SERIAL PORT

Serial port

   

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 port).[1] Throughout most of the history of personal computers, data was transferred through serial ports to devices such as modems, terminals and various peripherals.

While such interfaces as Ethernet, FireWire, and USB all send data as a serialstream, the term "serial port" usually identifies hardware more or less compliant to the RS-232 standard, intended to interface with a modem or with a similar communication device.

Modern computers without serial ports may require serial-to-USB converters to allow compatibility with RS 232 serial devices. Serial ports are still used in applications 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. Network equipment (such as routers and switches) often use serial console for configuration. Serial ports are still used in these areas as they are simple, cheap and their console functions are highly standardized and widespread. A serial port requires very little supporting software from the host system.

Contents

 

•             1 Hardware

o             1.1 DTE and DCE

o             1.2 Connectors

o             1.3 Pinouts

o             1.4 Hardware abstraction

•             2 Common applications for serial ports

•             3 Settings

o             3.1 Speed

o             3.2 Data bits

o             3.3 Parity

o             3.4 Stop bits

o             3.5 Conventional notation

o             3.6 Flow control

•             4 "Virtual" serial ports

•             5 See also

•             6 References

•             7 Further reading

•             8 External links

Hardware

Some computers, such as the IBM PC, used an integrated circuit called a UART, that converted characters to (and from) asynchronous serial form, and automatically looked after the timing and framing of data. Very low-cost systems, such as some early home computers, would instead use the CPU to send the data through anoutput pin, using the bit-banging technique. Before large-scale integration (LSI) UART integrated circuits were common, a minicomputer or microcomputer would have a serial port made of multiple small-scale integrated circuits to implement shift registers, logic gates, counters, and all the other logic for a serial port.

Early home computers often had proprietary serial ports with pinouts and voltage levels incompatible with RS-232. Inter-operation with RS-232 devices may be impossible as the serial port cannot withstand the voltage levels produced and may have other differences that "lock in" the user to products of a particular manufacturer.

Low-cost processors now allow higher-speed, but more complex, serial communication standards such as USB and FireWireto replace RS-232. These make it possible to connect devices that would not have operated feasibly over slower serial connections, such as mass storage, sound, and video devices.

Many personal computer motherboards still have at least one serial port, even if accessible only through a pin header. Small-form-factor systems and laptops may omit RS-232 connector ports to conserve space, but the electronics are still there. RS-232 has been standard for so long that the circuits needed to control a serial port became very cheap and often exist on a single chip, sometimes also with circuitry for a parallel port.

DTE and DCE

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. Conventionally, computers and terminals are DTE while modems and peripherals are DCE.

If it is necessary to connect two DTE devices (or two DCE devices but that is more unusual) a cross-over null modem, in the form of either an adapter or a cable, must be used.

Connectors

While the RS-232 standard originally specified a 25-pin D-type connector, many designers of personal computers chose to implement only a subset of the full standard: they traded off compatibility with the standard against the use of less costly and more compact connectors (in particular the DE-9 version used by the original IBM PC-AT). The desire to supply serial interface cards with two ports required that IBM reduce the size of the connector to fit onto a single card back panel. A DE-9 connector also fits onto a card with a second DB-25 connector that was similarly changed from the original Centronics-style connector. Starting around the time of the introduction of the IBM PC-AT, serial ports were commonly built with a 9-pin connector to save cost and space. However, presence of a 9-pin D-subminiature connector is not sufficient to indicate the connection is in fact a serial port, since this connector was also used for video, joysticks, and other purposes.

Some miniaturized electronics, particularly graphing calculators and hand-heldamateur and two-way radio equipment, have serial ports using a phone connector, usually the smaller 2.5 or 3.5 mm connectors and use the most basic 3-wire interface.

Many models of Macintosh favored the related RS-422 standard, mostly using German Mini-DIN connectors, except in the earliest models. The Macintosh included a standard set of two ports for connection to a printer and a modem, but somePowerBook laptops had only one combined port to save space.

The standard specifies 20 different signal connections. Since most devices use only a few signals, smaller connectors can often be used. For example, the 9-pin DE-9 connector was used by most IBM-compatible PCs since the IBM PC AT, and has been standardized as TIA-574. More recently, modular connectors have been used. Most common are 8P8C connectors. Standard EIA/TIA 561 specifies a pin assignment, but the "Yost Serial Device Wiring Standard"[2] invented by Dave Yost (and popularized by the Unix System Administration Handbook) is common on Unixcomputers and newer devices from Cisco Systems. Many devices don't use either of these standards. 10P10C connectors can be found on some devices as well. Digital Equipment Corporation defined their own DECconnect connection system which was based on the Modified Modular Jack (MMJ) connector. This is a 6-pin modular jack where the key is offset from the center position. As with the Yost standard, DECconnect uses a symmetrical pin layout which enables the direct connection between two DTEs. Another common connector is the DH10 header connector common on motherboards and add-in cards which is usually converted via a cable to the more standard 9-pin DE-9 connector (and frequently mounted on a free slot plate or other part of the housing).

Pinouts

The following table lists commonly used RS-232 signals and pin assignments.[3]

Signal    Origin    DB-25

DE-9

(TIA-574)

MMJ

8P8C ("RJ45")

10P10C ("RJ50")

Name    Abbreviation      DTE

DCE

                                                EIA/TIA-561

Yost (DTE)           Yost (DCE)           Cyclades[4]

Digi(ALTPIN option)[5]

National Instruments[6]

Cyclades[4]

Digi[7]

Transmitted Data             TxD        ?                              2              3              2              6              6              3              3              4              8              4                5

Received Data   RxD                        ?              3              2              5              5              3              6              6              5              9              7              6

Data Terminal Ready      DTR        ?                              20           4              1              3              7              2              2              8              7              3                9

Data Carrier Detect         DCD                       ?              8              1              N/A        2              2              7              7              1              10           8                10

Data Set Ready DSR                        ?              6              6              6              1                                              8              N/A        5              9              2

Ring Indicator    RI                            ?              22           9              N/A                        N/A        N/A        N/A        N/A        2              10           1

Request To Send              RTS         ?                              4              7              N/A        8              8              1              1              2              4              2                3

Clear To Send    CTS                         ?              5              8              N/A        7              1              8              5              7              3              6              8

Signal Ground    G             common              7              5              3,4          4              4,5          4,5          4              6              6              5              7

Protective Ground          PG          common              1              N/A        N/A        N/A        N/A        N/A        N/A        3              N/A        1                4

The signals are named from the standpoint of the DTE, for example, an IBM-PC compatible serial port. The ground signal is a common return for the other connections; it appears on two pins in the Yost standard but is the same signal. The DB-25 connector includes a second "protective ground" on pin 1. Connecting this to pin 7 (signal reference ground) is a common practice but not essential.

Note that EIA/TIA 561 combines DSR and RI,[8][9] and the Yost standard combines DSR and DCD.

Hardware abstraction

Operating systems usually use a symbolic name to refer to the serial ports of a computer. Unix-like operating systems usually label the serial port devices/dev/tty* (TTY is a common trademark-free abbreviation for teletype) where *represents a string identifying the terminal device; the syntax of that string depends on the operating system and the device. On Linux, 8250/16550 UARThardware 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.

The Microsoft MS-DOS and Windows environments refer to serial ports as COMports: COM1, COM2,..etc. Ports numbered greater than COM9 should be referred to using the \\.\COM10 syntax.[10]

Common applications for serial ports

The RS-232 standard is used by many specialized and custom-built devices. 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.

•             Dial-up modems

•             Configuration and management of networking equipment such as routers, switches, firewalls, load balancers

•             GPS receivers (typically NMEA 0183 at 4,800 bit/s)

•             Bar code scanners and other point of sale devices

•             LED and LCD text displays

•             Satellite phones, low-speed satellite modems and other satellite based transceiver devices

•             Flat-screen (LCD and Plasma) monitors to control screen functions by external computer, other AV components or remotes

•             Test and measuring equipment such as digital multimeters and weighing systems

•             Updating firmware on various consumer devices.

•             Some CNC controllers

•             Uninterruptible power supply

•             Stenography or Stenotype machines.

•             Software debuggers that run on a second computer.

•             Industrial field buses

•             Printers

•             Computer terminal, teletype

•             Older digital cameras

•             Networking (Macintosh AppleTalk using RS-422 at 230.4 kbit/s)

•             Serial mouse

•             Older GSM mobile phones

•             Some Telescopes

•             IDE hard drive[11][12] repair[13][14]

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.