RS232与RS485串行信息-英文翻译.doc

RS232与RS485串行信息1 RS232接口指标以及介绍在RS232标准中进行的通信是一种异步串行通信的方法。串行的意思是，每一次只发送一位数据的信息。异步是指，信息不会在预定的的时段内发送。数据传输可以在任何特定时间开始，并且接收器的任务是能检测到一个信息的开始和结束。异步通信既有优点，又有缺点。2 RS232比特流在物理信道上，标准的RS232串口传输信息是通过一位一位发送的方式进行传输的，那就必须分离一个数据字的信息。而且数据字的长度是可变的，在PC机上可以选择5到8位的长度。为用于同步和错误检查的目的，可以适当增加一些位。发射器和接收器使用相同的位数是很重要的，否则，字数据可能会被误解或不承认。 在同步通信中，必须有时钟或触发信号来表示每次传输的开始。没有时钟信号，可以使异步通信渠道运行起来更便宜，减掉一些必要的电缆。一个缺点是，接收器可能会在错误的时刻接收信息，这样就需要时间使它们再同步运行。在同步期间里，所有收到的数据会丢失。另一个不利之处是需要额外的比特的数据流，以显示有用的信息的开始和结束。而这些额外的位要占用带宽。 数据位发送预先的频率，即波特率。发射器和接收器通过编程来使用相同的位频率。当第一次收到一个位时，接收器就会计算出其他数据位将在哪个时间段收到。在这些时刻，将检查线电压。 RS232接口的线电压有两种状态。打开状态称作1，关闭状态称作0，也可以既不处于打开状态又不处于关闭状态。当线处于闲置状态，它将保持在高电平。 2.1 起始位 RS232接口定义为一个异步的通信方式。这意味着，能在任意时刻发送一个数据字。如果任意时刻都有可能的话，这可能会为接收器确定哪一位是起始位造成一些问题。为了解决这个问题，每个字数据都有一个注意位作为开始。这个注意位，也被称为起始位，始终被定义为低电平。由于当线路为空闲时为高电平，起始位很容易被接收器发现。2.2 数据位在数据发送过程中，直接紧跟着起始位的是数据位。位值为1产生高电平，位值为0为低电平。最低位始终是在第一位发送。2.3 奇偶校验位为了检测错误，有可能在字数据中自动地增加额外的位。发送器依靠发送的信息来计算每一位的值。如果实际的奇偶校验位的值与计算的值相对应，接收机会进行同样的计算和检查。2.4 停止位由于在传输线上噪声的干扰，假定接收器已经错过了起始位，它将以一个空值来作为第一个数据位的开始，这会导致到达接收器的信息为乱码，必须出现一个机制使通信再同步。要做到这一点，引进一种帧格式，帧是指所有的数据位和奇偶校验位包含一帧的起始位和停止位。在起始位和停止位之间的时间段是一个常数，由波特率，数据位和奇偶校验位来确定。起始位总是低电平，停止位总是高电平。当停止位在线上传输时，如果接收器检测到的值不是高电平，就会导致同步失败，在UART接收端会导致帧错误。该装置会在新的传输位上进行再同步。对于再同步 ，接收器会对有效的启动和停止位进行数据扫描。如果数据0值被反复发送，同步是不可能的。例如：确定数据帧结束的停止位可以有不同的长度。事实上，它不是一个真实的位而是在每一个字结束时都有一个最小时间的高电平。在PC机上此时间段有三种长度：时间等于1 ， 1.5或2位。 1.5位仅用于5位长度的数据字。对于所有数据字的大小1位停止位的长度是有可能的。3 RS232接口的物理特性RS232接口标准描述了一个在不同的环境中能够进行通信的方法。在引脚上，这已影响了所能允许的最大电压值。在最初的定义中，当时就已经考虑到了技术的可能性。例如，最大波特率为20 kbps。随着目前的设备，如16550A UART接口的出现，最大速度可达到1.5 Mbps。3.1 电压 RS232引脚的信号电平有两个状态。高电位由负电压表示，而低电位由正电压表示。这可能会有点不适应，因为在正常情况下，逻辑值高界定为高电压。电压范围如下所示。RS232接口电压值电平发送器电压值(V)接收器电压值 (V)低电平 (0)+5 . +15+3 . +25高电平(1)-5 . -15-3 . -25未定义-3 . +3计算机端口可以产生的最大电压幅值能影响到最大的电缆长度和通信速度。此外，如果电压差很小，数据失真迟早会发生。例如，与-11.5 V的台式计算机相比，东芝笔记本高电压为-9.3V。在高噪音的工业环境中，相同的电缆传输下，笔记本电脑很难与三菱PLC进行通信，而台式电脑在传输过程中却没有任何数据错误。因此，即使远远超出了最低限度的电压电平， 2V的差值可以对通信质量产生巨大影响。尽管存在高电压，但是不可能由于短路而毁坏串行端口。只有在高电压与高电流下，可能烧坏驱动芯片。仍旧，在大多数情况下UART不会被毁坏。3.2 最大电缆长度在RS232领域，电缆长度是讨论的最多的项目之一。该标准有一个明确的答案，最大电缆长度为50英尺，或电缆长度相当于一个2500 pF的电容。后者的规定往往被遗忘。这意味着，在没有超出标准的限制下，使用一个带有低电容的电缆覆盖更远的距离。例如，如果双绞线CAT - 5电缆使用一个17V/英尺的电容，所允许的最大电缆长度为147英尺。在这个标准中所提到的电缆长度允许最高通信速度产生。如果速度减少2倍或4倍，最大长度急剧增加。多年前德州仪器公司做了一些切实可行的实验，不同的传输速率对应所允许的最大电缆长度。请记住， RS232标准最初是20 kbps。由于有最高通讯速度，允许电缆长度增加10倍！根据德州仪器所测试的RS232电缆长度波特率最大电缆长度（英尺）1920050960050048001000240030004 RS485接口RS232 ， RS422 ， RS423和RS485是用于计算机和设备进行串行通信的方法。 RS232串口众所周知，因为在今天，这种串行接口几乎应用在所有的电脑上。然而，其他一些接口也引起了人们的关注，因为它们可以用于RS232接口所不能用的情况下。我们将重点介绍RS485接口。RS232接口用于数据终端设备与数据通信设备的连接，最高通信速率为20 kbps，最大通信线长度为50英尺。在过去，几乎所有的计算机设备使用调制解调器进行连接就足以进行通信。但是，不久之后，人们开始寻求通信能力更强的接口，如下所示：1 不需要调制解调器而直接连接数据终端设备2 在一个网络结构中连接多个终端3 通信距离更远4 通信速率更快RS485是由EIA所定义的最通用的通信标准，因为它有以下四个优点。这就是为什么在有多个节点进行通信的数据采集和控制应用中，RS485是目前使用最广泛的通信接口。5 RS485的差分信号：更远距离和更高比特率RS232的一个主要问题是在信号传输过程中抗干扰能力不强。发送器和接收器的传输线上的电压值是以地线作为标准的。接地电平的改变可能带来不利影响。因此，RS232接口的触发电平为相对较高的3V。很容易产生干扰信号，因而限制了最大传输距离和通信速度。相反，RS485没有接地端来作为参考点。它利用两线之间的电压差值进行发送和接收不会引起任何问题。 RS485传输信号是变化的，每个信号的传送通过一根+信号线和一根-信号线。 RS485接收的信号是通过比较两线之间的电压差得到的，而不是只由一根信号线上的电压值表示。这样能防止存在的接地环路（通信中存在的一个共同问题），使得运行良好。如果+信号线和-信号线使用双绞线，就能达到最好的效果。下面的图解释了其原因。 在中直电缆和双绞线电缆中所引起的噪声 在上面的图中，噪声产生于磁场的环境中。由于磁场的存在，上图显示的是在485数据线中的磁场线和噪声电流。中直电缆，所有噪声电流流动的方向是一致的，特别像在一个普通的变压器中产生的环路电流。当电缆交织时，我们看到，某些信号线上的噪声电流方向与其他部分的电缆的电流方向相反。由于这一原因，所造成的噪音电流比普通直电缆引起的电流要弱。屏蔽这是一种常用的防止RS232线上噪音的方法试图保持相反的磁场远离信号线。然而，在RS485通信中的双绞线抗干扰能力更强，磁场可以通过，但不会产生干扰。如果要具备高抗噪性，往往是将双绞和屏蔽相结合。例如在STP中，像屏蔽双绞线，FTP 以及箔层双绞线网络电缆等等。差分信号和双绞线使RS485通信距离比RS232通信距离更远。用RS485进行通信，可达1200米。差分信号线比非差分连接的信号线有更高的比特率。因此， RS485可以克服在RS232通信中速度的限制。目前生产的RS485设备可以实现的比特率为35 Mbps。 6 RS485与RS232， RS422和RS423的特点RS232 ， RS422 ， RS423和RS485的特点RS232RS423RS422RS485差分否否是是最大发送器节点数11132最大接收器节点数1101032运行方式半双工半双工半双工半双工全双工网络拓扑结构点对点多点多点多点最大距离(标准)15 m1200 m1200 m1200 m在12m时的最大速度20 kbs100 kbs10 Mbs35 Mbs在1200m时的最大速度(1 kbs)1 kbs100 kbs100 kbs最大摆率30 V/s可调节n/an/a接收器输入电阻3.7 k 4 k 4 k 12 k3.7 k 450 100 54 接收器输入门限3 V200 mV200 mV200 mV接收器输入电压范围15 V12 V10 V7.12 V发送器输出最大电压值25 V6 V6 V7.12 V发送器输出最小电压值(有负载)5 V3.6 V2.0 V1.5 V此表的信息告诉我们：首先，可以看到进行差分传输的RS422和RS485 远远优于RS232和RS423 。其次，在RS232和RS423里有一个最高转换率的定义，这样做是为了避免反射信号，而最高转换率也限制了线上的最高通信速度。像RS422及RS485接口则没有最高转换率的限制。为了避免在更长电缆上的反射信号，有必要使用合适的终端电阻。我们还看到，对于所有接口所允许的最大电压等级都在同一范围内，但该信号电平对于更快的接口而言就更低。由于这一原因，在同一高比特率下，RS485和其他接口在几V电压的变化下仍然是可以用的，因为逻辑0和逻辑1之间的转换只有几百毫伏。值得注意的是， RS232接口能够进行全双工通信。这是因为在其他接口中，通信渠道由多个接收端和多个发送端共享，如RS485接口。良好的书面协议使得RS232接口有一个单独的通信线路用于发送和接收，在相同比特率下，它比其他接口有更高的数据传输速率。在大多数协议中所需要的数据并不占用RS232接口上主要数据通道的带宽。7 由RS485构成的网络拓扑网络的拓扑结构可能是目前RS485在数据采集和控制应用中最受欢迎的原因。 在同一网络中，只有RS485接口，能够互联多个发射机和接收机。当在RS485接收器里加入12 k的输入电阻就有可能将32个装置连接到网络。目前抗干扰能力更强的RS485可以扩展到256个 。 运用RS485中继器还可以增加几千个节点，覆盖几公里的范围。并有一个不需要智能网络硬件的接口：在软件方面的执行并没有RS232接口困难。这就是为什么RS485在计算机， PLC，微控制器和智能传感器的科技应用中如此广泛的原因吧。RS485总线网络的拓扑结构RS485的网络拓扑图如上图所示。N个节点连接在RS485多点网络中。对于更高的速度和更远的距离，有必要在线的两端接上100的终接电阻，以消除反射信号。RS485网络的设计必须是一点对多点。虽然多点对多点的结构可使总电缆长度变短，但会明显降低信号的质量。 8 RS485的功能现在最重要的问题是RS485在实际运行中的功能怎么样。所有在RS485总线上的发送端具有三种状态，包括高阻态。在更高级别的协议中，其中一个节点在RS485总线上用于发出查询或命令，而所有其他节点接收这些数据。根据传送过来的数据信息，零个或多个节点对主机作出响应。在这种情况下，带宽利用率几乎为100 。在RS485网络上还能实现每个节点都可以开始发送自己的数据的功能，这与以太网网络功能相似。因为在传输过程中数据可能会丢失，在这种情况下，理论上，只有37 的带宽将得到有效利用。在RS485网络上进行通信时，更高级的通信协议是实行差错控制去检测数据的错误并且之后重发息，这一点是很有必要的。对于发送器来说没有必要去打开或关闭RS485。当数据发送后，在数微秒后RS485驱动器会自动地返回到高阻态。因此，在RS485总线上，在数据包之间不需要延迟。RS485作为众所周知的接口标准被用作电气层，包括现场和Modbus 。因此，在未来几十年中RS485仍将被广泛使用。9 错误检测一个检测错误的方法已经讨论，就是帧检测机制。如果输入位被起始位和停止位包围，就可用它来检测。为进一步检查错误，可以使用一个奇偶校验位。但使用此位不是强制性的。如果错误位很少出现（例如，当与一个内部调制解调器通信）或如果更高级别的协议被用于错误检测和纠正（ Z型调制解调器，RAS等） ，在UART上，通过不使用奇偶特征位就能提高通信速度。奇偶是使编码字数据成为一种机制来检测错误信息的一种简单方法。就是在串行通信中添加一个位给每个数据字，这个位的值取决于数据字的值。有必要这两个发射器和接收器使用相同的算法去计算出奇偶校验位的值。否则，接收器可能会检测到不存在的错误。9.1 偶校验基本上，奇偶校验位可以有两种方法来计算。当使用偶校验位时，发送的信息位的值将总是包含偶数个1。9.2 奇校验奇校验系统与偶校验系统很相似，但在这种情况下，高位的值总是奇数。9.3 奇偶校验系统的缺点奇偶校验系统为每个数据字使用一位奇偶校验位并不能发现所有的错误，只有当各位发生奇数个值的变化才能被检测出来。第二个问题是，没有办法知道哪一位出错。所以，使用更高级别的协议来通知发送器必须重发一些信息是有必要的。因此，在有噪声的线路上，使用其他检测系统来确保发出的信息能正确收到。这些系统大多数在单个的数据字上并不起作用，但在检测许多数据字时就会起到很大的作用了。8RS232 and RS485 serial information1 RS232 specifications, introductionCommunication as defined in the RS232 standard is an asynchronous serial communication method. The word serial means, that the information is sent one bit at a time. Asynchronous tells us that the information is not sent in predefined time slots. Data transfer can start at any given time and it is the task of the receiver to detect when a message starts and ends. Asynchronous communication has some advantages and disadvantages .2 RS232 bit streamsThe RS232 standard describes a communication method where information is sent bit by bit on a physical channel. The information must be broken up in data words. The length of a data word is variable. On PCs a length between 5 and 8 bits can be selected. For proper transfer additional bits are added for synchronisation and error checking purposes. It is important, that the transmitter and receiver use the same number of bits. Otherwise, the data word may be misinterpreted, or not recognized at all. With synchronous communication, a clock or trigger signal must be present which indicates the beginning of each transfer. The absence of a clock signal makes an asynchronous communication channel cheaper to operate. Less lines are necessary in the cable. A disadvantage is, that the receiver can start at the wrong moment receiving the information. Resynchronization is then needed which costs time. All data received in the resynchronization period is lost. Another disadvantage is that extra bits are needed in the data stream to indicate the start and end of useful information. These extra bits take up bandwidth. Data bits are sent with a predefined frequency, the baud rate. Both the transmitter and receiver must be programmed to use the same bit frequency. After the first bit is received, the receiver calculates at which moments the other data bits will be received. It will check the line voltage levels at those moments. With RS232, the line voltage level can have two states. The on state is also known as mark, the off state as space. No other line states are possible. When the line is idle, it is kept in the mark state. 本文摘自http:/www.lammertbies.nl/comm/info2.1 Start bitRS232 defines an asynchronous type of communication. This means, that sending of a data word can start on each moment. If starting at each moment is possible, this can pose some problems for the receiver to know which is the first bit to receive. To overcome this problem, each data word is started with an attention bit. This attention bit, also known as the start bit, is always identified by the space line level. Because the line is in mark state when idle, the start bit is easily recognized by the receiver. 2.2 Data bitsDirectly following the start bit, the data bits are sent. A bit value 1 causes the line to go in mark state, the bit value 0 is represented by a space. The least significant bit is always the first bit sent. 2.3 Parity bitFor error detecting purposes, it is possible to add an extra bit to the data word automatically. The transmitter calculates the value of the bit depending on the information sent. The receiver performs the same calculation and checks if the actual parity bit value corresponds to the calculated value. 2.4 Stop bitsSuppose that the receiver has missed the start bit because of noise on the transmission line. It started on the first following data bit with a space value. This causes garbled date to reach the receiver. A mechanism must be present to resynchronize the communication. To do this, framing is introduced. Framing means, that all the data bits and parity bit are contained in a frame of start and stop bits. The period of time lying between the start and stop bits is a constant defined by the baud rate and number of data and parity bits. The start bit has always space value, the stop bit always mark value. If the receiver detects a value other than mark when the stop bit should be present on the line, it knows that there is a synchronization failure. This causes a framing error condition in the receiving UART. The device then tries to resynchronize on new incomming bits. For resynchronizing, the receiver scans the incomming data for valid start and stop bit pairs. If data value zero is sent repeatedly, resynchronization is not possible for example. The stop bit identifying the end of a data frame can have different lengths. Actually, it is not a real bit but a minimum period of time the line must be idle (mark state) at the end of each word. On PCs this period can have three lengths: the time equal to 1, 1.5 or 2 bits. 1.5 bits is only used with data words of 5 bits length. A stop bit length of 1 bit is possible for all data word sizes. 3 RS232 physical propertiesThe RS232 standard describes a communication method capable of communicating in different environments. This has had its impact on the maximum allowable voltages etc. on the pins. In the original definition, the technical possibilities of that time were taken into account. The maximum baud rate defined for example is 20kbps. With current devices like the 16550A UART, maximum speeds of 1.5Mbps are allowed. 3.1 VoltagesThe signal level of the RS232 pins can have two states. A high bit, or mark state is identified by a negative voltage and a low bit or space state uses a positive value. This might be a bit confusing, because in normal circumstances, high logical values are defined by high voltages also. The voltage limits are shown below. RS232 voltage valuesLevelTransmittercapable (V)Receivercapable (V)Space state (0)+5 . +15+3 . +25Mark state (1)-5 . -15-3 . -25Undefined-3 . +3The maximum voltage swing the computer can generate on its port can have influence on the maximum cable length and communication speed that is allowed. Also, if the voltage difference is small, data distortion will occur sooner. For example, my Toshiba laptop marks voltage is -9.3V, compared to -11.5V on my desktop computer. The laptop has difficulties to communicate with Mitsubishi PLCs in industrial environments with high noise levels where the desktop computer has no data errors at all using the same cable. Thus, even far beyond the minimum voltage levels, 2volts extra can make a huge difference in communication quality. Despite the high voltages present, it is not possible to destroy the serial port by short circuiting. Only applying external voltages with high currents may eventually burn out the driver chips. Still then, the UART wont be damaged in most cases. 3.2 Maximum cable lengthsCable length is one of the most discussed items in RS232 world. The standard has a clear answer, the maximum cable length is 50feet, or the cable length equal to a capacitance of 2500pF. The latter rule is often forgotten. This means that using a cable with low capacitance allows you to span longer distances without going beyond the limitations of the standard. If for example UTP CAT-5 cable is used with a typical capacitance of 17pF/ft, the maximum allowed cable length is 147feet. The cable length mentioned in the standard allows maximum communication speed to occur. If speed is reduced by a factor 2 or 4, the maximum length increases dramatically. Texas Instruments has done some practical experiments years ago at different baud rates to test the maximum allowed cable lengths. Keep in mind, that the RS232 standard was originally developed for 20kbps. By halving the maximum communication speed, the allowed cable length increases a factor ten! RS232 cable length according to Texas InstrumentsBaud rateMaximum cable length (ft)1920050960050048001000240030004 RS485RS232, RS422, RS423 and RS485 are serial communication methods for computers and devices. RS232 is without doubt the best known interface, because this serial interface is implemented on almost all computers available today. But some of the other interfaces are certainly interesting because they can be used in situations where RS232 is not appropriate. We will concentrate on the RS485 interface here. RS232 is an interface to connect one DTE, data terminal equipment to one DCE, data communication equipment at a maximum speed of 20kbps with a maximum cable length of 50 feet. This was sufficient in the old days where almost all computer equipment were connected using modems, but soon after people started to look for interfaces capable of one or more of the following: 1 Connect DTEs directly without the need of modems 2 Connect several DTEs in a network structure 3 Ability to communicate over longer distances 4 Ability to communicate at faster communication rates RS485 is the most versatile communication standard in the standard series defined by the EIA, as it performs well on all four points. That is why RS485 is currently a widely used communication interface in data acquisition and control applications where multiple nodes communicate with each other. 5 Differential signals with RS485:Longer distances and higher bit ratesOne of the main problems with RS232 is the lack of immunity for noise on the signal lines. The transmitter and receiver compare the voltages of the data- and handshake lines with one common zero line. Shifts in the ground level can have disastrous effects. Therefore the trigger level of the RS232 interface is set relatively high at 3Volt. Noise is easily picked up and limits both the maximum distance and communication speed. With RS485 on the contrary there is no such thing as a common zero as a signal reference. Several volts difference in the ground level of the RS485 transmitter and receiver does not cause any problems. The RS485 signals are floating and each signal is transmitted over a Sig+ line and a Sig- line. The RS485 receiver compares the voltage difference between both lines, instead of the absolute voltage level on a signal line. This works well and prevents the existence of ground loops, a common source of communication problems. The best results are achieved if the Sig+ and Sig- lines are twisted. The image below explains why. Noise in straight and twisted pair cablesIn the picture above, noise is generated by magnetic fields from the environment. The picture shows the magnetic field lines and the noise current in the RS485 data lines that is the result of that magnetic field. In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer. When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable. Shieldingwhich is a common method to prevent noise in RS232 linestries to keep hostile magnetic fields away from the signal lines. Twisted pairs in RS485 communication however adds immunity which is a much better way to fight noise. The magnetic fields are allowed to pass, but do no harm. If high noise immunity is needed, often a combination of twisting and shielding is used as for example in STP, shielded twisted pair and FTP, foiled twisted pair networking cables. Differential signals and twisting allows RS485