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. If this driver is not installed properly or is not compatible with a serial port, it may cease to perform.
Re-installing the affected drivers can help a user overcome this problem. In addition, a user may be able to alter the serial port settings to ensure this issue is fully resolved. If a connected system needs to receive a response from a controlled device, however, a third wire also may be connected as well.
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DCE devices will have the voltage on Pin 2. Straight-through cables are used to connect a DTE e. When connecting two DTE devices directly with no modem in between, a crossover, or null-modem cable is used.
This type of cable cross transmits and receives data signals between the two sides. There is no standard and many variations on how the other control signals are wired. Below is an example of one of them:. RS connects the Ground of 2 different devices together, which is the so-called "Unbalanced" connection. Unbalanced connections have a distance limitation of 50 ft approximately 15 meters and are very susceptible to noise.
Eltima IBC uses cookies to personalize your experience on our website. By continuing to use this site, you agree to our cookie policy. Click here to learn more. Toggle navigation Virtual Serial Port. Olga Weis Nov 28, Contents What does RS stand for? In bit mode, the new displayed value is written to the register whenever the user clicks on and toggles an LED. In byte mode, the user can toggle LEDs or enter hex data directly by keyboard, and the new displayed value is written to the data register when the user clicks on the Write button.
If the user clicks on the Strobe data button, the displayed value is written to the data register and the nStrobe signal pin 1 is strobed. This mode is useful for writing data to a printer. Regardless of operating mode, the user can always toggle the Bi-Directional Enable bit in the control register.
The Bi-directional Enable bit is an internal signal used to control direction on the data register of most newer PC printer ports most laptops and and Pentium systems have a printer port that allows bi-directional data flow. VBPortTest was developed to meet specific requirements to test and debug parallel port interfaces. Online Help with useful parallel port reference material includes signal descriptions and Centronics handshake timing waveform.
A beta version of VBPortTest is available for download as freeware. Windows 98 and ME compatible. Download VBPortTest about 1. PC Serial and Parallel.
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