View source for Serial port

{{Short description|Communication interface transmitting information sequentially}}
[[File:Serial port (9-pin).jpg|thumb|upright|A [[Engineering:Gender of connectors and fasteners|male]] D-subminiature connector used for a serial port on an IBM PC compatible computer along with the serial port symbol]]

On [[Computer|computer]]s, a '''serial port''' is a [[Serial communication|serial communication]] [[Interface (computing)|interface]] through which information transfers in or out sequentially one [[Bit|bit]] at a time.<ref>{{cite dictionary |dictionary=Webopedia |url=https://www.webopedia.com/definitions/serial-port/ |title=Serial Port Definition & Meaning |first=Vangie |last=Beal |date=September 1996 |access-date=2021-03-08 }}</ref>  This is in contrast to a [[Parallel port|parallel port]], which communicates multiple bits simultaneously in [[Parallel communication|parallel]]. Throughout most of the history of [[Engineering:Personal computer|personal computer]]s, data has been transferred through serial ports to devices such as [[Modem|modem]]s, [[Computer terminal|terminal]]s, various [[Engineering:Peripheral|peripheral]]s, and directly between computers.

While interfaces such as [[Ethernet]], FireWire, and USB also send data as a serial [[Stream (computing)|stream]], the term ''serial port'' usually denotes [[Computer hardware|hardware]] compliant with [[RS-232]] or a related standard, such as [[RS-485]] or [[RS-422]].

Modern consumer [[Engineering:Personal computer|personal computer]]s (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|point of sale]] systems and some industrial and consumer products.

[[Server (computing)|Server]] computers may use a serial port as a control console for diagnostics, while [[Networking hardware|networking hardware]] (such as [[Engineering:Router (computing)|router]]s and [[Network switch|switch]]es) 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.

==Hardware==

Modern devices use an [[Engineering:Integrated circuit|integrated circuit]] called a [[Universal asynchronous receiver-transmitter|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 [[Engineering:Home computer|home computer]]s, would instead use the [[Central processing unit|CPU]] to send the data through an [[Input output|output]] pin, using the [[Bit banging|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 [[Mainframe computer|mainframes]] and [[Minicomputer|minicomputer]]s, 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 [[Engineering:Chipset|chipset]].

{{Gallery
| IBM PC Serial Card.jpg | An IBM PC serial card with a 25-pin connector (obsolete 8-bit [[Engineering:Industry Standard Architecture|ISA]] card)
| RS232 PCI-E.jpg | A [[Engineering:PCI Express|PCI Express]]&nbsp;×1 card with one serial port
| Four-port serial card with an octopus cable.jpg | A four-port serial (RS-232) PCI Express&nbsp;×1 [[Engineering:Expansion card|expansion card]] with an octopus cable that breaks the card's DC-37 connector into four standard DE-9 connectors
| FTDI USB SERIAL.jpg | A converter from USB to an RS-232 compatible serial port—more than a physical transition, it requires a driver in the host system software and a built-in processor to emulate the functions of the IBM XT compatible serial port hardware.
|UART to USB adapter.jpg|UART to USB adapter}}

===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|data terminal equipment]]'' (DTE) and ''[[Data circuit-terminating equipment|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.

Conventionally, computers and terminals are DTE, while peripherals such as [[Modem|modem]]s are DCE. If it is necessary to connect two DTE (or DCE) devices together, a cable with reversed TX and RX lines, known as a cross-over, roll-over or null modem cable must be used.

===Gender===
Generally, serial port connectors are [[Engineering:Gender of connectors and fasteners|gendered]], only allowing connectors to mate with a connector of the opposite gender. With [[Engineering:D-subminiature|D-subminiature]] connectors, the male connectors have protruding pins, and female connectors have corresponding round sockets.<ref>{{cite web |url=http://www.cisco.com/c/en/us/support/docs/routers/7200-series-routers/12219-17.html |title=Serial Cable Connection Guide |date=2006-08-01 |publisher=CISCO |access-date=2016-01-31}}</ref> Either type of connector can be mounted on equipment or a panel; or terminate a cable.

Connectors mounted on DTE are likely to be male, and those mounted on DCE are likely to be female (with the cable connectors being the opposite). However, this is far from universal; for instance, most serial printers have a female DB25 connector, but they are DTEs.<ref>{{cite web |url=http://ltxfaq.custhelp.com/app/answers/detail/a_id/1216/~/rs232---dte-and-dce-connectors |title=RS232 - DTE and DCE connectors |date=2006-03-29 |publisher=Lantronix |access-date=2016-01-31 |archive-url=https://web.archive.org/web/20151214054632/http://ltxfaq.custhelp.com/app/answers/detail/a_id/1216/~/rs232---dte-and-dce-connectors |archive-date=2015-12-14 |url-status=dead }}</ref> In this circumstance, the appropriately gendered connectors on the cable or a [[Engineering:Gender changer|gender changer]] can be used to correct the mismatch.

===Connectors===
The only connector specified in the original RS-232 standard was the 25-pin D-subminiature, however, many other connectors have been used to save money or save on physical space, among other reasons. In particular, since many devices do not use all of the 20 signals that are defined by the standard, connectors with fewer pins are often used. While specific examples follow, countless other connectors have been used for RS-232 connections.

The 9-pin [[Engineering:D-subminiature|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.<ref>{{Cite web|title=IBM PC AT Serial/Parallel Adapter|url=http://www.minuszerodegrees.net/oa/OA%20-%20IBM%20PC%20AT%20Serial_Parallel%20Adapter.pdf|url-status=dead|archive-url=https://web.archive.org/web/20200224102106/http://minuszerodegrees.net/oa/OA%20-%20IBM%20PC%20AT%20Serial_Parallel%20Adapter.pdf|archive-date=2020-02-24}}</ref> This connector has been standardized for RS-232 as TIA-574.

Some miniaturized electronics, particularly [[Graphing calculator|graphing calculator]]s<ref>{{Cite web|title=TI-73...92+/V200 TI Link Guide|url=http://merthsoft.com/linkguide/cable.html|access-date=2020-08-14|website=merthsoft.com}}</ref> and hand-held amateur and two-way radio equipment,<ref>{{Cite web|title=Technical Section |url=https://www.miklor.com/COM/UV_Technical.php#progcable|access-date=2020-08-14|website=Miklor.com}}</ref> have serial ports using a [[Engineering:Phone connector (audio)|phone connector]], usually the smaller 2.5 or 3.5&nbsp;mm connectors and the most basic 3-wire interface—transmit, receive and ground.

[[File:CiscoConsoleCable.jpg|left|thumb|A Cisco rollover cable using the 8P8C Yost standard]]

8P8C connectors are also used in many devices. The EIA/TIA-561 standard defines a pinout using this connector, while the [[Rollover cable|rollover cable]] (or Yost standard) is commonly used on [[Software:Unix|Unix]] computers and network devices, such as equipment from [[Company:Cisco Systems|Cisco Systems]].<ref>{{Cite web|title=Cabling Guide for Console and AUX Ports|url=https://www.cisco.com/c/en/us/support/docs/routers/7000-series-routers/12223-14.html|access-date=2020-08-14|website=Cisco }}</ref>

[[File:Mac lc printer modem ports.jpg|thumb|upright=0.5|Pair of female Mini DIN-8 connectors used for [[RS-422|RS-422]] serial ports on a Macintosh LC computer]]

Many models of [[Engineering:Macintosh|Macintosh]] favor the related RS-422 standard, mostly using circular [[Engineering:Mini-DIN connector|mini-DIN connector]]s. The Macintosh included a standard set of two ports for connection to a printer and a modem, but some PowerBook laptops had only one combined port to save space.<ref>{{Cite web |title=Classic Mac Ports |url=https://whitefiles.org/tec/pgs/h10b.htm |access-date=2020-08-14 |website=WhiteFiles.org }}</ref>

10P10C connectors can be found on some devices.<ref name="National Instruments 2013">{{cite web |url=http://www.ni.com/pdf/manuals/371253e.pdf |publisher=National Instruments |title=Serial Quick Reference Guide |website=NI.com |date=July 2013 |access-date=2021-06-18 }}</ref>

Another common connector is a {{resx|10|2}} [[Engineering:Pin header|pin header]] common on motherboards and add-in cards which is usually converted via a ribbon cable to the more standard 9-pin DE-9 connector (and frequently mounted on a free slot plate or other part of the housing).<ref>{{Cite book |url=https://www.intel.com/content/dam/support/us/en/documents/motherboards/server/s5000pal/sb/s5000pal_s5000xal_tps_r2_0.pdf |title=Intel Server Board S5000PAL/S5000XAL Technical Product Specification |pages=38 }}</ref>

===Pinouts===
The following table lists commonly used RS-232 signals and pin assignments:<ref>{{cite web |url=http://www.hardwarebook.info/Serial_(PC_9) |title=Serial (PC 9) |author-first=Joakim |author-last=Ögren |access-date=2010-07-07 |archive-url=https://web.archive.org/web/20100811173526/http://www.hardwarebook.info/Serial_%28PC_9%29 |archive-date=2010-08-11 }}</ref>

{| class="wikitable sortable" style="margin:auto; text-align:center;"
! colspan="3"|Signal !! colspan="2"|Direction !! colspan="11"|Connector pin
|-
! rowspan="2"|Name !! rowspan="2"|V.24 circuit !! rowspan="2"|Abbreviation !! rowspan="2"|[[Data terminal equipment|DTE]] !! rowspan="2"|[[Data circuit-terminating equipment|DCE]] !! rowspan="2"|DB-25 !! rowspan="2"|{{nowrap|DE-9}} {{nowrap|(TIA-574)}} !! rowspan="2"|[[Modified Modular Jack|MMJ]] !! colspan="5"|8P8C ("RJ45") !! colspan="3"|10P10C ("RJ50")
|-
! EIA/TIA-561 !! Yost (DTE)<ref name="Yost">{{cite web |url=http://yost.com/computers/RJ45-serial/ |title=Yost Serial Device Wiring Standard |access-date=2020-05-10 |archive-url=https://web.archive.org/web/20200617132523/http://yost.com/computers/RJ45-serial/ |archive-date=2020-06-17 |url-status=dead }}</ref> !! Yost (DCE)<ref name="Yost"/> !! Cyclades !! [[Company:Digi International|Digi]] (ALTPIN option) !! National Instruments<ref name="National Instruments 2013">{{cite web |url=http://www.ni.com/pdf/manuals/371253e.pdf |publisher=National Instruments |title=Serial Quick Reference Guide |website=NI.com |date=July 2013 |access-date=2021-06-18 }}</ref> !! Cyclades !! Digi
|-
| Transmitted Data || 103 || TxD
| Out || In || 2 || 3 || 2 || 6 || 6 || 3 || 3 || 4 || 8 || 4 || 5
|-
| Received Data || 104 || RxD
| In || Out || 3 || 2 || 5 || 5 || 3 || 6 || 6 || 5 || 9 || 7 || 6
|-
| Data Terminal Ready || 108/2 || DTR
| Out || In || 20 || 4 || 1 || 3 || 7 || 2 || 2 || 8 || 7 || 3 || 9
|-
| Data Carrier Detect || 109 || DCD
| In || Out || 8 || 1 || {{n/a}} || 2 || rowspan="2"|2 || 7 || 7 || 1 || 10 || 8 || 10
|-
| Data Set Ready || 107 || DSR
| In || Out || 6 || 6 || 6 || rowspan="2"|1 || {{n/a}} || 8 || {{n/a}} || 5 || 9 || 2
|-
| Ring Indicator || 125 || RI
| In || Out || 22 || 9 || {{n/a}} || {{n/a}} || {{n/a}} || {{n/a}} || {{n/a}} || 2 || 10 || 1
|-
| Request To Send || 105 || RTS
| Out || In || 4 || 7 || {{n/a}} || 8 || 8 || 1 || 1 || 2 || 4 || 2 || 3
|-
| Clear To Send || 106 || CTS
| In || Out || 5 || 8 || {{n/a}} || 7 || 1 || 8 || 5 || 7 || 3 || 6 || 8
|-
| Signal Ground || 102 || G
| colspan="2" | Common || 7 || 5 || 3, 4 || 4 || 4, 5 || 4, 5 || 4 || 6 || 6 || 5 || 7
|-
| Protective Ground || 101 || PG
| colspan="2" | Common || 1 || {{n/a}} || {{n/a}} || {{n/a}} || {{n/a}} || {{n/a}} || {{n/a}} || 3 || {{n/a}} || 1 || 4
|}

''Signal Ground'' is a [[Engineering:Single-ended signaling|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, which is intended to be connected by each device to its own frame ground or similar. Connecting Protective Ground to Signal Ground is a common practice but not recommended.

Note that EIA/TIA 561 combines DSR and RI,<ref>{{cite web| url = http://www.hardwarebook.info/RS-232D| title = Hardware Book RS-232D}}</ref><ref>{{cite web| url = http://www.t0rchthe.net/rj45console/index.html| title = RS-232D EIA/TIA-561 RJ45 Pinout}}</ref> and the Yost standard combines DSR and DCD.

===Hardware abstraction===
Operating systems usually create symbolic names for the serial ports of a computer, rather than requiring programs to refer to them by hardware address.

[[Unix-like]] operating systems usually label the serial port devices {{mono|/dev/tty*}}. ''TTY'' is a common trademark-free abbreviation for ''[[Engineering:Teleprinter|teletype]]'', a device commonly attached to early computers' serial ports, and {{mono|*}} represents a string identifying the specific port; the syntax of that string depends on the operating system and the device. On [[Software:Linux|Linux]], 8250/16550 UART hardware serial ports are named {{mono|/dev/ttyS*}}, USB adapters appear as {{mono|/dev/ttyUSB*}} and various types of virtual serial ports do not necessarily have names starting with {{mono|tty}}.

The [[Engineering:DOS|DOS]] and [[Software:Microsoft Windows|Windows]] environments refer to serial ports as [[Engineering:COM (hardware interface)|COM]] ports: COM1, COM2,..etc.<ref>{{cite web |title=What Is a Com1 Port? |url=https://www.techwalla.com/articles/what-is-a-com1-port |author=Stephen Byron Cooper |access-date=2021-09-30}}</ref>

==Common applications for serial ports==
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 [[Engineering:Microcontroller|microcontroller]], where they can be used to communicate with a PC or other serial devices.

{{Div col|colwidth=30em}}
* Dial-up [[Modem|modem]]s
* Configuration and management of [[Computer network|networking]] equipment such as [[Engineering:Router (computing)|routers]], [[Network switch|switches]], [[Firewall (computing)|firewalls]], load balancers
* [[Engineering:Global Positioning System|GPS]] receivers (typically [[NMEA 0183]] at {{nowrap|4,800 bit/s}})
* [[Barcode reader|Bar code scanner]]s and other [[Point of sale|point of sale]] devices
* [[Engineering:Light-emitting diode|LED]] and [[Engineering:Liquid-crystal display|LCD]] text displays
* [[Engineering:Satellite phone|Satellite phone]]s, low-speed satellite modems and other satellite based transceiver devices
* [[Flat-panel display]]s to control screen functions by external computer, other AV components or remotes
* Test and measuring equipment such as digital [[Engineering:Multimeter|multimeter]]s and weighing systems
* Updating [[Firmware|firmware]] on various consumer devices
* CNC controllers
* [[Uninterruptible power supply]] management and status reporting
* Stenography or Stenotype machines
* Software debuggers that run on a second computer
* Console or debugger interface to [[Engineering:Microprocessor development board|microprocessor development or evaluation board]]s
* Industrial field buses
* [[Engineering:Printer (computing)|Printer]]s
* [[Computer terminal]], [[Engineering:Teleprinter|teletype]]
* [[Computer network|Networking]] (Macintosh [[AppleTalk]] using RS-422 at {{nowrap|230.4 kbit/s}})
* Serial mouse
{{Div col end}}

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|uninterruptible power supply]] which used the control lines to signal loss of power, low battery, and other status information. At least some [[Engineering:Morse code|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. 

Serial computer mice may draw their operating power from the received data or control signals.<ref>{{cite web |last1=Chan |first1=Alvin |title=AN-681 PC Mouse Implementation Using COP800 |url=https://www.ti.com/lit/an/snaa005/snaa005.pdf |publisher=National Semiconductor |access-date=29 July 2023 |date=1990}}</ref><ref>{{cite web |title=AN519   Implementing a Simple Serial Mouse Controller |url=https://ww1.microchip.com/downloads/en/Appnotes/00519c.pdf |publisher=Microchip Technology Inc. |access-date=29 July 2023 |date=1997}}</ref>

==Settings==
{| class="wikitable floatright" style="width: 33%;"
|+ Common serial port speeds
! style=max-width:5em | [[Bit rate|Bit rate (bit/s)]] !! style=max-width:5em | Time per bit (μs) !! style=max-width:6em | [[Software:Microsoft Windows|Windows]] predefined serial port speed<ref>{{Cite web | url=https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/content/ntddser/ns-ntddser-_serial_commprop | title=SERIAL_COMMPROP structure | publisher=[[Company:Microsoft|Microsoft]] | date=2018-04-22 | access-date=2019-09-28 | archive-url=https://web.archive.org/web/20190928060324/https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/content/ntddser/ns-ntddser-_serial_commprop | archive-date=2019-09-28 }}</ref><ref name="winbase.h">{{cite web |title=DCB Structure |url=https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-dcb |work=Windows Dev Center |publisher=[[Company:Microsoft|Microsoft]] |date=2018-12-04 |access-date=2019-09-28 |archive-url=https://web.archive.org/web/20190928055944/https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-dcb |archive-date=2019-09-28 |url-status=dead }}</ref> || Common applications
|-
| 75 || 13333.3 || {{yes}} ||
|-
| 110 || 9090.9 || {{yes}} || Bell 101 modem
|-
| 134.5 || 7434.9 || {{yes}} ||
|-
| 150 || 6666.6 || {{yes}} ||
|-
| 300 || 3333.3 || {{yes}} || Bell 103 modem or V.21 modem
|-
| 600 || 1666.7 || {{yes}} ||
|-
| 1,200 || 833.3 || {{yes}} || Bell 202, [[Bell 212A]], or V.22 modem
|-
| 1,800 || 555.6 || {{yes}} ||
|-
| 2,400 || 416.7 || {{yes}} || V.22bis modem
|-
| 4,800 || 208.3 || {{yes}} || V.27ter modem
|-
| 7,200 || 138.9 || {{yes}} ||
|-
| 9,600 || 104.2 || {{yes}} || V.32 modem
|-
| 14,400 || 69.4 || {{yes}} || V.32bis modem
|-
| 19,200 || 52.1 || {{yes}} ||
|-
| 31,250 || 32 || {{no}} || [[MIDI]] port
|-
| 38,400 || 26.0 || {{yes}} ||
|-
| 56,000 || 17.9 || {{yes}} || V.90/V.92 modem
|-
| 57,600 || 17.4 || {{yes}} || V.32bis modem with V.42bis compression
|-
| 76,800 || 13.0 || {{no}} || [[BACnet]] MS/TP networks<ref>{{Cite web | title = BACnet MS/TP Overview Manual | url = http://www.neptronic.com/Controls/PDF/EVC/BACnetModbus/BACnet%20MSTP%20Overview%20Manual-160405.pdf | publisher = Neptronic | access-date = September 26, 2019 | archive-url = https://web.archive.org/web/20200110133346/http://www.neptronic.com/Controls/PDF/EVC/BACnetModbus/BACnet%20MSTP%20Overview%20Manual-160405.pdf | archive-date = January 10, 2020 | url-status = live }}</ref>
|-
| 115,200 || 8.68 || {{yes}} || V.34 modem with V.42bis compression, low cost serial V.90/V.92 modem with V.42bis or V.44 compression
|-
| 128,000 || 7.81 || {{yes}} || Basic Rate Interface [[Integrated Services Digital Network|ISDN]] terminal adapter
|-
| 230,400 || 4.34 || {{no}} || [[LocalTalk]], high end serial V.90/V.92 modem with V.42bis or V.44 compression<ref>{{Cite web | url = https://www.multitech.com/documents/publications/data-sheets/86002093.pdf | title = MultiModem ZBA | publisher = Multi-Tech Systems, Inc | date = January 2019 | access-date = September 26, 2019 | archive-url = https://web.archive.org/web/20190303121617/https://www.multitech.com/documents/publications/data-sheets/86002093.pdf | archive-date = March 3, 2019 | url-status = dead }}</ref><ref>{{Cite web | url = https://unicom.usr.com/support/3453c/3453c-ug/control_data_rates.html | title = Courier 56K Business Modem: User Guide: Controlling Data Rates | publisher = [[Company:USRobotics|USRobotics]] | date = 2007 | access-date = September 26, 2019 | archive-url = https://web.archive.org/web/20170804100605/http://unicom.usr.com/support/3453c/3453c-ug/control_data_rates.html | archive-date = August 4, 2017 | url-status = live }}</ref>
|-
| 250,000 || 4.0 || {{no}} || [[DMX512]], stage lighting and effects network
|-
| 256,000 || 3.91 || {{Yes}} ||
|}

Serial standards provide for many different operating speeds as well as adjustments to the protocol to account for different operating conditions. The most well-known options are 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 these settings can be software-controlled. Hardware from the 1980s and earlier may require setting switches or jumpers on a circuit board.

The configuration for serial ports designed to be connected to a PC has become a de facto standard, usually stated as [[8-N-1|9600/8-N-1]].

===Speed===
Serial ports use two-level (binary) signaling, so the data rate in bits per second is equal to the symbol rate in [[Baud|baud]]. The total 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|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.

A standard series of rates is based on multiples of the rates for electromechanical [[Engineering:Teleprinter|teleprinter]]s; 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&nbsp;bit/s.<ref name="winbase.h"/> 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).<ref>{{cite web |url=https://cdn.hackaday.io/files/1614916909230944/SX1272_DS_V4.pdf |title=SX1272/73 - 860 MHz to 1020 MHz Low Power Long Range Transceiver Datasheet |publisher=[[Company:Semtech|Semtech]] |date=January 2019 }}</ref> [[Engineering:Crystal oscillator|Crystal oscillator]]s with a frequency of 1.843200&nbsp;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.

===Data bits===
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|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 [[Least significant bit|least significant bit]] first. Also possible, but rarely used, is [[Most significant bit|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===
{{Main|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. 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 protocol|communication protocol]]s 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.

The parity bit in each character can be set to one of the following:

* '''None (N)''' means that no parity bit is sent and the transmission is shortened.
* '''Odd (O)''' means that the parity bit is set so that the number of 1 bits is odd.
* '''Even (E)''' means that the parity bit is set so that the number of 1 bits is even.
* '''Mark (M)''' parity means that the parity bit is always set to the mark signal condition (1 bit value).
* '''Space (S)''' parity always sends the parity bit in the space signal condition (0 bit value).

Aside from uncommon applications that use the last bit (usually the 9th) for some form of addressing or special signaling, mark or space parity is uncommon, as it adds no error detection information.

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 that could be caused by serial port speed mismatches. The most common parity setting, however, is ''none'', with error detection handled by a communication protocol.

To allow detection of messages damaged by line noise, electromechanical teleprinters were arranged to print a special character when received data contained a parity error.

===Stop bits===
Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronize with the character stream. Electronic devices usually use one stop bit. If slow electromechanical [[Engineering:Teleprinter|teleprinter]]s are used, one-and-one half or two stop bits may be required.

===Conventional notation===
The data/parity/stop (D/P/S) 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 7 data bits for a total of 8 bits between the start and stop bits.

===Flow control===
[[Flow control (data)|Flow control]] is used in circumstances where a transmitter might be able to send data faster than the receiver is able to process it. To cope with this, serial lines often incorporate a [[Handshake (computing)|handshaking]] method. There are ''hardware'' and ''software'' handshaking methods.

'''Hardware handshaking''' is done with extra signals, often the RS-232 RTS/CTS or DTR/DSR signal circuits. RTS and CTS are used to control data flow, signaling, for instance, when a buffer is almost full. Per the RS-232 standard and its successors, DTR and DSR are used to signal that equipment is present and powered up so are usually asserted at all times. However, non-standard implementations exist, for example, printers that use DTR as flow control.

'''Software handshaking''' is done for example with [[ASCII]] control characters XON/XOFF to control the flow of data. The XON and 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 system 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. It is an example of [[In-band signaling|in-band signaling]], where control information is sent over the same channel as its data.
 
The advantage of hardware handshaking is that it can be extremely fast, it works independently of imposed meaning such as ASCII on the transferred data and it is [[State (computer science)|stateless]]. Its disadvantage is that it requires more hardware and cabling, and both ends of the connection must support the hardware handshaking protocol used.
 
The advantage of software handshaking is that it can be done with absent or incompatible hardware handshaking circuits and cabling. The disadvantage, common to all in-band control signaling, is that it introduces complexities in ensuring that control messages get through even when data messages are blocked, and data can never be mistaken for control signals. The former is normally dealt with by the operating system or device driver; the latter normally by ensuring that control codes are escaped (such as in the [[Kermit (protocol)|Kermit protocol]]) or omitted by design (such as in [[ANSI escape code|ANSI terminal control]]).
 
If '''no handshaking''' is employed, an overrun receiver might simply fail to receive data from the transmitter. Approaches for preventing this include reducing the speed of the connection so that the receiver can always keep up, increasing the size of [[Data buffer|buffers]] so it can keep up averaged over a longer time, using delays after time-consuming operations (e.g. in [[Termcap|termcap]]) or employing a mechanism to resend data which has not been received correctly (e.g. [[Transmission Control Protocol|TCP]]).

==See also==
* {{Section link|List of Bluetooth profiles|Serial Port Profile (SPP)}}
* {{Annotated link|Serial over LAN}}
* [[Engineering:Teleprinter|Teleprinter]]{{snd}}Device that motivated serial port development
* {{Annotated link|Virtual COM port}}

==References==
{{Reflist}}

==Further reading==
* ''Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems''; 2nd Edition; Jan Axelson; Lakeview Research; 380 pages; 2007; {{ISBN|978-1-931-44806-2}}.

==External links==
* [http://pinouts.ru/SerialPorts/ RS-232 and other serial port pinouts list]

{{Basic computer components}}

[[Category:Computer connectors]]
[[Category:Serial buses]]

{{Sourceattribution|Serial port}}