Serial Programming HOWTO
Linux 串口编程 中英文简体对照 beta 版
翻译：Carol Li
原作：Gary Frerking gary@frerking.org
Peter Baumann
This document describes how to program communications with devices over a serial port on a Linux box.
本文档记述了如何在Linux设备上通过串口进行通信的程序开发
中文简体版以英文 1.01 版为原文，同时参考了繁体中文的串口编程HOWTO
1. Introduction 简介
This is the Linux Serial Programming HOWTO. All about how to program communications with other devices / computers over a serial line under Linux. Different techniques are explained: Canonical I/O (only complete lines are transmitted/received), asyncronous I/O, and waiting for input from multiple sources.
This is the first update to the initial release of the Linux Serial Programming HOWTO. The primary purpose of this update is to change the author information and convert the document to DocBook format. In terms of technical content, very little if anything has changed at this time. Sweeping changes to the technical content aren't going to happen overnight, but I'll work on it as much as time allows.
If you've been waiting in the wings for someone to take over this HOWTO, you've gotten your wish. Please send me any and all feedback you have, it'd be very much appreciated.
All examples were tested using a i386 Linux Kernel 2.0.29.
本文是为 Linux 串口程序编写的 HOWTO. 主要讨论如何在 Linux 环境下，编写串口与其它计算机设备进行通讯的程序。文中所谈到的技术包括: 标准的 I/O (只具备 传送/接收 线的), 异步 I/O, 以及 等待来自多信号源输入 的程序。
本文是初始的 linux serial programming howto 的第一个升级版。主要升级了一些作者信息，把文件转换为 DocBook 格式。就技术内容而言，几乎没什么大的改变。大规模的技术内容的改变是不可能一夜之间发生的，如果时间允许，我会尽量做一些工作。
如果你正在一边等着有谁来接管这份 HOWTO,那你的心愿达成了。我会感谢你发来的任何反馈信息。
所有的示例都在 i386 Linux Kernel 2.0.29 下测试通过。
1.1. Copyright Information 版权信息
This document is copyrighted (c) 1997 Peter Baumann, (c) 2001 Gary Frerking and is distributed under the terms of the Linux Documentation Project (LDP) license, stated below.
Unless otherwise stated, Linux HOWTO documents are copyrighted by their respective authors. Linux HOWTO documents may be reproduced and distributed in whole or in part, in any medium physical or electronic, as long as this copyright notice is retained on all copies. Commercial redistribution is allowed and encouraged; however, the author would like to be notified of any such distributions.
Linux Serial-Programming-HOWTO 的版權(C) 1997 归 Peter Baumann 所有，(C) 2001 归 Gary Frerking 所有，并且以 LDP lisence (附后)发布。
除非另做申明，Linux HOWTO 文件的版权归各自的作者所有。Linux HOWTO 文件可以完整或部份的以實物或電子版形式复制或者发布, 只要能在所有的拷贝中保留版權申明即可。我们鼓励允许商業的发布，不過, 如果以此形式发布的话，请告知作者。
All translations, derivative works, or aggregate works incorporating any Linux HOWTO documents must be covered under this copyright notice. That is, you may not produce a derivative work from a HOWTO and impose additional restrictions on its distribution. Exceptions to these rules may be granted under certain conditions; please contact the Linux HOWTO coordinator at the address given below.
In short, we wish to promote dissemination of this information through as many channels as possible. However, we do wish to retain copyright on the HOWTO documents, and would like to be notified of any plans to redistribute the HOWTOs.
If you have any questions, please contact <linux-howto@metalab.unc.edu>
所有的翻譯, 以及其他派生的工作, 或整合合併任何 Linux HOWTO 文件都必須在此版權申明的規範下进行. 也就是说, 你不可以在从 HOWTO 所衍生的工作中, 散佈的文件上附加額外的限制條款。 除了這些規則，都可获得某種條件的授與; 請见后面的地址来聯絡 Linux HOWTO 協調員。
簡而言之, 我們希望儘可能得透過各種渠道来促進這份资料的流通, 不過, 我们強烈希望將此版權宣告置於 HOWTO 的文件上, 并且希望任何想重新发佈 HOWTO 的人可以通知我們一下。
如果你有問題, 請通过 email linux-howto@metalab.unc.edu 进行联系.
1.2. Disclaimer 申明
No liability for the contents of this documents can be accepted. Use the concepts, examples and other content at your own risk. As this is a new edition of this document, there may be errors and inaccuracies, that may of course be damaging to your system. Proceed with caution, and although this is highly unlikely, the author(s) do not take any responsibility for that.
All copyrights are held by their by their respective owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark.
Naming of particular products or brands should not be seen as endorsements.
You are strongly recommended to take a backup of your system before major installation and backups at regular intervals.
使用本文的概念，例子及其他内容的风险由您自己承担，我们对此造成的后果不负责任。由于这是份新的文档，可能存在着错误或误差，而有可能导致对您的系统的损害。请小心操作，虽然这是几乎不可能发生的，作者不对此承担任何责任。
除非特别标注，所有的版权归其各自的作者。使用此文档不可标住任何商标或服务标记。
特定的产品或品牌的命名不可被理解为是认可的。（晕，这一段应该请律师来翻）。
强烈推荐您在重大的安装前备份系统，并且做到定期备份。
1.3. New Versions 版本更新
As previously mentioned, not much is new in terms of technical content yet.
如前面所提到的，此版本在技术内容上较前一版本并没有什么大的更新。
1.4. Credits 感谢
The original author thanked Mr. Strudthoff, Michael Carter, Peter Waltenberg, Antonino Ianella, Greg Hankins, Dave Pfaltzgraff, Sean Lincolne, Michael Wiedmann, and Adrey Bonar.
原作者感谢 Strudthoff, Michael Carter, Peter Waltenberg, Antonino Ianella, Greg Hankins, Dave Pfaltzgraff, Sean Lincolne, Michael Wiedmann, and Adrey Bonar 诸位先生。
1.5. Feedback 意见反馈
Feedback is most certainly welcome for this document. Without your submissions and input, this document wouldn't exist. Please send your additions, comments and criticisms to the following email address : <gary@frerking.org>.
非常欢迎对此文档提出反馈意见。有任何补充，评论，批评请发信到： gary@frerking.org
2. Getting started 入门
2.1. Debugging 调试
The best way to debug your code is to set up another Linux box, and connect the two computers via a null-modem cable. Use miniterm (available from the LDP programmers guide (ftp://sunsite.unc.edu/pub/Linux/docs/LDP/programmers-guide/lpg-0.4.tar.gz in the examples directory) to transmit characters to your Linux box. Miniterm can be compiled very easily and will transmit all keyboard input raw over the serial port. Only the define statement #define MODEMDEVICE "/dev/ttyS0" has to be checked. Set it to ttyS0 for COM1, ttyS1 for COM2, etc.. It is essential for testing, that all characters are transmitted raw (without output processing) over the line. To test your connection, start miniterm on both computers and just type away. The characters input on one computer should appear on the other computer and vice versa. The input will not be echoed to the attached screen.
调试代码最好的方法，是另外建立一台Linux主机（Linux box），采用非调制解调器的串口线（null-modem）连接两台机器。还可以使用minicom (可以从 LDP 编程指南上获得：ftp://sunsite.unc.edu/pub/Linux/docs/LDP/programmers-guide/lpg-0.4.tar.gz 里的examples 目录)来传输字符到你的 Linux 主机。Miniterm 很容易编译而且会直接把键盘的输入不做处理(raw 方式)地传到串口。只需要把 定义申明 #define MODEMDIVICE “/dev/ttyS0” 改一下。 COM1 口就设置成 ttyS0, COM2 口就是 ttyS1, 以此类推… 测试是必需的，所有的字符直接通过缆线传输，不进行任何输出处理。为了测试你的连接，在你的两台机器上开启minicom，然后随意输入一些字符。从一台电脑中输入的字符应该能显示在另一台设备上，反之亦然。输入的字符不会回显(echo) 在本地的屏幕上.
To make a null-modem cable you have to cross the TxD (transmit) and RxD (receive) lines. For a description of a cable see sect. 7 of the Serial-HOWTO.
自制非调制解调器的串口连接线（null-modem cable）时，你需要将一端的传送端（TxD）与另一端的接收端（RxD）连接，一端的接收端与另外一端的传送端连接，详情见Serial-HOWTO第七节。
It is also possible to perform this testing with only one computer, if you have two unused serial ports. You can then run two miniterms off two virtual consoles. If you free a serial port by disconnecting the mouse, remember to redirect /dev/mouse if it exists. If you use a multiport serial card, be sure to configure it correctly. I had mine configured wrong and everything worked fine as long as I was testing only on my computer. When I connected to another computer, the port started loosing characters. Executing two programs on one computer just isn't fully asynchronous.
如果你的电脑有两个空闲的串口端口的话，那么只要使用一台机器就可以做这些试验了，你可以在两个虚拟控制台上各运行一个miniterm，分别用来发送和接收结果。如果你使用串口鼠标，记得在试验前将 /dev/mouse 重定向。如果你使用多端口的串口卡（multiport serial card）,一定要确保此设备配置正确，我的电脑就曾因为配置错误，而出现这样的问题：当我在自己的机器上测试的时候一切正常，而连接到其他电脑上的时候，端口开始丢失数据。注意，在一台机器上运行两个串口应用程序，并不是完全异步的。
2.2. Port Settings 端口设置
The devices /dev/ttyS* are intended to hook up terminals to your Linux box, and are configured for this use after startup. This has to be kept in mind when programming communication with a raw device. E.g. the ports are configured to echo characters sent from the device back to it, which normally has to be changed for data transmission.
设备 /dev/ttyS* 会被当作连接到你的 Linux 机器的终端设备，而且在系统启动后就已经配置好了。這一点是在你寫 raw 设备的串口通信程式時必需牢記的. 举例来说，這個串口被設定為回显（echo）所有自此设备送出的字符, 通常在做数据传输时，需要改變這種工作模式。
All parameters can be easily configured from within a program. The configuration is stored in a structure struct termios, which is defined in <asm/termbits.h>:
#define NCCS 19
struct termios {
tcflag_t c_iflag; /* input mode flags */
tcflag_t c_oflag; /* output mode flags */
tcflag_t c_cflag; /* control mode flags */
tcflag_t c_lflag; /* local mode flags */
cc_t c_line; /* line discipline */
cc_t c_cc[NCCS]; /* control characters */
}
所有的参数都可以在程序中轻松配置。配置保存在结构体 struct termios 中，这个结构是在 <asm/termbits.h> 中定义的
#define NCCS 19
struct termios {
tcflag_t c_iflag; /* 输入模式标志 */
tcflag_t c_oflag; /* 输出模式标志 */
tcflag_t c_cflag; /* 控制模式标志 */
tcflag_t c_lflag; /* 本地模式标志 */
cc_t c_line; /* 行控制 line discipline */
cc_t c_cc[NCCS]; /* 控制字符 control characters */
}
This file also includes all flag definitions. The input mode flags in c_iflag handle all input processing, which means that the characters sent from the device can be processed before they are read with read. Similarly c_oflag handles the output processing. c_cflag contains the settings for the port, as the baudrate, bits per character, stop bits, etc.. The local mode flags stored in c_lflag determine if characters are echoed, signals are sent to your program, etc.. Finally the array c_cc defines the control characters for end of file, stop, etc.. Default values for the control characters are defined in <asm/termios.h>. The flags are described in the manual page termios(3). The structure termios contains the c_line (line discipline) element, which is not used in POSIX compliant systems.
这个文件也包括了所有标志 (flag) 的定义。c_iflag 中的输入模式标志，进行所有的输入处理，也就是说从其他设备传来的字符，在被read函数读入之前，会先按照标志进行预处理。同样的，c_oflag 定义了所有的输出处理。c_cflag 包括了对端口的设置，比如波特率，停止符号等等。c_lflag 包括了，决定了字符是否回显，信号是否发送到你的程序，等等所有的本地工作方式。c_cc 定义了所有的控制符号，例如文件结束符和停止符等等，所有的这些参数的默认值都定义在<asm/termios.h>中，man手册页termios(3)中有这些参数的具体描述。termio结构体中还包括在不能在 POSIX 系统中使用的c_line参数(行控制)
2.3. Input Concepts for Serial Devices 串口设备的输入概念
Here three different input concepts will be presented. The appropriate concept has to be chosen for the intended application. Whenever possible, do not loop reading single characters to get a complete string. When I did this, I lost characters, whereas a read for the whole string did not show any errors.
这里将介绍串行设备三种不同的输入方式，你需要为你的程序选择合适的工作方式。任何可能的情况下，不要采用循环读取单字符的方式来获取一个字符串。我以前这样做的时候，就丢失了字符，而读取整个字符串的 read 方法，则没有这种错误。
2.3.1. Canonical Input Processing 标准输入模式
This is the normal processing mode for terminals, but can also be useful for communicating with other dl input is processed in units of lines, which means that a read will only return a full line of input. A line is by default terminated by a NL (ASCII LF), an end of file, or an end of line character. A CR (the DOS/Windows default end-of-line) will not terminate a line with the default settings.
Canonical input processing can also handle the erase, delete word, and reprint characters, translate CR to NL, etc..
这是终端设备的标准处理模式, 在与其它 dl 的以行为单位的输入通讯中也很有用。这种方式中， read 会传回一整行完整的输入. 一行的结束，默认是以 NL (ASCII值 LF), 文件结束符, 或是一个行结束字符。默认设置中, CR ( DOS/Windows 里的默认行结束符) 并不是行结束标志。
标准的输入处理还可以处理 清除, 删除字, 重画字符, 转换 CR 为 NL 等等功能。
2.3.2. Non-Canonical Input Processing 非标准输入模式
Non-Canonical Input Processing will handle a fixed amount of characters per read, and allows for a character timer. This mode should be used if your application will always read a fixed number of characters, or if the connected device sends bursts of characters.
非标准输入处理可以用于需要每次读取固定数量字符的情况下, 并允许使用字符接收时间的定时器。这种模式可以用在每次读取固定长度字符串的程序中, 或者所连接的设备会突然送出大量字符的情况下。
2.3.3. Asynchronous Input 异步输入模式
The two modes described above can be used in synchronous and asynchronous mode. Synchronous is the default, where a read statement will block, until the read is satisfied. In asynchronous mode the read statement will return immediatly and send a signal to the calling program upon completion. This signal can be received by a signal handler.
前面敘述的兩種模式都可以用在同步與异步的傳輸模式。默认是在同步的模式下工作的, 也就是在尚未讀完数据之前, read 的狀態會被阻塞(block)。而在异步模式下，read 的狀態會立即返回並送出一个信号到所调用的程式直到完成工作。這個信号可以由信号处理程式 handler來接收。
2.3.4. Waiting for Input from Multiple Sources 等待来自多信号源的输入
This is not a different input mode, but might be useful, if you are handling multiple devices. In my application I was handling input over a TCP/IP socket and input over a serial connection from another computer quasi-simultaneously. The program example given below will wait for input from two different input sources. If input from one source becomes available, it will be processed, and the program will then wait for new input.
The approach presented below seems rather complex, but it is important to keep in mind that Linux is a multi-processing operating system. The select system call will not load the CPU while waiting for input, whereas looping until input becomes available would slow down other processes executing at the same time.
本节介绍的不是另一个输入模式，不过如果你要处理来自多个设备的数据的话，可能会很有用。在我的应用程序中，我需要同时通过一个 TCP/IP socket 和一个串口来处理其它计算机传来的输入。下面给出的示例程序将等待来自两个不同输入源的输入。如果其中一个信号源出现, 程序就会进行相应处理, 同时程序会继续等待新的输入。
后面提出的方法看起来相当覆杂, 但请记住 Linux 是一个多进程的操作系统。 系统调用 select 并不会在等待输入信号时增加 CPU 的负担，而如果使用轮询方式来等待输入信号的话，则将拖慢其它正在执行的进程。
