Linux串口编程指南：POSIX系统详解

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《POSIX操作系统串行编程指南》第五版由Michael R. Sweet撰写，版权保护日期为1994-1999年。这本书是关于Linux系统下的串口编程入门和进阶教程，特别适合那些希望在POSIX兼容的环境下掌握串口通信技术的专业人士。以下章节概述了书中的关键知识点： 1. **基础串行通信**：本章介绍了串行通信的基本原理，包括什么是串行通信、RS-232标准以及信号定义。串行通信是通过单条数据线在设备间进行点对点传输，RS-232是一种广泛使用的标准接口协议。作者解释了异步通信（如UART）的概念，区分了全双工和半双工通信方式，并讨论了流控制，如硬件和软件中断。 2. **配置串口**：这一章重点讲解如何在Linux系统中配置串口。涵盖了POSIX终端接口的使用，包括控制选项、本地选项、输入选项和输出选项的设置。此外，还详细介绍了控制字符，这些是用于设备间的交互和通信控制的重要元素。 3. **调制解调器通信**：对于涉及远距离通信的情况，如使用MODEM，本章探讨了MODEM的原理，如何与MODEM进行通信，以及常见的通信问题及其解决方案。标准的MODEM命令在此部分被详细介绍。 4. **高级串行编程**：这部分内容深入到更复杂的编程技巧，如使用系统调用（System Call）`ioctl()`来设置和获取串口控制信号，获取串口中的可用字节数，以及如何选择从串口接收数据。此外，还介绍了如何利用`SELECT`系统调用来处理多个文件描述符，增强程序的并发性和效率。《POSIX操作系统串行编程指南》不仅提供了一套全面的基础知识，而且通过实例和实用建议帮助读者理解和应用串口编程技术。无论是初次接触串口通信的开发者还是经验丰富的技术专家，都能从中找到所需的信息和指导。对于那些在Linux环境下工作并需要处理串口通信任务的人来说，这是一份不可或缺的参考资料。

Figure 1 - Asynchronous Data Transmission

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 1's in the character. With odd parity, the parity bit is 0 if there is an odd

number of 1'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", "7E1", and so forth. These stand for "8 data bits, no parity, 1 stop bit" and "7

data bits, even parity, 1 stop bit" respectively.

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.

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 DC1, 021 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.

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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.

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.

Like all devices, UNIX provides access to serial ports via device files. To access a serial port you simply open the corresponding device

file.

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

System Port 1 Port 2

IRIX® /dev/ttyf1 /dev/ttyf2

HP-UX /dev/tty1p0 /dev/tty2p0

Solaris®/SunOS® /dev/ttya /dev/ttyb

Linux® /dev/ttyS0 /dev/ttyS1

Digital UNIX® /dev/tty01 /dev/tty02

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.

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