflections caused by improperly terminated cables. Figure 1 shows the driver output waveform and its Fourier analysis of a 150kHz signal transmitted by a MAX481, MAX485,MAX490, MAX491, or MAX1487. High-frequency harmonics with large amplitudes are evident. Figure 13 shows the same information displayed for a MAX483,MAX487, MAX488, or MAX489 transmitting under the same conditions. Figure2s high-frequency harmonics have much lower amplitudes, and the potential for EMI is significantly reduced. Figure 1 Driver Output Waveform and FFT Plot of MAX481/MAX485/MAX490/MAX491/MAX1487 Transmitting a 150kHzSignal Figure 2. Driver Output Waveform and FFT Plot of MAX483/MAX487MAX489 Transmitting a 150kHz SignalLow-Power Shutdown Mode(MAX481/MAX483/MAX487)A low-power shutdown mode is initiated by bringing both RE high and DE low. The devices will not shut down unless both the driver and receiver are disabled .In shutdown, the devices typically draw only 0.1A of supply current . REand DE may be driven simultaneously; the parts are guaranteed not to enter shutdown if RE is high and DE is low for less than 50ns. If the inputs are in this state for at least 600ns, the parts are guaranteed to enter shutdown.For the MAX481, MAX483, and MAX487, the t ZH and t ZL enable times assume the part was not in the low-power shutdown state (the MAX485/MAX488MAX491 and MAX1487 can not be shut down). The tZH(SHDN) and t enable times assume the parts were shut ZL(SHDN) .down (see Electrical Characteristics). It takes the drivers and receivers longer to become enabled from the low-power shutdown state (t ), t ) than from the operating mode (tzH,Tzl) .(The parts are in operating mode if the RE ,DE inputs equal a logical 0,1 or 1,1 or 0, 0.)Driver Output Protection Excessive output current and power dissipation caused by faults or by bus contention are prevented by two mechanisms. A fold back current limit on the output stage provides immediate protection against short circuits over the whole common-mode voltage range (see Typical Operating Characteristics). In addition, a thermal shutdown circuit forces the driver outputs into a high-impedance state if the die temperature rises excessively . Propagation Delay Many digital encoding schemes depend on the difference between the driver and receiver propagation delay times. Typical propagation delays are shown in Figures 1518 using Figure 14s test circuit.The difference in receiver delay times, | t PLH - t PHL|, is typically under 13ns for the MAX490, MAX491, and MAX1487 and is typically less than 100ns for the MAX483 and MAX487MAX489.The driver skew times are typically 5ns (10ns max) for the MAX481, MAX485, MAX490, MAX491, and MAX1487, and are typically 100ns (800ns max) for the MAX483 and MAX487MAX489. Analog signal conditioningThe type of signal conditioning required for a particular measurement depends on the electrical characteristics of the primary element and the component that will receive the signal .Typical conditioning includes galvanic isolation, common-mode isolation , impedance transformation , amplification , noise reduction , signal conversion , linearization , compensation ,and calibration . In the trend toward distributed control , the signal conditioning circuitry has moved close to the sensor on the process floor . Signal conditioning is done in the individual measuring transmitters , and it is done in signal conditioning systems using plug-in modules for various functions . The plug-in modules are rack mounted in card cages that can accept 4 , 8 , or 16 plug-in modules . The following is a brief description of some typical plug-in modules .A millivolt / thermocouple module accepts low-level dc voltage signals with a span as low as 2 mV or as high as 55 mV . The zero can be adjusted from -5 to 25 mV . The module converts isolation from dc and ac common-mode voltage of several hundred volts , and a common-cold rejection ratio of 140 dB at 60 Hz . The module provides noise rejection , linearization , cold junction compensation , and break detection for thermocouple inputs. The module can be factory calibrated for a specified zero and span.A resistance temperature detector (RTD)module accept input from 100- platinum or 10-copper RTD sensors . The input signal is converted to a high-level input such as a 4-to 20-mA current signal .The input signal is common-mode isolation from ac and dc voltages RTD sensor ,lead wire compensation , noise reduction ,linearization , and break detection . A typical 98% response time is 0.25 s.A frequency input module accepts pulse signals from digital tachometers . turbine flow meters , and other sensors that produce a series of pulses . The frequency of the input pulses is converted to a high-level voltage or current signal . The frequency range is selectable from as low as 25 Hz to as high as 12 KHz . The 98% response time varies from 10 s for the low-frequency range to 0.2 s for the high-frequency range .The remainder of this section covers details of various signal conditioning operations .Isolation and Impedance ConversionTo quote Lord Kelvin:“The act of measuring destroys that which is measured.”Consider the problem of measuring the temperature of a small container of water with a thermometer .Lets assume that the water temperature is 40 and the temperature of the thermometer is 22 before the measurement takes place .The measurement consists of placing the thermometer in the water and reading the thermometer after a temperature equilibrium has been reached between the water and the thermometer .The equilibrium temperature depends on the ratio of the heat capacities of the thermometer and the water . If the heat capacities are equal, the equilibrium temperature will be 31, the average .Whatever the equilibrium temperature is , it will be less than 40 and greater than 22. Thus the act of measuring the temperature of the water has changed the temperature .One purpose of isolation and impedance conversion is to avoid , or at least minimize ,the destruction of “that which is measured.” An equally important purpose is to protect the measuring instrument from “that which is measured .”For example , a high common-mode voltage can easily destroy an op-amp circuit unless the circuit is isolated from the high voltage . Impedance transformation is one method of protecting the measured variable and the measuring instrument from each other .The voltage follower , introduced in Section 6.3 , does this job quite well (refer to Section 6.3 for further details). When galvanic isolation is required , special amplifier are used . There are two methods that can be used to obtain galvanic isolation: transformer coupling and optical coupling . A typical isolation amplifier with transformer coupling has a capacitance of about 120 dB . Optical coupling uses a light beam to transmit the signal from the source circuit to the receiver circuit . This makes it possible to remove all electrical connections between the circuits .Amplification and Analog-to-Analog ConversionChanging the level of analog signal is accomplished by either an inverting amplifier , a non-inverting amplifier , or a differential amplifier . Sometimes it is necessary to convert the signal from a voltage to a current or from a current to a voltage . Bridge CircuitsA number of primary elements convert changes in the measured variable into small changes in the resistance temperature detectors are three examples .A bridge circuit is the traditional method of measuring small changes in the resistance of an element . The operation of a bridge falls into two categories , balanced and unbalanced operation . In the balanced operation , the resistance of the sensor is determined from the values of three other resistors whose values are known with precision . In the unbalanced operation , the change in the sensor resistance from a base value produces a small different between two voltage . A different amplifier is used to amplify the difference between the two voltage .Noise ReductionControl system exist in an environment filled with high levels of electromagnetic energy just waiting to produce noise in electric signal lines . The best noise reduction system is one that prevents the noise from ever getting to the signal . Noise prevention consists of careful grounding of signal lines and shielding of cables to keep the signals as free of noise as possible . Despite the best noise prevention effort , some noise will appear in the control signals . Special circuits called filters are designed to reduce the level of the noise in the signals . Actually , filters can be used to reduce everything in a specific range (or band ) of frequencies . The terms low-pass filter , band-pass filter ,high-pass filter , and notch filter name various filters according to the band of frequencies they allow to pass through unaffected .Before going on , we should pause a moment and consider the frequency components of a sign . Everyone is familiar with the fact that we can tune a radio to receive different stations . Each station is assigned a carrier frequency on which it superimpose its voice and music signals . A radio antenna receives the carrier signals from all stations in its vicinity , but the radio receiver is sensitive to only one carrier at a time , depending on the position of the tuner . You select a different station by moving the tuner to a different position .低功耗、限摆率RS-485/RS-422收发器概述MAX481 、MAX483 、MAX485 、MAX487-MAX491以及MAX1487是用于RS-485 与RS-422 通信的低功耗收发器，每个器件中都具有一个驱动器和一个接收器MAX487、MAX488、 MAX489以及MAX483具有限摆率驱动器，可以减小EMI，并降低由不恰当的终端匹配电缆引起的反射,实现最高250kbps的无差错数据传输。MAX481 、MAX485、 MAX490、 MAX491、 MAX1487的驱动器摆率不受限制，可以实现最高的传输速率。这些收发器在驱动器禁用的空载或满载状态下，吸取的电源电流在120(A至500(A 之间。另外，MAX481与MAX483、MAX487 具有低电流关断模式，仅消耗单电源下0.1A 。所有器件都工作在5V单电源下。驱动器具有短路电流限制，并可以通过热关断电路将驱动器输出置为高阻状态，防止过度的功率损耗。接收器输输入具有失效保护特性，当输入开路时，可以确保逻辑高电平输出。MAX487与MAX1487具有四分之一单位负载的入阻抗，使得总线上最多可以有 个128 MAX487/ MAX1487收发器。使用MAX488-MAX491可以实现全双工通信，而MAX481 、MAX483、 MAX485 与MAX487则为半双工应用设计。应用低功耗 收发器RS-485低功耗 收发器RS-422电平转换器用于敏感应用的收发器EMI工业控制局域网容错应用MAX3430: ±80V故障保护、失效保护、1/4单位负载、 RS-485收发器MAX3440E-MAX3444E: ±15kV ESD保护、±60V故障保护、10Mbps、失效保护RS-485/J1708收发器对于空间受限应用MAX3460-MAX3464: +5V、失效保护、 20Mbps、Profibus RS-485/RS-422 收发器、高速、 RS-485/RS-422收发器，采用SOT23封装MAX3280E-MAX3284E: ±15kV ESD保护、52Mbps、+3V至、SOT23、RS-485/RS-422、真失效保护接收器MAX3293/MAX3294/MAX3295: 20Mbps、 +3.3V 、SOT23 、RS-485/RS-422发送器对于多通道收发器应用MAX3030E-MAX3033E: ±15kV ESD保护、四路RS-422发送器对于失效保护应用MAX3080-MAX3089: 失效保护、高速(10Mbps)、限摆率RS-485/RS-422收发器对于低电压应用MAX3483E/MAX3485E/MAX3486E/MAX3488E/供电、 ±15kV ESD保护、12Mbps 、限摆率、真正的RS-485/RS-422收发器MAX481/MAX483/MAX485/MAX487-MAX491以及MAX1487是用于RS-485 与RS-422 通信的低功耗收发器。MAX481、MAX485、MAX490、MAX491以及MAX1487能够以最高2.5Mbps的数据速率发送并接收数据；而MAX483 、MAX487、 MAX488以及MAX489 则用于最高250kbps的数据速率。MAX488-MAX491是全双工收发器，MAX481 、MAX483 、MAX485 、MAX487以及MAX1487是半双工收发器。 另外，MAX481、MAX483、MAX485 MAX487 、MAX489、MAX491以及MAX1487中包含驱动器使能(DE)与接收器使能(RE)控制引脚，被禁用时，驱动器或接收器输出为高阻态。与标准RS-485驱动器(最多32个收发器)的单位负载(12k输入阻抗)相比，MAX487 与MAX1487具有48k输入电阻，1/4单位负载的接收器输入阻抗，在一条总线上允许最多挂接128个收发器。MAX487/MAX1487与其他RS-485收发器的任意组合可以允许32个收发器或更少的收发器连接在同一条总线上。MAX481/MAX483/MAX485与MAX488-MAX491具有标准的12k接收器输入阻抗。测试电路MAX483/MAX487/MAX488/MAX489:降低EMI和反射MAX483以及MAX487-MAX489