Serial Programming Guide for POSIX Operating Syste

文件名称: Serial Programming Guide for POSIX Operating Systems.pdf

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详细说明：linux 串口编程，串口设置。老外文档。介绍了linux串口的基本操作，国内很多博客都引用和翻译此文。 1.基本串口通信 2.如何配置串口 3.调制解调通信 4.高级串口编程Serial programming Guide for posiX Operating Systems https://www.cmrr.umnedu/strupp/serial.htm1#contents When referring to serial devices or ports, they are either labeled as Dala Communications Equipmen(Dce")or Dula Terminal Equipment(DTE). The difference between these is simple- every signal pair, like transmit and receive, is swapped. When connecting two DtE or two DCE interfaces together, a serial null-MODEM cable or adapter is used that swaps the signal pairs What Is rs-232? RS-232 is a standard electrical interface for serial communications defined by the Electronic Industries Association C"EIA". RS-232 actually comes in 3 different flavors(A, B, and C)with each one defining a different voltage range for the on and off levels. The most commonly used variety is rs-232C, which defines a mark(on) bit as a voltage between -3V and -12V and a space(off) bit as a voltage between+3V and +12V. The rS-232C specification says these signals can go about 25 feet( 8m) before they become unusable. You can usually send signals a bit farther than this as long as the baud is low enough Besides wires for incoming and outgoing data, there are others that provide timing, status, and handshaking Table 1-Rs-232 Pin assignments Pin Description Pin Description Pin Description Pin Description Pin Descriptio Igna Earth Ground 6 DSR-Data Secondary 1Unassigned16 Set ready RXD 21 Quality Detect TXD 2Transmitted 7- Logic Secondar 17 Receiver Clock 22 Ring Detect Ground DCD Data 3 XD /8DCD-Data Received Data Carrier Detect 13 Secondary CTS 18 Unassigned 123 Data Rate Select /4 RTS-RO quest Secondar 9 Reserved To Send 19 Secondary RTS 24Iransmit TXD Clock DTR-Data 15CTS-Clear Transmit Reserved 15 To Send Clock 20 Terminal 25 Unassigned Read Two standards for serial interfaces you may also see are rs-422 and rs-574 Rs-422 uses lower voltages and differential signals to allow cable lengths up to about 1000ft(300m). Rs-574 defines the 9-pin PC serial connector and voltages Signal Definitions The rs-232 standard defines some 18 different signals for serial communications. Of these, only six are generally available in the UNIX environment GND- Logic Ground Technically the logic ground is not a signal, but without it none of the other signals will operate. Basically, the logic ground acts as a reference voltage so that the electronics know which voltages are positive or negative TXD- Transmitted data 第3页共26页 Serial programming Guide for posiX Operating Systems https://www.cmrr.umn.edustrupp/serial.html#contents The TXD Signal carries data transmitted from your workstation to the computer or device on the other end (like a MODEm). a mark voltage is interpreted as a value of l, while a space voltage is interpreted as a value of o RXD Received data The rXd signal carries data transmitted from the computer or device on the other end to your workstation. Like TXD, mark and space voltages are interpreted as l and 0, respectively DCD- Data Carrier Detect The dcd signal is received from the computer or device on the other end of your serial cable. a space voltage on this signal line indicates that the computer or device is currently connected or on line dCd is not always used or available DTR- Data Terminal Ready The dTR signal is generated by your workstation and tells the computer or device on the other end that you are ready (a space voltage) or not-ready(a mark voltage). DtR is usually enabled automatically whenever you open the serial interface on the workstation CTS- Clear To send The CTS signal is received from the other end of the serial cable. A space voltage indicates that is alright to send more serial data from your workstation. CTS is usually used to regulate the flow of serial data from your workstation to the other end RTS- Request To send The rts signal is set to the space voltage by your workstation to indicate that more data is ready to be sent Like cts, rts helps to regulate the flow of data between your workstation and the computer or device on the other end of the serial cable. Most workstations leave this signal set to the space voltage all the time Asynchronous Communications For the computer to understand the serial data coming into it, it needs some way to determine where one character ends and the next begins. This guide deals exclusively with asynchronous serial data In asynchronous mode the serial data line stays in the mark (1)state until a character is transmitted a start bit preceeds each character and is followed immediately by each bit in the character, an optional parity bit, and one or more stop bits. The start bit is always a space (O)and tells the computer that new serial data is available. Data can be sent or received at any time, thus the name asynchronous 第4页共26页 Serial programming Guide for posiX Operating Systems https://www.cmrr.umnedu/strupp/serial.htm1#contents Figure 1- Asynchronous Data Transmission SPAGE +12V MARK 12v The optional parity bit is a simple sum of the data bits indicating whether or not the data contains an even or odd number of 1 bits. With even parity, the parity bit is 0 if there is an even number of l's in the character. With odd parity, the parity bit is 0 if there is an odd number of I's in the data. You may also hear the terms space parity, mark parity, and no parity space parity means that the parity bit is always 0, while mark parity means the bit is always 1. No parity means that no parity bit is present or transmitted The remaining bits are called stop bits. There can be 1,1.5, or 2 stop bits between characters and they always have a value of 1. Stop bits traditionally were used to give the computer time to process the previous character, but now only serve to synchronize the receiving computer to the incoming characters Asynchronous data formats are usually expressed as 8N1", 7El", and so forth These stand for"8 data bits, no parity, I stop bit "and"7 data bits, even parity, I stop bit" respectively What Are Full Duplex and Half duplex? Full duplex means that the computer can send and receive data simultaneously -there are two separate data channels(one coming in, one going out Half duplex means that the computer cannot send or receive data at the same time. Usually this means there is only a single data channel to talk over. This does not mean that any of the rs-232 signals are not used. Rather, it usually means that the communications link uses some standard other than rs-232 that does not support full duplex operation Flow control It is often necessary to regulate the flow of data when transferring data between two serial interfaces. This can be due to limitations in an intermediate serial communications link, one of the serial interfaces, or some storage media. Two methods are commonly used for asynchronous data The first method is often called "software" flow control and uses special characters to start (Xon or dcl, 02l octal or stop (Xoff or DC3, 023 octal)the flow of data. These characters are defined in the American Standard Code for Information Interchange("ASCII" While these codes are useful when transferring textual information, they cannot be used when transferring other types of information without special programming The second method is called"hardware"flow control and uses the rs-232 CTS and RTS Signals instead of special characters. The receiver sets CTs to the space voltage when it is ready to receive more data and to the mark voltage when it is not ready. Likewise, the sender sets rts to the space voltage when it is ready to send more data. Because hardware flow control uses a separate set of signals, it is much faster than software flow control which needs to send or receive multiple bits of information to do the same thing CTS/ RTS flow control is not supported by all hardware or operating systems What s a break? 第5页共26页 Serial programming Guide for posiX Operating Systems https://www.cmrr.umn.edustrupp/serial.html#contents Normally a receive or transmit data signal stays at the mark voltage until a new character is transferred. If the signal is dropped to the space voltage for a long period of time, usually 1/4 to 1/2 second, then a break condition is said to exist a break is sometimes used to reset a communications line or change the operating mode of communications hardware like a modem Chapter 3, Talking to MODEMs covers these applications in more depth Synchronous Communications Unlike asynchronous data, synchronous data appears as a constant stream of bits. To read the data on the line, the computer must provide or receive a common bit clock so that both the sender and receiver are synchronized Even with this synchronization, the computer must mark the beginning of the data somehow. The most common way of doing this is to use a data packet protocol like Serial Data Link Control ("SDlC )or High-Speed Data Link Control ("HDLC") Each protocol defines certain bit sequences to represent the beginning and end of a data packet Each also defines a bit sequence that is used when there is no data. These bit sequences allow the computer see the beginning of a data packet Because synchronous protocols do not use per-character synchronization bits they typically provide at least a 25% improvement in performance over asynchronous communications and are suitable for remote networking and configurations with more than two serial interfaces Despite the speed advantages of synchronous communications, most Rs-232 hardware does not support it due to the extra hardware and software required Accessing Serial Ports Like all devices, UNIX provides access to serial ports via device files. To access a serial port you simply open the corresponding device file Serial Port filles Each serial port on a UNIX System has one or more device files(files in the /dev directory) associated with it Table 2-Serial port Device files sⅤstem Port 1 Port 2 IRIXR Vdev/tyfl /dev/ty f2 HP-UX /dev/tty l po/dev/tty 2pO Solaris R/SunOS(R/ dev/ttya /dev/ttyb LinuX R /dev/ttys0 /dev/ttyS1 Digital uNiX dev/ttyo1 /dev/tty02 Opening a serial Port Since a serial port is a file, the open (2) function is used to access it. The one hitch with uNiX is that device files are usually not accessable by normal users. Workarounds include changing the access permissions to the file(s)in question, running your program as the super-user(root), or making your program set-userid so that it runs as the owner of the device file 第6页共26页 Serial programming Guide for posiX Operating Systems https://www.cmrr.umn.edustrupp/serial.html#contents For now we ll assume that the file is accessable by all users. The code to open serial port 1 on an sgiB workstation running IrIX is Listing 1-Opening a serial port #include k standard input/output de=initions */ #include /* String function definitions * #include /* UNIx standard function definitions #include /* File control definitions * #include errno.h> /* Error number definitions #include /* posix terminal control definitions * x open port()-open serial port 1 Returns the file descriptor on success or -1 on error int open port(void) int fd: / File descriptor for the port x fd oper("/dev/ttyfl",O RDWR I C NOCTTY O NDELAY) if(fd==-1) 大Cou1 d not open the port perrcr("open port: Unable to open /dev/ttyf1 } else fcntl(fd, F stFt o)i return (fd) Other systems would require the corresponding device file name, but otherwise the code is the same Open Options You' ll notice that when we opened the device file we used two other flags along with the read-+ write mode fd= open("/dev/ttyf1", O RDWR I O NOCTTY I O NDE-AY)i The O NOCTTY flag tells UNIX that this program doesn't want to be the"controlling terminal "for that port. If you don' t specify this then any input(such as keyboard abort signals and so forth)will affect your process. Programs like getty(1M/8 )use this feature when starting the login process, but normally a user program does not want this behavior The O NDELAy flag tells UNIX that this program doesn't care what state the dCd signal line is in-whether the other end of the port is up and running. If you do not specify this flag, your process will be put to sleep until the dCd signal line is the space voltage Writing data to the port Writing data to the port is easy-just use the write(2) system call to send data it n= write(fd,AZ\2,4) f(n<0) puts("write()of 1 bytes failed! \n", stderr)i 第7页共26页 Serial programming Guide for posiX Operating Systems https://www.cmrr.umnedu/strupp/serial.html#contents The write function returns the number of bytes sent or-l if an error occurred Usually the only error you ll run into is elo when a MODEM or data link drops the Data Carrier Detect (DCD) line. This condition will persist until you close the port Reading data from the Port Reading data from a port is a little trickier. When you operate the port in raw data mode, each read(2) system call will return however many characters are actually available in the serial input buffers. If no characters are available, the call will block(wait)until characters come in, an interval timer expires, or an error occurs. The read function can be made to return immediately by doing the following fcntl(fd, F SETFL, FNDELAY The FNDELAY option causes the read function to return 0 if no characters are available on the port. To restore normal (blocking) behavior, call cnllo without the FNDELAY option fcntl(fd, F SETFL, 0) This is also used after opening a serial port with theo ndelay option Closing a serial Port To close the serial port, just use the close system call close(fd)i Closing a serial port will also usually set the dtR signal low which causes most MODEMs to hang up Chapter 2, Configuring the Serial Port This chapter discusses how to configure a serial port from C using the PosiX termios interface The posix terminal Interface Most systems support the POSIX terminal(serial) interface for changing parameters such as baud rate, character size, and so on. The first thing you need to do is include the file ; this defines the terminal control structure as well as the POsIX control functions The two most important POSIX functions are tcgetattr (3) and tcsetattr (3) These get and set terminal attributes, respectively; you provide a pointer to a termios structure that contains all of the serial options available Table 3- Termios Structure members Member Description c cflag Control options c lfas 8 Line options ifl g put options oflag Output options c cC Control characters Ic_ispeed Input baud(new interface) 第8页共26页 Serial programming Guide for posiX Operating Systems https://www.cmrr.umnedu/strupp/serial.html#contents c spee Output baud (new interface) Control Options The c cflag member controls the baud rate, number of data bits, parity, stop bits, and hardware flow control. There are constants for all of the supported configurations Table 4-Constants for the c cflag member Constant Description BAUD Bit mask for baud rate Bo 0 baud( drop DtR B50 50 baud B75 75 baud B110 110 baud B134 134.5 baud B150 150 baud B200 200 baud B300 300 baud B600 600 baud B1200 200 baud Bl800 1800 baud B2400 2400 baud B4800 4800 baud B9600 9600 baud B19200 19200 baud B38400 38400 baud B57600 57,600baud B76800 76800 baud B115200 115,200baud EXTA External rate clock EXTB External rate clock CSIZE Bit mask for data bits CS5 data bits CS6 6 data bits CS7 7 data bits CS8 8 data bits CSTOPB 2 stop bits(I otherwise) CREAD Enable receiver PARenT Enable parity bit PARODI Use odd parity instead of even HUPCL Hangup( drop DTR)on last close 第9页共26页 Serial programming Guide for posiX Operating Systems https://www.cmrr.umnedu/strupp/serial.htm1#contents CLOCAL Local line -do not change"owner"of port LOBLK Block job control output CNEW RTSCTS Enable hardware flow control(not CRTSCTS supported on all platforms) The c cflag member contains two options that should always be enabled, CLOCAL and CREAD. These will ensure that your program does not become the ownerof the port subject to sporatic job control and hangup signals, and also that the serial interface driver will read incoming data bytes The baud rate constantS(CBAUD, B9600, etc. ) are used for older interfaces that lack the c ispeed and c speed members. See the next section for in formation on the posix functions used to set the baud rate Never initialize the c cflag(or any other flag) member directly; you should always use the bitwise AND, OR, and not operators to set or clear bits in the members. Different operating system versions(and even patches) can and do use the bits differently, so using the bitwise operators will prevent you from clobbering a bit flag that is needed in a newer serial driver Setting the Baud Rate using one of the baud rate constants in table 4, while newer implementations provide the c ispeed and c-ospeed members that contai The baud rate is stored in different places depending on the operating system. Older interfaces store the baud rate in the c cflag member the actual baud rate value The cfsetospeed (3) and cfsetispeed functions are provided to set the baud rate in the termios structure regardless of the underlying operating system interface. Typically you'd use the following code to set the baud rate Listing 2- Sctting the baud rate struct termios options Cet the current opions for the pcrt tcgetattr(fd, &options)i x Set the baud rates = o 19200. cfsetispeed(&options, B19200 cfsetospeed(&options, B19200) Enable the receiver and set local mode options.c cflag (CLOCAL I CREAD); Set the new options for the port tcsctattr(fd, TCSANOW, options)i The tcgetattr() function fills the termios structure you provide with the current serial port configuration. After we set the baud rates and enable local mode and serial data receipt, we select the new configuration using tcsetattr(3). The TCSANOW constant specifies that all changes should occur immediately without waiting for output data to finish sending or input data to finish receiving. There are other constants to wait for input and output to finish or to flush the input and output buffers 第10页共26页

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