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RS-422 & RS-485 Translation

Chapter 1: Overview

Introduction



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The purpose of this application note is to describe the main elements of an RS-422 and RS-485 system. This application note attempts to cover enough technical details so that the system designer will have considered all the important aspects in his data system design. Since both RS-422 and RS-485 are data transmission systems that use balanced differential signals, it is appropriate to discuss both systems in the same application note. Throughout this application note the generic terms of RS-422 and RS-485 will be used to represent the EIA/TIA-422 and EIA/TIA-485 Standards.
 
本应用笔记的目的是描述RS-422和RS-485系统的主要原理。这应用笔记试图包括足够的技术细节以便系统设计师在数据系统设计时能考虑到所有重要方面。因RS-422和RS-485再者皆是使用平衡差分信号的数据传输系统,故将它们放在同一应用笔记中讨论是适宜的。本应用笔记涉及到的RS-422 和RS-485通用术语都被用于表述EIA/TIA-422和EIA/TIA-485标准。
 

Data Transmission Signals

Unbalanced Line Drivers


不平衡线路驱动器

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Each signal that transmits in an RS-232 unbalanced data transmission system appears on the interface connector as a voltage with reference to a signal ground. For example, the transmitted data (TD) from a DTE device appears on pin 2 with respect to pin 7 (signal ground) on a DB-25 connector. This voltage will be negative if the line is idle and alternate between that negative level and a positive level when data is sent with a magnitude of ±5 to ±15 volts. The RS-232 receiver typically operates within the voltage range of +3 to +12 and -3 to -12 volts as shown in Figure 1.1.
 
在不平衡数据传输系统RS-232中传输的各个信号，出现在接口连接器的电压是以信号地为参考的。例如, 对于DB-25连接器，从数据终端设备DTE设备传送的数据(TD)是2脚对应信号地7脚（的电压）。（该引脚的电平），如在总线空闲状态，其电压为负，如在数据发送状态，其电压则在±5 t～±15伏的正电平和负电平之间反复切换。如图1所示，RS-232接收器的典型工作电压为+3～+12V和-3～-12V以内。



Figure 1.1: RS-232 Interface Circuit
图1.1：RS-232接口电路

Balanced Line Drivers


平衡线路驱动器

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In a balanced differential system the voltage produced by the driver appears across a pair of signal lines that transmit only one signal. Figure 1.2 shows a schematic symbol for a balanced line driver and the voltages that exist. A balanced line driver will produce a voltage from 2 to 6 volts across its A and B output terminals and will have a signal ground (C) connection. Although proper connection to the signal ground is important, it isn't used by a balanced line receiver in determining the logic state of the data line. A balanced line driver can also have an input signal called an “Enable” signal. The purpose of this signal is to connect the driver to its output terminals, A and B. If the “Enable” signal is OFF, one can consider the driver as disconnected from the transmission line. An RS-485 driver must have the “Enable” control signal. An RS-422 driver may have this signal, but it is not always required. The disconnected or "disabled" condition of the line driver usually is referred to as the “tristate1” condition of the driver.
 
平衡差分系统中信号电压，则由出现在传输一个信号的一对信号线的驱动器提供。如图1.2所示的就是平衡线路驱动器的原理符号和线路上所存在的电压。平衡线路驱动器将产生一个2～6V的电压在线路的A和B两个输出端，且有一信号地C的连接。虽然适当地接信号地接很重要，但它并不是用来决定数据线上平衡线路接收器的逻辑状态的。一个平衡线驱动器也有一个被称为“使能(Enable)”的输入信号，该信号的目的控制驱动器的输出端 A和B。如果使能信号(Enable)为关闭(状态)，就可以认为驱动器已从传输线上退出(分开)。RS-485与RS-422不同，RS-485的驱动器必须有一个“使能(Enable)”的控制信号，而RS-422驱动器可以有这个信号，但并非必须要的。驱动器在被分离或“禁止”的情况下，通常指的就是驱动器的“第三态”的情形。

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1 The term “tristate” comes from the fact that there is a third output state of an RS-485 driver, in addition to the output states of “1” and “0.”
 
术语“第三态”源自这样的事实：RS-485驱动器输出除“0”和“1”之外的第三种状态。



Figure 1.2: Balanced Differential Output Line Driver
图1.2：平衡差分输出驱动器
图注：关于使能端，对RS-422是不一定必要的(Optional for RS-485)；对RS-485则是必须的(Required for RS-485)。
注意：图中的共模电压范围并不代表RS-422或RS-485任何一种许可的共模电压范围，此处只是起到说明什么是“Balanced Differential Output Line Driver”的作用。实际上，RS-422的共模电压范围是-7V≤Vcm≤+7V；而RS-485的共模电压范围则是-7V≤Vcm≤+12V。参见文章附录A中所列出的数据表。

Balanced Line Receivers


平衡线路接收器

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A balanced differential line receiver senses the voltage state of the transmission line across two signal input lines, A and B. It will also have a signal ground (C) that is necessary in making the proper interface connection. Figure 1.3 is a schematic symbol for a balanced differential line receiver. Figure 1.3 also shows the voltages that are important to the balanced line receiver. If the differential input voltage Vab is greater than +200 mV the receiver will have a specific logic state on its output terminal. If the input voltage is reversed to less than -200 mV the receiver will create the opposite logic state on its output terminal. The input voltages that a balanced line receiver must sense are shown in Figure 1.3. The 200 mV to 6 V range is required to allow for attenuation on the transmission line.
 
平衡差分线路接收器感测的是两个信号输入端上的传输线A和B的电压状态。它也有信号接地C，这是做适当的界面衔接所必须的。图1.3是一条平衡差分线路接收器的原理符号。图中显示了平衡线路接收器电压的重要性。如果有差分输入电压Vab 大于+200 mV，接收器的输出端将有一个明确的逻辑状态，反过来，如果输入电压小于-200 mV，接收器的输出端将产生相反逻辑状态。一个平衡线路接收器必须能够感测出如图1.3所示的输入电压。因必须要允许传输线的衰减，故而(输入电压要有200mV～6V的范围。
 



Figure 1.3: Balanced Differential Input Line Receiver
图1.3：平衡差分输入线路接收器

EIA Standard RS-422 Data Transmission



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The EIA Standard RS-422-A entitled “Electrical Characteristics of Balanced Voltage Digital Interface Circuits” defines the characteristics of RS-422 interface circuits. Figure 1.4 is a typical RS-422 four-wire interface. Notice that five conductors are used. Each generator or driver can drive up to ten (10) receivers. The two signaling states of the line are defined as follows:
 
已授权的“平衡电压数字接口电路的电气特性”EIA RS-422-A标准定义了RS-422接口电路的特性。图1.4是一个典型的RS-422四线接口，请注意这里要用到五根导体(导线)，各发生器或驱动器能驱动多达十个接收器。导线的两个信号的状态定义如下:

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a. When the “A” terminal of the driver is negative with respect to the “B” terminal, the line is in a binary 1 (MARK or OFF) state.

b. When the “A” terminal of the driver is positive with respect to the “B” terminal, the line is in a binary 0 (SPACE or ON) state.
 
A、当驱动器的A端相对于B端为负时，线路为一个二进制1(MARK or OFF)的状态。

B、当驱动器的A端相对于B端为正时，线路为一个二进制0(SPACE or ON)的状态。

［译者语］这里MARK 和SPACE怎样译呢？

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Figure 1.5 shows the condition of the voltage of the balanced line for an RS-232 to RS-422 converter when the line is in the “idle” condition or OFF state. It also shows the relationship of the “A” and “B” terminals of an RS-422 system and the “-“ and “+” terminal markings used on many types of equipment. The “A” terminal is equivalent to the “-“ designation, and the “B” terminal equivalent to the “+” designation. The same relationship shown in Figure 1.5 also applies for RS-485 systems. RS-422 can withstand a common mode voltage (Vcm) of ±7 volts.  Common mode voltage is defined as the mean voltage of the A and B terminals with respect to signal ground.
 
如图1.5所示，是线路处于空闲情况或关断状态OFF时的RS-232到RS-422转换器平衡线路电压情况。它也表明了RS-422系统的终端A和B的关系以及用于许多类型装置的＋端和－端标识的关系。A端等同于“－”的标识，而B端则等同于“＋”端的标识。图1.5所示的关系，对RS－485系统也是一样的。 RS-422可以抵御±7V的共模电压，共模电压定义为AB两端对应于信号地的“the mean voltage”。

［译者语］麻烦，还真想不出“the mean voltage”应该怎样译？！



Figure 1.4: Typical RS-422 Four Wire Network
图1.4：典型的4线制RS－422网络



Figure 1.5: Relationship between EIA Standard “A” and “B” terminals on RS-422 or RS-485 Device and “+”and “-” Identification Convention
图1.5：EIA RS-422或RS-485装置的AB端的关系以及＋、－端的关系
【图注】 NOTE： Under “idle” conditions it is possible to determine with terminal is “A” and which is “B” //务必注意：只有在空闲状态下才能确定哪一端是A端或B端// Note: Under 'idle' conditions it is possible to determine which terminal is 'A' and which is 'B'

 


EIA Standard RS-485 Data Transmission


EIA标准RS－485数据传输

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The RS-485 Standard permits a balanced transmission line to be shared in a party line or multidrop mode.  As many as 32 driver/receiver pairs can share a multidrop network. Many characteristics of the drivers and receivers are the same as RS-422. The range of the common mode voltage Vcm that the driver and receiver can tolerate is expanded to +12 to -7 volts. Since the driver can be disconnected or tristated from the line, it must withstand this common mode voltage range while in the tristate condition.  Some RS-422 drivers, even with tristate capability, will not withstand the full Vcm voltage range of +12 to -7 volts.
 
RS-485允许一条平衡传输线共享一条公共电话线或使用多节点模式，一个多节点网络可以有多达32个节点共享一个多站点网络。RS-485接收器和发送器的许多特性和RS-422是一样的。它（RS-485接收器和发送器）容许的共模电压范围Vcm允许扩展到+12～-7V。因为RS-485的发送器/驱动器可以从总线上分离或以第三态退出，所以，它(接收器)必须能够第三态时抵御得住这样的共模电压范围。而有的RS-422驱动器虽然可能也有第三态的能力，但并非抵御+12～-7V的全部共模电压范围。
［译者语］to be shared in a party line的意思应当是不与电话的使用发生冲突。

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Figure 1.6 shows a typical two-wire multidrop network.  Note that the transmission line is terminated on both ends of the line but not at drop points in the middle of the line. Termination should only be used with high data rates and long wiring runs. A detailed discussion of termination can be found in Chapter 2 of this application note. The signal ground line is also recommended in an RS-485 system to keep the common mode voltage that the receiver must accept within the -7 to +12 volt range. Further discussion of grounding can be found in Chapter 3 of this application note.
 
如图1.6所示的典型2线制多节点网络。请注意传输线终止于线路的两端而不是中止于线路的中间。Termination should only be used with high data rates and long wiring runs.关于终端的详细描述，可到本应用笔记的第二章中查阅。RS-485系统推荐的接收器信号地线共模电压也必须在-7V～+12V范围之内，关于更详细的接地论述，可查阅本应用笔记的第三章。
［译者语］Termination should only be used with high data rates and long wiring runs.由于不是很理解它的意思，没法译(现在的理解是：终端电阻只用于数据速度高和使用长电缆的场合。不知道有没有局限性？)。但关于一词，有这样的解释供参考：The process of selecting and connecting resistors to “terminate” a line or cable.(选择并连接电阻以“终止”线路或电缆的过程)。

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An RS-485 network can also be connected in a four-wire mode as shown in Figure 1.7. Note that four data wires and an additional signal ground wire are used in a “four-wire” connection. In a four-wire network it is necessary that one node be a master node and all others be slaves. The network is connected so that the master node communicates to all slave nodes. All slave nodes communicate only with the master node. This network has some advantages with equipment with mixed protocol communications. Since the slave nodes never listen to another slave response to the master, a slave node cannot reply incorrectly to another slave node.
 
RS-485可以连接成如图1.7所示的4线制模式。请注意4线制连接的4条数据线和另外的一条信号地线是一并使用的。在四线制网络里，必须有一个节点中主节点而其余的为从节点。网络接成这样，为的是让主节点对所有从节点进行通讯，而所有从节点是只对主节点进行通讯。以固定协议通讯，这样的网络也有一些优点。因为从节点从不收听其它从节点而只对主节点作为出响应，从节点不能错误地对其它从节点作出应答。



Figure 1.6: typical RS-485 two wire multidrop network
图1.6：典型的2线制RS－485多节点网络


Figure 1.7: typical RS-485 four wire multidrop network
图1.7：典型的4线制RS－485多节点网络

Tristate Control of an RS-485 Device using RTS


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As discussed previously, an RS-485 system must have a driver that can be disconnected from the transmission line when a particular node is not transmitting. In an RS-232 to RS-485 converter or an RS-485 serial card, this may be implemented using the RTS control signal from an asynchronous serial port to enable the RS-485 driver. The RTS line is connected to the RS-485 driver enable such that setting the RTS line to a high (logic 1) state enables the RS-485 driver. Setting the RTS line low (logic 0) puts the driver into the tristate condition.  This in effect disconnects the driver from the bus, allowing other nodes to transmit over the same wire pair. Figure 1.8 shows a timing diagram for a typical RS-232 to RS-485 converter. The waveforms show what happens if the VRTS waveform is narrower than the data VSD. This is not the normal situation, but is shown here to illustrate the loss of a portion of the data waveform. When RTS control is used, it is important to be certain that RTS is set high before data is sent. Also, the RTS line must then be set low after the last data bit is sent. This timing is done by the software used to control the serial port and not by the converter.
 
如前所述，当一个特写节点不作传输时，RS-485系统必须让驱动器从传输线上脱离下来。对RS-232 to S-485转换器或RS-485串口卡,则可以这样实现：使用异步串口上的RTS控制信号使能RS-48驱动器。（具体是：）将RTS线连接到RS-485驱动器的使能端，用RTS线高电平状态(逻辑1)使能RS-485驱动器；用RTS线的低电平状态(逻辑0)使驱动器进入第三态----这里的作用就是让驱动器从总线上分开，允许其它节点在这一对绞线上传输（数据）。图1.8所示的是一张典型的RS-232 to RS-485转换器的时序图。从信号波形可以看出，如果VRTS信号波形比数据信号VSD窄（即时间短）。就是不正常的情况，但却已表明这里的数据信号波形有部份的损失。使用控制信号RTS时，发送数据前RTS置高电平无疑是重要的，同样，在发送数据的末位后RTS线必须被置低。这个时序不是由转换器（控制）完成的的而是由控制端口的软件完成的。

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When an RS-485 network is connected in a two-wire multidrop party line mode, the receiver at each node will be connected to the line (see Figure 1.6). The receiver can often be configured to receive an echo of its own data transmission. This is desirable in some systems, and troublesome in others. Be sure to check the data sheet for your converter to determine how the receiver “enable” function is connected.

 
当RS-485网络接成两线制的共用线路模式时，各个节点的接收器都被连接这对导线上到线(参见图1.6)。接收器经常被配置成已旧换新自身发送数据的返听(模式)。这在某些系统是需要的而在别的系统就可能变得烦人了。(为正确使用)你的转换器，请务必查对它的数据手册，以确认接收器“使能(enable)”端是如何连接的。
［译者语］party line和common line, shared line意义相似，具体时如何区分它们？

Send Data Control of an RS-485 Device


RS-485器件的发送数据控制

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［014］

Many of B&B Electronics’ RS-232 to RS-485 converters and RS-485 serial cards include special circuitry, which is triggered from the data signal to enable the RS-485 driver. Figure 1.9 is a timing diagram of the important signals used to control a converter of this type. It is important to note that the transmit data line is “disabled” at a fixed interval after the last bit, typically one character length. If this interval is too short, you can miss parts of each character being sent. If this time is too long, your system may try to turn the data line around from transmit to receive before the node (with the Send Data converter) is ready to receive data. If the latter is the case, you will miss portions (or complete characters) at the beginning of a response.
 
B&B公司中许多RS-232 to RS-485转换器和RS-485串口卡，包括有用数据信号触发、使能RS-485驱动器的特殊电路(special circuitry)。图1.9是用于控制此类转换器重要信号的时序图。要注意有一点尤为重要：末位发送后的一个固定间隔禁止(释放)传输线。该间隔典型为一个字长，如果该间隔是太短，所发送的各个字节就会部分丢失(即发生切尾现象)；如果间隔时间太长，系统就会在准备接收数据之前试图从发送状态返回接收状态(with the Send Data converter)。如果是是后一种情况，则有可能在开始反应之初就发生字符被切头甚或丢失整个字符的现象。

 



Figure 1.8: Timing Diagram for RS-232 to RS-485 Converter with RTS Control of RS-485 Driver and Receiver
图1.8：用RTS信号控制RS-485驱动器和接收器的RS-232 to RS-485转换器时序图
图注：1)Voltage here is determined by other device on the line. //此电压由线路上的器件决定。2)all peak values of voltage are approximate. //所有电压峰值均为近似值。
Note 1 - Voltage here is determined by other devices on the line
Note 2 - All peak values of voltages are approximate
Figure 1.8: Timing Diagram for RS-232 to RS-485 Converter with RTS Control of RS-485 Driver and Receiver


Figure 1.9 - Timing Diagram for RS-232 to RS-485 Converter with Send Data (SD) Control of RS-485 Driver and Receiver

图1.9：用SD信号控制RS-485驱动器和接收器的RS-232 to RS-485转换器时序图
图注：1)Voltage here is determined by other device on the line. //此电压由线路上的器件决定。2) This timing interval determined by components in timing circuit,. The start of this interval is determined by the leading edge of each data bit. //该时序的间隔由时序电路的元件决定，该间隔的启动由各数据位的上升沿确定。3) All peak values of voltage are approximate. //所有电压峰值均为近似值。
图中：RETIGGERABLE TIMING CIRCUIT//可重触发时序电路

Note 1 - Voltage here is determined by other devices on the line
Note 2 - This timing interval detremined by components in timing ciruit. The start of this interval is determined by the leading edge of each data bit
Note 3 - All peak values of voltages are approximate
Figure 1.9 - Timing Diagram for RS-232 to RS-485 Converter with Send Data (SD) Control of RS-485 Driver and Receiver

 

 


Chapter 2: System Configuration

Network Topologies


网络拓扑

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［015］

Network configuration isn’t defined in the RS-422 or RS-485 specification. In most cases the designer can use a configuration that best fits the physical requirements of the system.
 
RS-422或RS-485规范都不定义网络配置。多数情况下由设计师根据系统的物理要求采用最适宜的配置。
 

Two Wire or Four Wire Systems


两线或四线的系统

［016］
［016］

RS-422 systems require a dedicated pair of wires for each signal, a transmit pair, a receive pair and an additional pair for each handshake/control signal used (if required). The tristate capabilities of RS-485 allow a single pair of wires to share transmit and receive signals for half-duplex communications. This “two wire” configuration (note that an additional ground conductor should be used) reduces cabling cost. RS-485 devices may be internally or externally configured for two wire systems. Internally configured RS-485 devices simply provide A and B connections (sometimes labeled “-“ and “+”).
RS-422系统要求专门的一对导线给各个信号，即发送配对、接收对，另外的为用于各握手或控制信号的信号线对(如果有要求)。RS-485的三态能力允许以半双工通讯的方式分享传送和接收信号的一对导线。这种“双线”配置(注意，另外的一根地线并不能少)可以降低配线成本。对双线系统的配置，RS-485可以是内部或外部的，内部配置的RS-485，简单地提供A和B连接(有时标记为－和＋)。
 

［017］
［017］

Devices configured for four wire communications bring out A and B connections for both the transmit and the receive pairs. The user can connect the transmit lines to the receive lines to create a two wire configuration. The latter type device provides the system designer with the most configuration flexibility. Note that the signal ground line should also be connected in the system. This connection is necessary to keep the Vcm common mode voltage at the receiver within a safe range. The interface circuit may operate without the signal ground connection, but may sacrifice reliability and noise immunity. Figures 2.1 and 2.2 illustrate connections of two and four wire systems.
四线制通信配置的设备，分别用两组接线端A和B作为发送对和接收对。用户可以连接发送线到接收线，以形成2线制的配置。后者可以让系统设计师有更有配置灵活性。当然，系统的信号接地线也应该连接。这种为确保接收器共模电压Vcm在一个安全电压范围内的连接是必要的。接口电路可以在没有连接信号地时工作,但将牺牲可靠性和抗扰度。图2.1和图2.2分别示出了二线和四线系统的连接。
［译者语］黑体一句这样译不通。我看只能意译为：用户可以按极性将发送线和接收线接到一起，以形成两线制的配置。



Figure 2.1: type RS-485 Four wire Multdrop network
图2.1：4线制RS－485多节点网络



Figure 2.2: Type RS-485 Two Wire Multdrop network
图2.2：2线制RS－485多节点网络

Termination


终端电阻

［018］
［018］

Termination is used to match impedance of a node to the impedance of the transmission line being used. When impedance are mismatched, the transmitted signal is not completely absorbed by the load and a portion is reflected back into the transmission line. If the source, transmission line and load impedance are equal these reflections are eliminated. There are disadvantages of termination as well. Termination increases load on the drivers, increases installation complexity, changes biasing requirements and makes system modification more difficult.
 
终端电阻用于节点阻抗与传输线阻抗匹配。若阻抗失配，发送信号会不完全地为负载所吸收的同时又部分地反射回到传输线。如果源信号阻抗与传输线阻抗、负载阻抗相等，则这些反射就会被消除。使用终端电阻也有不利的一面，它(额外地)加重驱动器的负载，增加装置的复杂性，改变偏置条件和造成系统日后修改困难更大。

［译者语］关于termination参见010的“译者语”。

［019］
［019］

The decision whether or not to use termination should be based on the cable length and data rate used by the system. A good rule of thumb is if the propagation delay of the data line is much less than one bit width, termination is not needed. This rule makes the assumption that reflections will damp out in several trips up and down the data line. Since the receiving UART will sample the data in the middle of the bit, it is important that the signal level be solid at that point. For example, in a system with 2000 feet of data line the propagation delay can be calculated by multiplying the cable length by the propagation velocity of the cable. This value, typically 66 to 75% of the speed of light (c), is specified by the cable manufacture.
 
是否使用终端电阻，应该根据系统的电缆长度和数据速率决定。一个好经验法则是：如果数据线的传播延迟是少于一个位宽度,就不必使用终端电阻。这个规则,是（基于）假定反射做几次往返而衰减（为前提的）。由于接收UART( Universal Asynchronous Receiver通用异步接收器)是在数据位的中点抽样的，所以，这个点(即数据位的中点)信号电平此时真实稳定显得尤其重要。例如, 一个系统在2000英尺的数据线中传播延迟，可以用电缆长度乘以(信号在电缆中的传播速度，----这个速度典型为光速c的66～75%,它由电缆制造商给定。
 

［020］
［020］

For our example, a round trip covers 4000 feet of cable. Using a propagation velocity of 0.66×c, one round trip is completed in approximately 6.2 μs. If we assume the reflections will damp out in three “round trips” up and down the cable length, the signal will stabilize 18.6 μs after the leading edge of a bit. At 9600 baud one bit is 104 μs wide. Since the reflections are damped out much before the center of the bit, termination is not required.
 
以一个在4000英尺的电缆里的往返为例。设传播速度为0.66×c,完成一个往返大约为6.2μs。如果我们假设反射在电缆中做三次衰减的往返(round trips)，信号前沿在18.6μs后稳定。对9600的波特率，一个位的宽度为104μs。这样反射衰减在数据位的中点之前(就已经结束)，因而就不必加装终端电阻了。
［译者语］要注意这是在4000ft(～1220米)电缆里往返而不是在长度为2000ft的电缆里往返。
 

［021］
［021］

There are several methods of terminating data lines. The method recommended by B&B is parallel termination. A resistor is added in parallel with the receiver’s “A” and “B” lines in order to match the data line characteristic impedance specified by the cable manufacture (120 Ω is a common value). This value describes the intrinsic impedance of the transmission line and is not a function of the line length. A terminating resistor of less than 90 Ω should not be used. Termination resistors should be placed only at the extreme ends of the data line, and no more than two terminations should be placed in any system that does not use repeaters. This type of termination clearly adds heavy DC loading to a system and may overload port powered RS-232 to RS-485 converters. Another type of termination, AC coupled termination, adds a small capacitor in series with the termination resistor to eliminate the DC loading effect. Although this method eliminates DC loading, capacitor selection is highly dependent on the system properties. System designers interested in AC termination are encouraged to read National Semiconductors Application Note 9032 for further information. Figure 2.3 illustrates both parallel and AC termination on an RS-485 two-wire node. In four-wire systems, the termination is placed across the receiver of the node.
 
终止数据线有好几种方法。B&B公司推荐的方法是并联终端电阻。在接收器的A、B端并联电阻，目的是为了匹配电缆制造商所确定的数据线的特征阻抗(其值通常为120Ω)。描述传输线固有阻抗的值与线路长度无关。使用的终端电阻不应小于90Ω，且终端电阻应当安装在数据线的终端,不过，两个终端电阻应当安装在没有使用重复器的两端。这种类型的终端电阻明显地要加重系统的直流负载，和导致RS-232转RS-485转换器的供电口过载。另一种类型终端电阻----AC耦合的终端电阻，增加一只小电容器串联在终端电阻上就消除了它的DC负载效应。虽然这个方法消除了直流负载，但电容器选择对系统特性的依赖程度高。系统设计师如对AC终端电阻有兴趣，建议查阅NS公司的应用笔记9032。图2.3详细地说明了在两线制RS-485节点上并联终端电阻和并联交流(AC)终端电阻的情况。对四线制系统系统,终端电阻则并接到节点中的接收器上。
 

2 Refer to Chapter 7 for information on National Semiconductors Application Notes.
 
2 参见第7章关于NS半导体应用笔记的信息。
 




Figure 2.3: Parallel and AC Termination
图2.3：并联和交流耦合终端电阻
 

 

Biasing an RS-485 Network


RS485网络的偏置

［022］
［022］

When an RS-485 network is in an idle state, all nodes are in listen (receive) mode. Under this condition there are no active drivers on the network, all drivers are tristated. Without anything driving the network, the state of the line is unknown. If the voltage level at the receiver’s A and B inputs is less than ±200 mV the logic level at the output of the receivers will be the value of the last bit received. In order to maintain the proper idle voltage state, bias resistors must be applied to force the data lines to the idle condition. Bias resistors are nothing more than a pullup resistor on the data B line (typically to 5 volts) and a pulldown (to ground) on the data A line. Figure 2.4 illustrates the placement of bias resistors on a transceiver in a two-wire configuration. Note that in an RS-485 four-wire configuration, the bias resistors should be placed on the receiver lines. The value of the bias resistors is dependent on termination and number of nodes in the system. The goal is to generate enough DC bias current in the network to maintain a minimum of 200 mV between the B and A data line. Consider the following two examples of bias resistor calculation.
 
RS-485网络处于空闲状态时，所有的节点都成为听模式(接收模式)。在这种情况下，所有发送器都是第三态，网络上没有任何工作的驱动器。没有任何(驱动器)驱动网络，则线路状态不明(未知)。如果接收器输入端A和B间的电平低于±200mV，接收器输出的逻辑电平将被当作末位值接收起来。为保持适当的空闲电压状态，就必须使用偏置电阻强迫数据线维持线路(处于正确的)空闲条件。偏置电阻不过是数据线B(典型为+5V)的上拉电阻和数据线A(接到“地”)的下拉电阻，如图2.4所示的双线系统，详细示出了收发器的偏置电阻。但要注意，对于4线系统，偏置电阻应当放在接收器一端。偏置电阻的值，取决于是不是在终端(termination)和由系统节点数(的多少)来确定。使用终端电阻的目的，是让网络发生足够的直流偏置电流，以维持数据线A和B间的最小电压在200mV(以上)。参见以下两个有关偏置电阻计算的例子。
［译者语］The value of the bias resistors is dependent on termination and number of nodes in the system一句似乎很有E文特色，就着它“译”出来。只是其中关键是“termination”一词如何能与“value”有关？



Figure 2.4 - Transceiver with Bias Resistors

Example 1. 10 node, RS-485 network with two 120 W termination resistors


例1 含两个120Ω终端电阻的10节点RS-485网络

［023］
［023］

Each RS-485 node has a load impedance of 12KΩ. 10 nodes in parallel give a load of 1200 Ω. Additionally, the two 120 Ω termination resistors result in another 60 W load, for a total load of 57 Ω. Clearly the termination resistors are responsible for a majority of the loading. In order to maintain at least 200mV between the B and A line, we need a bias current of 3.5 mA to flow through the load. To create this bias from a 5V supply a total series resistance of 1428 Ω or less is required. Subtract the 57 Ω that is already part of the load, and we are left with 1371 Ω. Placing half of this value as a pullup to 5V and half as a pulldown to ground gives a maximum bias resistor value of 685Ω for each of the two biasing resistors.
 
各RS-485节点有负载阻抗为12kΩ，10个节点并联后得出的阻抗为1200Ω，另外，两个120Ω的线路终端电阻形成的另一个负载为60Ω，因此，总负载电阻为57Ω。明显地，终端电阻将是线路的主要负载。为保持A、B线间至少有200mV的电压，这就需要3.5mA的偏置电流流过(这57Ω的)负载。用5V电源进行偏置时，所要串联的电阻最大为1428Ω。扣除已经有的57Ω负载电阻，剩下的电阻为1371Ω。如将其值的一半接到5V电源做上拉电阻，将另外的一半拉接到地做下拉电阻，则各个节点的两边偏置电阻最大总共为685Ω。
［译者语］最后一句说的是上拉或下拉电阻的并联值而不是直接利用这个值。这一点比较容易误会，而目前看到的资料又没有对此加以说明的，因而特别强调之。

Example 2. 32 node, RS-485 network without termination


例2：没有终端电阻的32节点网络

［024］
［024］

Each RS-485 node has a load impedance of 12KΩ. 32 nodes in parallel gives a total load of 375 Ω. In order to maintain at least 200 mV across 375Ω we need a current of 0.53 mA.  To generate this current from a 5V supply requires a total resistance of 9375W maximum. Since 375 Ω of this total is in the receiver load, our bias resistors must add to 9KΩ or less. Notice that very little bias current is required in systems without termination.
 
各RS-485节点的负载阻抗为12kΩ，32节点并联所得的总负载为375Ω。为保持375Ω负载的电压至少为200mV，需要的电流为0.53mA。通过5V电源发生这样的电流，所需的负载电阻最大为9375Ω。因有375Ω是接收口器总负载(固有)，因此，总的偏置电阻必须增加到9kΩ或更小。注意，系统没有终端电阻时所需的偏置电流非常小。

 

［025］
［025］

Bias resistors can be placed anywhere in the network or can be split among multiple nodes. The parallel combination of all bias resistors in a system must be equal to or less than the calculated biasing requirements. B&B Electronics uses 4.7KΩ bias resistors in all RS-485 products. This value is adequate for most systems without termination. The system designer should always calculate the biasing requirements of the network. Symptoms of under biasing range from decreased noise immunity to complete data failure. Over biasing has less effect on a system, the primary result is increased load on the drivers. Systems using port powered RS-232 to RS-485 converters can be sensitive to over biasing.
 
偏置电阻可以放在网络的任何位置或分别放到网络中的多处节点。系统中，所有偏置电阻并联后的总阻抗必须等于或小于计算出的偏置要求值。B&B Electonics公司所有的RS-485产品用4.7kΩ偏置电阻。此值适用于多数没有终端电阻的系统。系统设计师务必计算网络的偏置要求(值)。在偏置范围以下，则存在降低噪声的抗干扰性导致数据失效的symptoms。
 

Extending the Specification



［026］
［026］

Some systems require longer distances or higher numbers of nodes than supported by RS-422 or RS-485. Repeaters are commonly used to overcome these barriers. An RS-485 repeater such as B&B Electronics’ 485OP can be placed in a system to divide the load into multiple segments. Each “refreshed” signal is capable of driving another 4000 feet of cable and an additional 31 RS-485 loads.
 
性能扩展
往往有一些系统需要比RS-422或RS-485所支持的有更长的距离或更多的节点数。要突破这类局限，通常要用到重复器。象B&B公司的485OP型这样的RS-485总线重复器，可以插入到系统中将负载分割成许多段，各“复原”后的信号有驱动另一段4000ft电缆和带动另外31个RS-485负载的能力。
 

［027］
［027］

Another method of increasing the number of RS-485 nodes is to use low load type RS-485 receivers. These receivers use a higher input impedance to reduce the load on the RS-485 drivers to increase the total number of nodes. There are currently half and quarter load integrated circuit receivers available, extending the total allowable number of nodes to 64 and 128.