Serial Port Storage Devices

A serial port server, also called a serial server or port redirector, is a device that transfers data between a computer serial port COM port and an Ethernet local.

A serial port is an interface on a computer system with which information is transferred in or out one bit at a time. An interface is a connection between two subsystems.

Serial buses edit

Many communication systems were generally originally designed to connect two integrated circuits on the same printed circuit board, connected by signal traces on that board rather than external cables.

Integrated circuits are more expensive when they have more pins. To reduce the number of pins in a package, many ICs use a serial bus to transfer data when speed is not important. Some examples of such low-cost serial buses include SPI, I C, UNI/O, and 1-Wire.

Serial versus parallel edit

The communication links across which computers or parts of computers talk to one another may be either serial or parallel. A parallel link transmits several streams of data simultaneously along multiple channels e.g., wires, printed circuit tracks, or optical fibres ; whereas, a serial link transmits only a single stream of data.

Although a serial link may seem inferior to a parallel one, since it can transmit less data per clock cycle, it is often the case that serial links can be clocked considerably faster than parallel links in order to achieve a higher data rate. A number of factors allow serial to be clocked at a higher rate:

Clock skew between different channels is not an issue for unclocked asynchronous serial communication links.

A serial connection requires fewer interconnecting cables e.g., wires/fibres and hence occupies less space. The extra space allows for better isolation of the channel from its surroundings.

Crosstalk is less of an issue, because there are fewer conductors in proximity.

In many cases, serial is a better option because it is cheaper to implement. Many ICs have serial interfaces, as opposed to parallel ones, so that they have fewer pins and are therefore less expensive.

Examples of serial communication architectures edit

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In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time in contrast to a parallel.

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Since the control signals for a serial port can be easily turned on and off by a switch, 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, battery low alarm 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.

Settings edit

A four-port serial RS-232 PCI Express  1 expansion card with an octopus cable that breaks the card s DC-37 connector into four standard DE-9 connectors

Many settings are required for serial connections used for asynchronous start-stop communication, to select speed, number of data bits per character, parity, and number of stop bits per character. In modern serial ports using a UART integrated circuit, all settings are usually software-controlled; hardware from the 1980s and earlier may require setting switches or jumpers on a circuit board. One of the simplifications made in such serial bus standards as Ethernet, FireWire, and USB is that many of those parameters have fixed values so that users can not and need not change the configuration; the speed is either fixed or automatically negotiated. Often if the settings are entered incorrectly the connection will not be dropped; however, any data sent will be received on the other end as nonsense.

Speed edit

Serial ports use two-level binary signaling, so the data rate in bits per second is equal to the symbol rate in bauds. 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. The port speed and device speed must match. 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 series. Some serial port systems can automatically detect the bit rate.

The speed includes bits for framing stop bits, parity, etc. and so the effective data rate is lower than the bit transmission rate. For example, with 8-N-1 character framing only 80 of the bits are available for data for every eight bits of data, two more framing bits are sent.

Bit rates commonly supported include 75, 110, 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bit/s. 15 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.

Data bits edit

The number of data bits in each character can be 5 for Baudot code , 6 rarely used, 7 for true ASCII , 8 for most kinds of data, as this size matches the size of a byte , or 9 rarely used. 8 data bits are almost universally used in newer applications. 5 or 7 bits generally only make sense with older equipment such as teleprinters.

Most serial communications designs send the data bits within each byte LSB Least significant bit first. This standard is also referred to as little endian. Also possible, but rarely used, is big endian or MSB Most Significant Bit first serial communications; this was used, for example, by the IBM 2741 printing terminal. See Bit numbering for more about bit ordering. The order of bits is not usually configurable within the serial port interface. 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.

Main article: Parity bit

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. However, an even number of errors can pass the parity check.

Electromechanical teleprinters were arranged to print a special character when received data contained a parity error, to allow detection of messages damaged by line noise. A single parity bit does not allow implementation of error correction on each character, and communication protocols working over serial data links will have higher-level mechanisms to ensure data validity and request retransmission of data that has been incorrectly received.

The parity bit in each character can be set to none N, odd O, even E, mark M, or space S. None means that no parity bit is sent at all. Mark parity means that the parity bit is always set to the mark signal condition logical 1 and likewise space parity always sends the parity bit in the space signal condition. Aside from uncommon applications that use the 9th parity bit for some form of addressing or special signalling, mark or space parity is uncommon, as it adds no error detection information. Odd parity is more useful than even, since it ensures that at least one state transition occurs in each character, which makes it more reliable. The most common parity setting, however, is none, with error detection handled by a communication protocol.

Stop bits edit

Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. If slow electromechanical teleprinters are used, one-and-one half or two stop bits are required.

Conventional notation edit

The D/P/S Data/Parity/Stop conventional notation specifies the framing of a serial connection. The most common usage on microcomputers is 8/N/1 8N1. This specifies 8 data bits, no parity, 1 stop bit. In this notation, the parity bit is not included in the data bits. 7/E/1 7E1 means that an even parity bit is added to the seven data bits for a total of eight bits between the start and stop bits. If a receiver of a 7/E/1 stream is expecting an 8/N/1 stream, half the possible bytes will be interpreted as having the high bit set.

Main article: Flow control data

A serial port may use signals in the interface to pause and resume the transmission of data. For example, a slow printer might need to handshake with the serial port to indicate that data should be paused while the mechanism advances a line.

Common hardware handshake signals hardware flow control use the RS-232 RTS/CTS or DTR/DSR signal circuits. Generally, the RTS and CTS are turned off and on from alternate ends to control data flow, for instance when a buffer is almost full. DTR and DSR are usually on all the time and, per the RS-232 standard and its successors, are used to signal from each end that the other equipment is actually present and powered-up. However, manufacturers have over the years built many devices that implemented non-standard variations on the standard, for example, printers that use DTR as flow control.

Another method of flow control software flow control uses special characters such as XON/XOFF to control the flow of data. The XON/XOFF characters are sent by the receiver to the sender to control when the sender will send data, that is, these characters go in the opposite direction to the data being sent. The circuit starts in the sending allowed state. When the receiver s buffers approach capacity, the receiver sends the XOFF character to tell the sender to stop sending data. Later, after the receiver has emptied its buffers, it sends an XON character to tell the sender to resume transmission. These are non-printing characters and are interpreted as handshake signals by printers, terminals, and computer systems.

XON/XOFF flow control is an example of in-band signaling, in which control information is sent over the same channel used for the data. XON/XOFF handshaking presents difficulties as XON and XOFF characters might appear in the data being sent and receivers may interpret them as flow control. Such characters sent as part of the data stream must be encoded in an escape sequence to prevent this, and the receiving and sending software must generate and interpret these escape sequences. On the other hand, since no extra signal circuits are required, XON/XOFF flow control can be done on a 3 wire interface.

Virtual serial ports edit

Main article: COM port redirector

A virtual serial port is an emulation of the standard serial port. This port is created by software which enable extra serial ports in an operating system without additional hardware installation such as expansion cards, etc.. It is possible to create a large number of virtual serial ports in a PC. The only limitation is the amount of resources, such as operating memory and computing power, needed to emulate many serial ports at the same time.

Virtual serial ports emulate all hardware serial port functionality, including Baud rate, Data bits, Parity bits, Stop bits, etc. Additionally they allow controlling the data flow, emulating all signal lines DTR/DSR/CTS/RTS/DCD/RI and customizing pinout. Virtual serial ports are common with Bluetooth and are the standard way of receiving data from Bluetooth-equipped GPS modules.

Virtual serial port emulation can be useful in case there is a lack of available physical serial ports or they do not meet the current requirements. For instance, virtual serial ports can share data between several applications from one GPS device connected to a serial port. Another option is to communicate with any other serial devices via internet or LAN as if they are locally connected to computer Serial over LAN /Serial-over-Ethernet technology. Two computers or applications can communicate through an emulated serial port link. Virtual serial port emulators are available for many operating systems including MacOS, Linux, and various mobile and desktop versions of Microsoft Windows.