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.
While these codes are useful when transferring textual information, they cannot be used when transferring other types of information without special programming. 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. Normally a receive or transmit data signal stays at the mark voltage until a new character is transferred. A break is sometimes used to reset a communications line or change the operating mode of communications hardware like a MODEM.
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. 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. Despite the speed advantages of synchronous communications, most RS 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. 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.
For now we'll assume that the file is accessable by all users. Other systems would require the corresponding device file name, but otherwise the code is the same.
If you don't specify this then any input such as keyboard abort signals and so forth will affect your process. If you do not specify this flag, your process will be put to sleep until the DCD signal line is the space voltage. The write function returns the number of bytes sent or -1 if an error occurred. This condition will persist until you close 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:. Most systems support the POSIX terminal serial interface for changing parameters such as baud rate, character size, and so on.
These will ensure that your program does not become the 'owner' of the port subject to sporatic job control and hangup signals, and also that the serial interface driver will read incoming data bytes. 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.
The baud rate is stored in different places depending on the operating system. The cfsetospeed 3 and cfsetispeed 3 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:. The tcgetattr 3 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. Most systems do not support different input and output speeds, so be sure to set both to the same value for maximum portability.
Unlike the baud rate, there is no convienience function to set the character size. Instead you must do a little bitmasking to set things up. The character size is specified in bits:. Like the character size you must manually set the parity enable and parity type bits. UNIX serial drivers support even, odd, and no parity bit generation. Space parity can be simulated with clever coding.
Do the following to enable hardware flow control:. Canonical input is line-oriented. Input characters are put into a buffer which can be edited interactively by the user until a CR carriage return or LF line feed character is received. Raw input is unprocessed. Input characters are passed through exactly as they are received, when they are received.
IGNPAR is a somewhat dangerous option that tells the serial driver to ignore parity errors and pass the incoming data through as if no errors had occurred. This can be useful for testing the quality of a communications link, but in general is not used for practical reasons. Like the input modes, you can select processed or raw data output. The rest of the output options are primarily historic and date back to the time when line printers and terminals could not keep up with the serial data stream!
Constants are defined for every element of this array. UNIX serial interface drivers provide the ability to specify character and packet timeouts. VMIN specifies the minimum number of characters to read. Note that this does not mean that a read call for N bytes will wait for N characters to come in. Rather, the timeout will apply to the first character and the read call will return the number of characters immediately available up to the number you request.
If a character is read within the time given, any read will block wait until all VMIN characters are read. That is, once the first character is read, the serial interface driver expects to receive an entire packet of characters VMIN bytes total. If no character is read within the time allowed, then the call to read returns 0.
This method allows you to tell the serial driver you need exactly N bytes and any read call will return 0 or N bytes. However, the timeout only applies to the first character read, so if for some reason the driver misses one character inside the N byte packet then the read call could block forever waiting for additional input characters. VTIME specifies the amount of time to wait for incoming characters in tenths of seconds.
MODEMs are devices that modulate serial data into frequencies that can be transferred over an analog data link such as a telephone line or cable TV connection. A standard telephone MODEM converts serial data into tones that can be passed over the phone lines; because of the speed and complexity of the conversion these tones sound more like loud screeching if you listen to them.
Telephone MODEMs are available today that can transfer data across a telephone line at nearly 53, bits per second, or 53kbps. In addition, most MODEMs use data compression technology that can increase the bit rate to well over kbps on some types of data. Each command is sent with the "AT" characters starting in the first column followed by the specific command and a carriage return CR, octal.
After processing the command the MODEM will reply with one of several textual messages depending on the command. The ATD command dials the specified number. In addition to numbers and dashes you can specify tone "T" or pulse "P" dialing, pause for one second "," , and wait for a dialtone "W" :.
First and foremost, don't forget to disable input echoing. This chapter covers advanced serial programming techniques using the ioctl 2 and select 2 system calls.
In Chapter 2, Configuring the Serial Port we used the tcgetattr and tcsetattr functions to configure the serial port. Under UNIX these functions use the ioctl 2 system call to do their magic. The fd argument specifies the serial port file descriptor. To drop the DTR signal you can do:. The bits that can be set depend on the operating system, driver, and modes in use. PonyProg serial device programmer.
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