Serial Port Programming Linux Pdf Tutorial [Extra Quality]
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Unluckily, using serial ports in Linux is not the easiest thing in the world. When dealing with the termios.h header, there are many finicky settings buried within multiple bytes worth of bitfields. This page is an attempt to help explain these settings and show you how to configure a serial port in Linux correctly.
To write to a serial port, you write to the file. To read from a serial port, you read from the file. Of course, this allows you to send/receive data, but how do you set the serial port parameters such as baud rate, parity, e.t.c This is set by a special tty configuration struct.
At this point we could technically read and write to the serial port, but it will likely not work, because the default configuration settings are not designed for serial port use. So now we will set the configuration correctly.
We need access to the termios struct in order to configure the serial port. We will create a new termios struct, and then write the existing configuration of the serial port to it using tcgetattr(), before modifying the parameters as needed and saving the settings with tcsetattr().
UNIX systems provide two basic modes of input, canonical and non-canonical mode. In canonical mode, input is processed when a new line character is received. The receiving application receives that data line-by-line. This is usually undesirable when dealing with a serial port, and so we normally want to disable canonical mode.
The c_oflag member of the termios struct contains low-level settings for output processing. When configuring a serial port, we want to disable any special handling of output chars/bytes, so do the following:
Both OXTABS and ONOEOT are not defined in Linux. Linux however does have the XTABS field which seems to be related. When compiling for Linux, I just exclude these two fields and the serial port still works fine.
Rather than use bit fields as with all the other settings, the serial port baud rate is set by calling the functions cfsetispeed() and cfsetospeed(), passing in a pointer to your tty struct and a enum:
You can use FIONREAD along with ioctl() to see if there are any bytes available in the OS input (receive) buffer for the serial port1. This can be useful in a polling-style method in where the application regularly checks for bytes before trying to read them.
In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time.[1] This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel. Throughout most of the history of personal computers, data has been transferred through serial ports to devices such as modems, terminals, various peripherals, and directly between computers.
While interfaces such as Ethernet, FireWire, and USB also send data as a serial stream, the term serial port usually denotes hardware compliant with RS-232 or a related standard, such as RS-485 or RS-422.
Modern consumer personal computers (PCs) have largely replaced serial ports with higher-speed standards, primarily USB. However, serial ports are still frequently used in applications demanding simple, low-speed interfaces, such as industrial automation systems, scientific instruments, point of sale systems and some industrial and consumer products.
Server computers may use a serial port as a control console for diagnostics, while networking hardware (such as routers and switches) commonly use serial console ports for configuration, diagnostics, and emergency maintenance access. To interface with these and other devices, USB-to-serial converters can quickly and easily add a serial port to a modern PC.
Modern devices use an integrated circuit called a UART to implement a serial port. This IC converts characters to and from asynchronous serial form, implementing the timing and framing of data specified by the serial protocol in hardware. The IBM PC implements its serial ports, when present, with one or more UARTs.
Very low-cost systems, such as some early home computers, would instead use the CPU to send the data through an output pin, using the bit banging technique. These early home computers often had proprietary serial ports with pinouts and voltage levels incompatible with RS-232.
Before large-scale integration (LSI) made UARTs common, serial ports were commonly used in mainframes and minicomputers, which would have multiple small-scale integrated circuits to implement shift registers, logic gates, counters, and all the other logic needed. As PCs evolved serial ports were included in the Super I/O chip and then in the chipset.
The individual signals on a serial port are unidirectional and when connecting two devices, the outputs of one device must be connected to the inputs of the other. Devices are divided into two categories: data terminal equipment (DTE) and data circuit-terminating equipment (DCE). A line that is an output on a DTE device is an input on a DCE device and vice versa, so a DCE device can be connected to a DTE device with a straight wired cable, in which each pin on one end goes to the same numbered pin on the other end.
Generally, serial port connectors are gendered, only allowing connectors to mate with a connector of the opposite gender. With D-subminiature connectors, the male connectors have protruding pins, and female connectors have corresponding round sockets.[2] Either type of connector can be mounted on equipment or a panel; or terminate a cable.
The 9-pin DE-9 connector has been used by most IBM-compatible PCs since the Serial/Parallel Adapter option for the PC-AT, where the 9-pin connector allowed a serial and parallel port to fit on the same card.[4] This connector has been standardized for RS-232 as TIA-574.
Unix-like operating systems usually label the serial port devices .mw-parser-output .monospaced{font-family:monospace,monospace}/dev/tty*. TTY is a common trademark-free abbreviation for teletype, a device commonly attached to early computers' serial ports, and * represents a string identifying the specific port; the syntax of that string depends on the operating system and the device. On Linux, 8250/16550 UART hardware serial ports are named /dev/ttyS*, USB adapters appear as /dev/ttyUSB* and various types of virtual serial ports do not necessarily have names starting with tty.
This list includes some of the more common devices that are connected to the serial port on a PC. Some of these such as modems and serial mice are falling into disuse while others are readily available. Serial ports are very common on most types of microcontroller, where they can be used to communicate with a PC or other serial devices.
Since the control signals for a serial port can be driven by any digital signal, some applications used the control lines of a serial port to monitor external devices, without exchanging serial data. A common commercial application of this principle was for some models of uninterruptible power supply which used the control lines to signal loss of power, low battery, and other status information. At least some Morse code training software used a code key connected to the serial port to simulate actual code use; the status bits of the serial port could be sampled very rapidly and at predictable times, making it possible for the software to decipher Morse code.
A standard series of rates is based on multiples of the rates for electromechanical teleprinters; some serial ports allow many arbitrary rates to be selected, but the speeds on both sides of the connection must match for data to be received correctly. Bit rates commonly supported include 75, 110, 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bit/s.[13] Many of these standard modem baud rates are multiples of either 0.9 kbps (e.g., 19200, 38400, 76800) or 1.2 kbps (e.g., 57600, 115200).[17] Crystal oscillators with a frequency of 1.843200 MHz are sold specifically for this purpose. This is 16 times the fastest bit rate, and the serial port circuit can easily divide this down to lower frequencies as required.
The capability to set a bit rate does not imply that a working connection will result. Not all bit rates are possible with all serial ports. Some special-purpose protocols such as MIDI for musical instrument control, use serial data rates other than the teleprinter standards. Some serial port implementations can automatically choose a bit rate by observing what a connected device is sending and synchronizing to it.
Most serial communications designs send the data bits within each byte least significant bit first. Also possible, but rarely used, is most significant bit first; this was used, for example, by the IBM 2741 printing terminal. The order of bits is not usually configurable within the serial port interface but is defined by the host system. To communicate with systems that require a different bit ordering than the local default, local software can re-order the bits within each byte just before sending and just after receiving.
Parity is a method of detecting errors in transmission. When parity is used with a serial port, an extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. Correct parity does not necessarily indicate absence of corruption as a corrupted transmission with an even number of errors will pass the parity check. A single parity bit does not allow implementation of error correction on each character, and communication protocols working over serial data links will typically have higher-level mechanisms to ensure data validity and request retransmission of data that has been incorrectly received.
Odd parity is more useful than even parity since it ensures that at least one state transition occurs in each character, which makes it more reliable at detecting errors like those