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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2.5Mbps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、+3.3V、 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MAX3362: +3.3V、高速、 RS-485/RS-422收发器，采用SOT23封装MAX3280E-MAX3284E: 15kV ESD保护、52Mbps、+3V至 +5.5V、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、+3.3V、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/MAX3490E/MAX3491E: +3.3V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具有摆率限制，可以减小EMI，并降低由不恰当的终端匹配电缆引起的反射。图1给出了驱动器输出波形以及使用MAX481 MAX485 MAX490 MAX491或MAX1487发送150kHz信号时的傅立叶分析。有明显的大幅值高频谐波。图13给出了MAX483、 MAX487、MAX488或MAX489在相同条件下发送数据时的测试结果。 图2的高频谐波幅值要低得多，因此，从根本上抑制了EM图 1. MAX481/MAX485/MAX490/MAX491/MAX1487 150kHz号时的驱动器输出波形与FFT图图2 MAX483/MAX487-MAX489发送150kHz信号时的驱动器输出波形与FFT图低功耗关断模式(MAX481/MAX483/MAX487)RE高电平、DE低电平使器件进入低功耗关断模式。驱动器与接收器都被禁用时，器件才进入关断模式式下，器件吸取电源电流的典型值为0.1A。可以同时驱动RE和DE；只要RE为高电平且DE为低电平的持续时间小于50ns，可以确保这些器件不会进入关断模式。若两个输入端维持这种状态至少600ns，则可确保这些器件进入关断模式对于MAX481、MAX483与MAX487 ，tZH与tZL使能期间假定器件并未处于低功耗关断状态(MAX485/MAX488- MAX491以及MAX1487不能被关断).tZH(SHDN) 与tZL(SHDN)使能期间假定器件已被关断.在低功耗关断状态下激活驱动器和接收器 (tZH(SHDN)，t ZL(SHDN)要比在工作模式下激活驱动器和接收器（tZH,tZL）需要更长的时间 若RE、DE输入等于逻辑0、1，或1、1，或0、0，则这些器件处于工作模式。).com/bbs可以通过两种机制避免由故障或总线冲突引起的过高的输出电流与功耗。输出级的折返式电流限制在整个共模电压范围内提供短路保护。另外，当管芯温度上升过高时，热关断电路强制驱动器输出进入高阻态。传输延时许多数字编码方案都取决于驱动器与发送器传输延时的差别。图15-图18给出了采用图14测试电路得到的传输延时典型值。接收器延时时间差|tPLH-tPHL|对MAX481、MAX485以及MAX1487来说，其典型值小于13ns；对MAX483与MAX487-MAX489来说，其典型值小于100ns。驱动器偏移时间，对MAX481、MAX485、MAX490、MAX491以及MAX1487 来说，典型值为 最大值为5ns(最大值为10ns)；对MAX483 与MAX487-MAX489来说，其典型值为100ns （最大值为800ns)。模拟信号分析 模拟信号的调制信号的分析需要对特定测量取决于其电气特性的主要原理和成分,将获得的信号. 典型的分析包括对偶隔离,共模隔离,阻抗变化,剔除噪声，放大器 ，线形化处理，补偿和校准.在分散控制的趋势中, 信号分析电路和紧挨该传感器的电路板相结合. 信号适应完成在各自的测量的发射机, 并且它在完成信号适应系统的各种作用下使用插入式模块。插入式模块是安装在机架上可能接受4 个, 8 个, 或16 个插入式模块的插件框架。下列是一些典型的插入式模块的一个简要的描述。一个毫伏/热电偶模块所能承受的低水平的直流电压信号跨度低至2mV或高达 55mv. 该模块的零点可以调整为-5至25mv. 这个变压器模块变压的范围在DC和AC共模电压下为数百伏, 与其共同冷拒绝比率在60赫兹下为140分贝. 该单元提供噪音剔除,线性化冷端补偿,并打破检测热电偶的投入. 该模块可以提供给工厂的校正系数为指定零和-8.33%. 电阻温度探测器(RTD)的单元接受输入从100欧或10欧的RTD传感器.这个输入信号转化为一个高性能的输入信号,如4-20mA电流信号. 这个输入信号是通用型交流和直流电压的RTD传感器,引线补偿,减少噪音,线性化和损坏检查. 一个典型的例子是98%的反应时间是0.25s. 频率输入设备接受脉冲信号从数字式流速计、涡轮流量计, 和检测一系列的脉冲的其它传感器。输入脉冲的频率被转换成一个高级电压或当前的信号。频率的输入脉冲转换成一个高频的电压或电流信号. 频率范围是可选择的最低25 赫兹到最高12 千赫。98% 反应时间变化从10 s 为低频率范围对0.2 s 为高频率范围。余下的这一部分包括详细的各种信号调理作业 .隔离和阻抗变换 引述凯尔文阁下:能量守恒的选择 。考虑问题的测量温度的小型容器的温度计. 让我们假设水温为40,温度计是22 在测量之前温度发生变化. 其测试分为温度计在水中位置和读温度计在温度计开始测试和温度计达到平衡时的温度. 平衡温度取决于温度计和水的热容量的比率。如果热容量是相等的, 平衡温度的平均值将是31。无论平衡温度是多少, 它将是在少于40并且大于22这个范围 。因而测量水的温度行动改变了温度。目的之一隔离和阻抗变换是为了避免 或至少减少,摧毁,这是衡量 一个同样重要的目的是要保护测量电路,这是衡量 举例来说, 高共模电压可以轻易摧毁一个运算放大器电路除非这个电路是分离了高电压.的 阻抗变换是一种保护测量变量和测量仪器,由对方. 跟随电压在第6.3节介绍,这工作不错(参考第6.3进一步的细节). 当偶隔离要求,特别是放大器使用. 有两种方法可以用来获取电流隔离:变压器耦合和光纤耦合. 一个典型的隔离放大器的变压器耦合电容约为120分贝. 光学耦合利用光束来传送信号,从源头上电路的接收电路. 这使得能够将所有电器连接电路. 放大作用和模式对模式转换改变模拟信号的标准由或一个倒置的放大器、一个非倒置的放大器, 或一个有差别的放大器完成。有时它是必要转换信号从电压成电流或从电流成电压。电桥一定数量的主要元件转换变化在被测量的可变物上的变化在电阻的温度探测器是三个例子。电桥是测量变化的传统方法在元件的电阻。电桥的操作分成二类, 平衡的和被失衡的操作。在平衡的操作, 传感器的电阻是坚定的从价值为人所知以精确度三个其它电阻的值。在失衡的操作, 变化在传感器电阻上二者之间电压从基本的电阻值的产生一小部分的不同。在二者之间电压一个另外的放大器被使用在放大区。降低噪声控制系统存在在环境里用高水平电磁式能量被填装等待即在电信号线产生噪声。最佳的噪声降低系统是一个防止噪声曾经得到对信号。噪声预防包括仔细着陆信号线和保护缆绳保持信号一样免于噪声尽可能。尽管最佳的噪声预防措施是, 将一些噪声将出现在控制信号中。在信号被设计减少噪声的水平特别的电路叫做滤波器。实际上,滤波器可能被使用减少一切在一个指定范围(或带) 频率。期限低通过滤器、带通滤波器、高通滤波器, 和波谷过滤器等各种各样的过滤器根据他们准许通过未受影响频率的带。在继续之前, 我们应该停留片刻和考虑标志的频率分组。大家通晓的事实是我们能调定收音机接受不同的波段。各个波段被分配它叠加它的声音和音乐信号的载波频率。无线电天线接受载体信号从所有波段在它的近处, 但根据各条频器的位置，无线电接收机只对一个载体是敏感的一次,。您选择一个另外波段由移动条频器向一个另外位置。 （注：文档可能无法思考全面，请浏览后下载，供参考。可复制、编制，期待你的好评与关注）
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