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110-1027: The RS-232 standard had been commonly used with serial ports and serial cables . It is still widely used in industrial communication devices. A serial port complying with the RS-232 standard was once a standard feature of many types of computers. Personal computers used them for connections not only to modems, but also to printers , computer mice , data storage, uninterruptible power supplies , and other peripheral devices. Compared with later interfaces such as RS-422 , RS-485 and Ethernet , RS-232 has lower transmission speed, shorter maximum cable length, larger voltage swing, larger standard connectors, no multipoint capability and limited multidrop capability. In modern personal computers, USB has displaced RS-232 from most of its peripheral interface roles. Thanks to their simplicity and past ubiquity, however, RS-232 interfaces are still used—particularly in industrial CNC machines, networking equipment and scientific instruments where

220-513: A cross-over , roll-over or null modem cable must be used. Generally, serial port connectors are gendered , only allowing connectors to mate with a connector of the opposite gender. With D-subminiature connectors, the male connectors have protruding pins, and female connectors have corresponding round sockets. 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

330-467: A loopback capability used for testing. When enabled, signals are echoed back to the sender rather than being sent on to the receiver. If supported, the DTE can signal the local DCE (the one it is connected to) to enter loopback mode by setting Local Loop (LL, pin 18) to ON, or the remote DCE (the one the local DCE is connected to) to enter loopback mode by setting Remote Loop (RL, pin 21) to ON. The latter tests

440-433: A phone connector , usually the smaller 2.5 or 3.5 mm connectors and the most basic 3-wire interface—transmit, receive and ground. 8P8C connectors are also used in many devices. The EIA/TIA-561 standard defines a pinout using this connector, while the rollover cable (or Yost standard) is commonly used on Unix computers and network devices, such as equipment from Cisco Systems . Many models of Macintosh favor

550-524: 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). The following table lists commonly used RS-232 signals and pin assignments: Signal Ground 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, which

660-426: A router ) as an alternative to monitoring over Ethernet. In RS-232, user data is sent as a time-series of bits . Both synchronous and asynchronous transmissions are supported by the standard. In addition to the data circuits, the standard defines a number of control circuits used to manage the connection between the DTE and DCE. Each data or control circuit only operates in one direction, that is, signaling from

770-486: A "high" or "low" signaling rate. The rates, as well as which device will select the rate, must be configured in both the DTE and DCE. The prearranged device selects the high rate by setting the Data Signal Rate Selector (DSRS, pin 23) signal to ON. Sometimes called Data Rate Select (DRS), this signal should not be confused with the more commonly used Data Set Ready (DSR, pin 6). Many DCE devices have

880-514: A 25-pin D-sub connector does not necessarily indicate an RS-232-C compliant interface. For example, on the original IBM PC, a male D-sub was an RS-232-C DTE port (with a non-standard current loop interface on reserved pins), but the female D-sub connector on the same PC model was used for the parallel "Centronics" printer port . Some personal computers put non-standard voltages or signals on some pins of their serial ports. The standard does not define

990-409: A DTE to the attached DCE or the reverse. Because transmit data and receive data are separate circuits, the interface can operate in a full duplex manner, supporting concurrent data flow in both directions. The standard does not define character framing within the data stream or character encoding. The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for

1100-500: 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 9600/8-N-1. Serial ports use two-level (binary) signaling, so the data rate in bits per second is equal to the symbol rate in baud . The total speed includes bits for framing (stop bits, parity, etc.) and so

1210-473: A bit rate by observing what a connected device is sending and synchronizing to it. 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

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1320-453: A cable that connects the corresponding pins according to the table below. Cables with 9 pins on one end and 25 on the other are common. Manufacturers of equipment with 8P8C connectors usually provide a cable with either a DB-25 or DE-9 connector (or sometimes interchangeable connectors so they can work with multiple devices). Poor-quality cables can cause false signals by crosstalk between data and control lines (such as Ring Indicator ). If

1430-470: A calendar issued by the TSB. SGs are augmented by Focus Groups (FGs), an instrument created by ITU-T, providing a way to quickly react to ICT standardization needs and allowing great flexibility in terms of participation and working methods. The key difference between SGs and FGs is that the latter have greater freedom to organize and finance themselves, and to involve non-members in their work, but they do not have

1540-421: A common ground limits RS-232 to applications with relatively short cables. If the two devices are far enough apart or on separate power systems, the local ground connections at either end of the cable will have differing voltages; this difference will reduce the noise margin of the signals. Balanced, differential serial connections such as RS-422 or RS-485 can tolerate larger ground voltage differences because of

1650-429: 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 protocols 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

1760-461: A few of the twenty signals specified in the standard, so connectors and cables with fewer pins are sufficient for most connections, more compact, and less expensive. Personal computer manufacturers replaced the DB-25M connector with the smaller DE-9M connector. This connector, with a different pinout (see Serial port pinouts ), is prevalent for personal computers and associated devices. Presence of

1870-495: A given cable will not allow a data connection, especially if a gender changer is in use, a null modem cable may be necessary. Gender changers and null modem cables are not mentioned in the standard, so there is no officially sanctioned design for them. The following table lists commonly used RS-232 signals (called "circuits" in the specifications) and their pin assignments on the recommended DB-25 connectors (see Serial port pinouts for other commonly used connectors not defined by

1980-416: A maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the voltages available to the line driver circuit. Many RS-232 driver chips have inbuilt charge pump circuitry to produce the required voltages from a 3 or 5 volt supply. RS-232 drivers and receivers must be able to withstand indefinite short circuits to

2090-501: A maximum cable length, but instead defines the maximum capacitance that a compliant drive circuit must tolerate. A widely used rule of thumb indicates that cables more than 15 m (50 ft) long will have too much capacitance, unless special cables are used. By using low-capacitance cables, communication can be maintained over larger distances up to about 300 m (1,000 ft). For longer distances, other signal standards, such as RS-422 , are better suited for higher speeds. Since

2200-757: A mechanism to resend data which has not been received correctly (e.g. TCP ). CCITT The International Telecommunication Union Telecommunication Standardization Sector ( ITU-T ) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Communication Technology , such as X.509 for cybersecurity, Y.3172 and Y.3173 for machine learning, and H.264/MPEG-4 AVC for video compression, between its Member States, Private Sector Members, and Academia Members. The World Telecommunication Standardization Assembly (WTSA),

2310-416: A profusion of software firms around the world were still furiously competing to shape the future of the electronic office , and was completed in 1999 long after Microsoft Office 's then-secret binary file formats had become established as the global de facto standard. The ITU-T now operates under much more streamlined processes. The time between an initial proposal of a draft document by a member company and

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2420-423: A reduced voltage. Some manufacturers therefore built transmitters that supplied +5 V and −5 V and labeled them as "RS-232 compatible". Later personal computers (and other devices) started to make use of the standard so that they could connect to existing equipment. For many years, an RS-232-compatible port was a standard feature for serial communications , such as modem connections, on many computers (with

2530-532: A serial stream , the term serial port usually denotes hardware compliant with RS-232 or a related standard, such as RS-485 or RS-422 . Modern consumer personal computers (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 systems and some industrial and consumer products. Server computers may use

2640-415: A serial port as a control console for diagnostics, while networking hardware (such as routers and switches ) 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. Modern devices use an integrated circuit called a UART to implement

2750-833: 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 home computers , would instead use the CPU to send the data through an output pin, using the 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 mainframes and minicomputers , which would have multiple small-scale integrated circuits to implement shift registers, logic gates, counters, and all

2860-447: A short-range, point-to-point, low-speed wired data connection is fully adequate. The Electronic Industries Association (EIA) standard RS-232-C as of 1969 defines: The standard does not define such elements as the character encoding (i.e. ASCII , EBCDIC , or others), the framing of characters (start or stop bits, etc.), transmission order of bits, or error detection protocols. The character format and transmission bit rate are set by

2970-461: A specialized DTE called a bit error rate tester (or BERT). Some synchronous devices provide a clock signal to synchronize data transmission, especially at higher data rates. Two timing signals are provided by the DCE. Pin 15 is the transmitter clock (TCK), or send timing (ST); the DTE puts the next bit on the transmit data line (pin 2) when this clock transitions from OFF to ON (so it is stable during

3080-418: A synchronization preamble to the receiver when they are re-enabled. The DTE asserts RTS to indicate a desire to transmit to the DCE, and in response the DCE asserts CTS to grant permission, once synchronization with the DCE at the far end is achieved. Such modems are no longer in common use. There is no corresponding signal that the DTE could use to temporarily halt incoming data from the DCE. Thus RS-232's use of

3190-484: A weight reading, or a GPS receiver that periodically sends position, if no configuration via RS-232 is necessary). When only hardware flow control is required in addition to two-way data, the RTS and CTS lines are added in a 5-wire version. The EIA-232 standard specifies connections for several features that are not used in most implementations. Their use requires 25-pin connectors and cables. The DTE or DCE can specify use of

3300-473: A wide array of topics in the field of information and communication technologies (ICT) and attract high-ranking experts as speakers, and attendees from engineers to high-level management from all industry sectors. The technical work, the development of Recommendations, of ITU-T is managed by Study Groups (SGs), such as Study Group 13 for network standards, Study Group 16 for multimedia standards, and Study Group 17 for security standards, which are created by

3410-630: Is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange , issued in 1997. Changes since Revision C have been in timing and details intended to improve harmonization with the CCITT standard ITU-T/CCITT V.24  [ de ] , but equipment built to the current standard will interoperate with older versions. Related ITU-T standards include V.24 (circuit identification) and ITU-T/CCITT V.28  [ de ] (signal voltage and timing characteristics). In revision D of EIA-232,

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3520-619: 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 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 teleprinters are used, one-and-one half or two stop bits may be required. The data/parity/stop (D/P/S) conventional notation specifies

3630-462: 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

3740-504: Is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port , which communicates multiple bits simultaneously in parallel . Throughout most of the history of personal computers , data has been transferred through serial ports to devices such as modems , terminals , various peripherals , and directly between computers. While interfaces such as Ethernet , FireWire , and USB also send data as

3850-582: Is a fast-track approval procedure that was developed to allow standards to be brought to market in the timeframe that industry now demands. The AAP is defined in ITU-T Recommendation A.8. This dramatic overhaul of standards-making by streamlining approval procedures was implemented in 2001 and is estimated to have cut the time involved in this critical aspect of the standardization process by 80 to 90 percent. This means that an average standard that took around four years to approve and publish until

3960-453: Is a signal sent from the DCE to the DTE device. It indicates to the terminal device that the phone line is ringing. In many computer serial ports, a hardware interrupt is generated when the RI signal changes state. Having support for this hardware interrupt means that a program or operating system can be informed of a change in state of the RI pin, without requiring the software to constantly "poll"

4070-435: 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 modems are DCE. If it is necessary to connect two DTE (or DCE) devices together, a cable with reversed TX and RX lines, known as

4180-838: Is faster, uses lower voltages, and has connectors that are simpler to connect and use. Disadvantages of USB compared to RS-232 are that USB is far less immune to electromagnetic interference (EMI) and that maximum cable length defined by standards is much shorter (15 meters for RS-232 versus 3–5 meters for USB, depending on the USB version and use of active cables). RS-232 cable lengths of 2000 meters are possible with appropriate line drivers. In fields such as laboratory automation or surveying, RS-232 devices continue to be used. Some types of programmable logic controllers , variable-frequency drives , servo drives , and computerized numerical control equipment are programmable via RS-232. Computer manufacturers have responded to this demand by re-introducing

4290-616: Is followed by a period and the Recommendation number, which uniquely identifies the Recommendation within the series. Often, a range of related Recommendations are further grouped within the series and given adjacent numbers, such as "H.200-H.499: Infrastructure of audiovisual services" or "H.260-H.279: Coding of moving video". Many numbers are "skipped" to give room for future Recommendations to be adjacent to related Recommendations. Recommendations can be revised or "superseded" and keep their existing Recommendation number. In addition to

4400-458: 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, and the Yost standard combines DSR and DCD. 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

4510-460: Is now free of charge online. (About 30 specifications jointly maintained by the ITU-T and ISO/IEC are not available for free to the public. ) ITU-T has moreover tried to facilitate cooperation between the various forums and standard-developing organizations (SDOs). This collaboration is necessary to avoid duplication of work and the consequent risk of conflicting standards in the market place. In

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4620-419: 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

4730-408: 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 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

4840-475: 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

4950-432: Is to ensure the efficient and timely production of standards covering all fields of telecommunications and Information Communication Technology (ICTs) on a worldwide basis, as well as defining tariff and accounting principles for international telecommunication services. The international standards that are produced by the ITU-T are referred to as " Recommendations " (with the word capitalized to distinguish its meaning from

5060-474: Is used beyond the original purpose of interconnecting a terminal with a modem, successor standards have been developed to address the limitations. Issues with the RS-232 standard include: In the book PC 97 Hardware Design Guide , Microsoft deprecated support for the RS-232 compatible serial port of the original IBM PC design. Today, RS-232 has mostly been replaced in personal computers by USB for local communications. Advantages compared to RS-232 are that USB

5170-531: Is used by some older uninterruptible power supplies (UPSs) to signal a power failure state to the computer. Certain personal computers can be configured for wake-on-ring , allowing a computer that is suspended to answer a phone call. The Request to Send (RTS) and Clear to Send (CTS) signals were originally defined for use with half-duplex (one direction at a time) modems such as the Bell 202 . These modems disable their transmitters when not required and must transmit

5280-476: The DE-9M connector on their computers or by making adapters available. RS-232 ports are also commonly used to communicate to headless systems such as servers , where no monitor or keyboard is installed, during boot when an operating system is not yet running and therefore no network connection is possible. A computer with an RS-232 serial port can communicate with the serial port of an embedded system (such as

5390-569: The Kermit protocol ) or omitted by design (such as in 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 buffers so it can keep up averaged over a longer time, using delays after time-consuming operations (e.g. in termcap ) or employing

5500-851: The Plenipotentiary Conference (the top policy-making conference of ITU) saw a reform of ITU, giving the Union greater flexibility to adapt to an increasingly complex, interactive and competitive environment. The CCITT was renamed the Telecommunication Standardization Sector (ITU-T), as one of three Sectors of the Union alongside the Radiocommunication Sector ( ITU-R ) and the Telecommunication Development Sector ( ITU-D ). Historically,

5610-642: The World Telecommunication Standardization Assembly (WTSA) which is held every four years. As part of the deliberations, WTSA has instructed ITU to hold the Global Standards Symposium , which unlike WTSA is open to public for participation. The people involved in these SGs are experts in telecommunications from all over the world. There are currently 11 SGs. Study groups meet face to face (or virtually under exceptional circumstances) according to

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5720-793: 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 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. This connector has been standardized for RS-232 as TIA-574 . Some miniaturized electronics, particularly graphing calculators and hand-held amateur and two-way radio equipment, have serial ports using

5830-560: The D-subminiature connector was formally included as part of the standard (it was only referenced in the appendix of RS-232-C). The voltage range was extended to ±25 volts, and the circuit capacitance limit was expressly stated as 2500 pF. Revision E of EIA-232 introduced a new, smaller, standard D-shell 26-pin "Alt A" connector, and made other changes to improve compatibility with CCITT standards V.24, V.28 and ISO 2110. Specification document revision history: Because RS-232

5940-407: The DTE. Unlike the original use of RTS and CTS with half-duplex modems, these two signals operate independently from one another. This is an example of hardware flow control . However, "hardware flow control" in the description of the options available on an RS-232-equipped device does not always mean RTS/CTS handshaking. Equipment using this protocol must be prepared to buffer some extra data, since

6050-751: The French government invited international participants to a conference in Paris in 1865 to facilitate and regulate international telegraph services. A result of the conference was the founding of the forerunner of the modern ITU. At the 1925 Paris conference, the ITU created two consultative committees to deal with the complexities of the international telephone services, known as CCIF ( Comité Consultatif International Téléphonique ) and with long-distance telegraphy CCIT ( Comité Consultatif International des Communications Téléphoniques à grande distance ). In view of

6160-868: The ITU-T Recommendations, which have non-mandatory status unless they are adopted in national laws, ITU-T is also the custodian of a binding international treaty, the International Telecommunication Regulations. The ITRs go back to the earliest days of the ITU when there were two separate treaties, dealing with telegraph and telephone. The ITRs were adopted, as a single treaty, at the World Administrative Telegraphy and Telephone Conference held in Melbourne, 1988 (WATTC-88). The ITRs comprise ten articles which deal, inter alia , with

6270-460: The ON to OFF transition when the DCE registers the bit). Pin 17 is the receiver clock (RCK), or receive timing (RT); the DTE reads the next bit from the receive data line (pin 3) when this clock transitions from ON to OFF. Alternatively, the DTE can provide a clock signal, called transmitter timing (TT, pin 24) for transmitted data. Data is changed when the clock transitions from OFF to ON, and read during

6380-409: The ON to OFF transition. TT can be used to overcome the problem of propagation delay in a long cable. ST must traverse a cable of unknown length and delay, clock a bit out of the DTE after another unknown delay, and return it to the DCE over the same unknown cable delay. When sending data at high speed, the data bit may not arrive in time for the ON to OFF transition of ST. Since the relation between

6490-421: The RS-232 device, can send data onto the bus. Modern communications environments use full-duplex (both directions simultaneously) modems. In that environment, DTEs have no reason to deassert RTS. However, due to the possibility of changing line quality, delays in processing of data, etc., there is a need for symmetric, bidirectional flow control . A symmetric alternative providing flow control in both directions

6600-414: The RS-232 interface, and provide sufficient current to comply with the slew rate requirements for data transmission. Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero. This may also cause a hazardous ground loop . Use of

6710-454: The RTS and CTS signals, per the older versions of the standard, is asymmetric. This scheme is also employed in present-day RS-232 to RS-485 converters. RS-485 is a multiple-access bus on which only one device can transmit at a time, a concept that is not provided for in RS-232. The RS-232 device asserts RTS to tell the converter to take control of the RS-485 bus so that the converter, and thus

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6820-472: The Recommendations of the CCITT were presented at plenary assemblies for endorsement, held every four years, and the full set of Recommendations were published after each plenary assembly. However, the delays in producing texts, and translating them into other working languages, did not suit the fast pace of change in the telecommunications industry. The rise of the personal computer industry in

6930-476: The SG chairman, in consultation with TSB, sets up a comment resolution process by the concerned experts. The revised text is then posted on the web for an additional review period of three weeks. Similar to the last call phase, in additional review the Recommendation is considered as approved if no comments are received. If comments are received, it is apparent that there are some issues that still need more work, and

7040-579: The amendment of ITRs through a World Conference on International Telecommunications (WCIT). Accordingly, in 1998 there began a process of review of the ITRs; and in 2009 extensive preparations began for such a conference, WCIT-12. In addition to "regional preparatory meetings", the ITU Secretariat developed 13 "Background Briefs on key issues" that were expected to be discussed at the conference. Convened by former ITU secretary-general Hamadoun Touré,

7150-429: The approval process by providing equal opportunities for both sector members and member states in the approval of technical standards. A panel of SG experts drafts a proposal that is then forwarded at an SG meeting to the appropriate body which decides if it is sufficiently ready to be designated a draft text and thus gives its consent for further review at the next level. After this Consent has been given, TSB announces

7260-595: The authority to approve Recommendations. Focus Groups can be created very quickly, are usually short-lived and can choose their own working methods, leadership, financing, and types of deliverables. Current Focus Groups include the ITU-WHO Focus Group on Artificial Intelligence for Health (FG-AI4H) as well as Machine Learning for 5G (which developed Y.3172 ), Quantum Information Technologies for Networks , and Artificial Intelligence for Assisted and Autonomous Driving . The Alternative Approval Process (AAP)

7370-711: The basic similarity of many of the technical problems faced by the CCIF and CCIT , a decision was taken in 1956 to merge them into a single entity, the International Telegraph and Telephone Consultative Committee ( CCITT , in French : Comité Consultatif International Téléphonique et Télégraphique ). The first Plenary Assembly of the new organization was held in Geneva, Switzerland in December 1956. In 1992,

7480-491: The bits within each byte just before sending and just after receiving. 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

7590-549: 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. In this circumstance, the appropriately gendered connectors on the cable or a gender changer can be used to correct the mismatch. 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

7700-421: The common parlance sense of the word "recommendation"), as they become mandatory only when adopted as part of a national law. Since the ITU-T is part of the ITU, which is a United Nations specialized agency, its standards carry more formal international weight than those of most other standards development organizations that publish technical specifications of a similar form. At the initiative of Napoleon III ,

7810-403: The communications link, as well as both DCEs. When the DCE is in test mode, it signals the DTE by setting Test Indicator (TI, pin 25) to ON. A commonly used version of loopback testing does not involve any special capability of either end. A hardware loopback is simply a wire connecting complementary pins together in the same connector (see loopback ). Loopback testing is often performed with

7920-457: The computer acting as the DTE). It remained in widespread use into the late 1990s. In personal computer peripherals, it has largely been supplanted by other interface standards, such as USB. RS-232 is still used to connect older designs of peripherals, industrial equipment (such as PLCs ), console ports, and special purpose equipment. The standard has been renamed several times during its history as

8030-431: 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 bit/s. Many of these standard modem baud rates are multiples of either 1.2 kbps (e.g., 19200, 38400, 76800) or 0.9 kbps (e.g., 57600, 115200). Crystal oscillators with a frequency of 1.843200 MHz are sold specifically for this purpose. This

8140-415: 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, low battery, 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;

8250-471: The data bits within each byte least significant bit first. Also possible, but rarely used, is 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

8360-403: The data transmission and the control signal lines. Valid signals are either in the range of +3 to +15 volts or the range −3 to −15 volts with respect to the "Common Ground" (GND) pin; consequently, the range between −3 and +3 volts is not a valid RS-232 level. For data transmission lines (TxD, RxD, and their secondary channel equivalents), logic one is represented as a negative voltage and

8470-522: The definition of international telecommunication services, cooperation between countries and national administrations, safety of life and priority of telecommunications and charging and accounting principles. The adoption of the ITRs in 1988 is often taken as the start of the wider liberalization process in international telecommunications, though a few countries, including United States and United Kingdom, had made steps to liberalize their markets before 1988. The Constitution and Convention of ITU provides for

8580-609: The differential signaling. Unused interface signals terminated to the ground will have an undefined logic state. Where it is necessary to permanently set a control signal to a defined state, it must be connected to a voltage source that asserts the logic 1 or logic 0 levels, for example with a pull-up resistor . Some devices provide test voltages on their interface connectors for this purpose. RS-232 devices may be classified as Data Terminal Equipment (DTE) or Data Circuit-terminating Equipment (DCE); this defines at each device which wires will be sending and receiving each signal. According to

8690-601: The draft text and all comments are sent to the next Study Group meeting for further discussion and possible approval. Those Recommendations considered as having policy or regulatory implications are approved through what is known as the Traditional Approval Process (TAP), which allows a longer period for reflection and commenting by member states. TAP Recommendations are also translated into the six working languages of ITU (Arabic, Chinese, English, French, Russian, and Spanish). ITU-T Recommendations are

8800-505: The early 1980s created a new common practice among both consumers and businesses of adopting " bleeding edge " communications technology even if it was not yet standardized. Thus, standards organizations had to put forth standards much faster, or find themselves ratifying de facto standards after the fact. One of the most prominent examples of this was the Open Document Architecture project, which began in 1985 when

8910-409: 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. 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, but the speeds on both sides of

9020-412: The final approval of a full-status ITU-T Recommendation can now be as short as a few months (or less in some cases). This makes the standardization approval process in the ITU-T much more responsive to the needs of rapid technology development than in the ITU's historical past. New and updated Recommendations are published on an almost daily basis, and nearly all of the library of over 3,270 Recommendations

9130-494: The following: 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,

9240-435: 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 is used in circumstances where a transmitter might be able to send data faster than

9350-417: The ground or to any voltage level up to ±25 volts. The slew rate , or how fast the signal changes between levels, is also controlled. Because the voltage levels are higher than logic levels typically used by integrated circuits, special intervening driver circuits are required to translate logic levels. These also protect the device's internal circuitry from short circuits or transients that may appear on

9460-518: The mid nineties, and two years until 1997, can now be approved in an average of two months, or as little as five weeks. Besides streamlining the underlying procedures involved in the approval process, an important contributory factor to the use of AAP is electronic document handling. Once the approval process has begun the rest of the process can be completed electronically, in the vast majority of cases, with no further physical meetings. The introduction of AAP also formalizes public/private partnership in

9570-421: 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. Since the control signals for a serial port can be driven by any digital signal , some applications used

9680-421: The names given to telecommunications and computer protocol specification documents published by ITU-T. ITU-T assigns each Recommendation a name based on the series and Recommendation number. The name starts with the letter of the series the Recommendation belongs to. Each series encompasses a broad category of Recommendations, such as "H-Series Recommendations: Audiovisual and multimedia systems". The series letter

9790-460: 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 , where control information

9900-430: The original DCEs were (usually) modems. When electronic terminals (smart and dumb) began to be used, they were often designed to be interchangeable with teletypewriters, and so supported RS-232. Because the standard did not foresee the requirements of devices such as computers, printers, test instruments, POS terminals , and so on, designers implementing an RS-232 compatible interface on their equipment often interpreted

10010-475: The other logic needed. As PCs evolved serial ports were included in the Super I/O chip and then in the chipset . 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

10120-524: The receiver is able to process it. To cope with this, serial lines often incorporate a 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

10230-416: The related RS-422 standard, mostly using circular mini-DIN connectors . 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. 10P10C connectors can be found on some devices. Another common connector is a 10 × 2 pin header common on motherboards and add-in cards which is usually converted via

10340-405: The remote system may have begun transmitting just before the local system de-asserts RTR. A minimal "3-wire" RS-232 connection consisting only of transmit data, receive data, and ground, is commonly used when the full facilities of RS-232 are not required. Even a two-wire connection (data and ground) can be used if the data flow is one way (for example, a digital postal scale that periodically sends

10450-399: The same pin as RTS (Request to Send), and that when 133 is in use, RTS is assumed by the DCE to be asserted at all times. In this scheme, commonly called "RTS/CTS flow control" or "RTS/CTS handshaking" (though the technically correct name would be "RTR/CTS"), the DTE asserts RTS whenever it is ready to receive data from the DCE, and the DCE asserts CTS whenever it is ready to receive data from

10560-614: The sector's governing conference, convenes every four years. ITU-T has a permanent secretariat called the Telecommunication Standardization Bureau (TSB), which is based at the ITU headquarters in Geneva, Switzerland . The current director of the TSB is Seizo Onoe (of Japan), whose 4-year term commenced on 1 January 2023. Seizo Onoe succeeded Chaesub Lee of South Korea, who was director from 1 January 2015 until 31 December 2022. The ITU-T mission

10670-637: The serial port devices /dev/tty* . TTY is a common trademark-free abbreviation for teletype , a device commonly attached to early computers' serial ports, and * represents a string identifying the specific port; the syntax of that string depends on the operating system and the device. On Linux , 8250 / 16550 UART hardware 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 DOS and Windows environments refer to serial ports as COM ports: COM1, COM2,..etc. This list includes some of

10780-563: The serial port hardware, typically a UART , which may also contain circuits to convert the internal logic levels to RS-232 compatible signal levels. The standard does not define bit rates for transmission, except that it says it is intended for bit rates lower than 20,000 bits per second. RS-232 was first introduced in 1960 by the Electronic Industries Association (EIA) as a Recommended Standard . The original DTEs were electromechanical teletypewriters , and

10890-439: The signal condition is called "mark". Logic zero is signaled with a positive voltage and the signal condition is termed "space". Control signals have the opposite polarity: the asserted or active state is positive voltage and the de-asserted or inactive state is negative voltage. Examples of control lines include request to send (RTS), clear to send (CTS), data terminal ready (DTR), and data set ready (DSR). The standard specifies

11000-495: The sponsoring organization changed its name, and has been variously known as EIA RS-232, EIA 232, and, most recently as TIA 232. The standard continued to be revised and updated by the Electronic Industries Association and since 1988 by the Telecommunications Industry Association (TIA). Revision C was issued in a document dated August 1969. Revision D was issued in 1986. The current revision

11110-553: The standard definitions are not always correctly applied, it is often necessary to consult documentation, test connections with a breakout box , or use trial and error to find a cable that works when interconnecting two devices. Connecting a fully standard-compliant DCE device and DTE device would use a cable that connects identical pin numbers in each connector (a so-called "straight cable"). " Gender changers " are available to solve gender mismatches between cables and connectors. Connecting devices with different types of connectors requires

11220-411: The standard idiosyncratically. The resulting common problems were non-standard pin assignment of circuits on connectors, and incorrect or missing control signals. The lack of adherence to the standards produced a thriving industry of breakout boxes , patch boxes, test equipment, books, and other aids for the connection of disparate equipment. A common deviation from the standard was to drive the signals at

11330-440: The standard). The signals are named from the standpoint of the DTE. The ground pin is a common return for the other connections, and establishes the "zero" voltage to which voltages on the other pins are referenced. The DB-25 connector includes a second "protective ground" on pin 1; this is connected internally to equipment frame ground, and should not be connected in the cable or connector to signal ground. Ring Indicator (RI)

11440-585: The standard, male connectors have DTE pin functions, and female connectors have DCE pin functions. Other devices may have any combination of connector gender and pin definitions. Many terminals were manufactured with female connectors but were sold with a cable with male connectors at each end; the terminal with its cable satisfied the recommendations in the standard. The standard recommends the D-subminiature 25-pin connector up to revision C, and makes it mandatory as of revision D. Most devices only implement

11550-549: The start of the AAP procedure by posting the draft text to the ITU-T website and calling for comments. This gives the opportunity for all members to review the text. This phase, called last call , is a four-week period in which comments can be submitted by member states and sector members. If no comments other than editorial corrections are received, the Recommendation is considered approved since no issues were identified that might need any further work. However, if there are any comments,

11660-492: The state of the pin. RI does not correspond to another signal that carries similar information the opposite way. On an external modem the status of the Ring Indicator pin is often coupled to the "AA" (auto answer) light, which flashes if the RI signal has detected a ring. The asserted RI signal follows the ringing pattern closely, which can permit software to detect distinctive ring patterns. The Ring Indicator signal

11770-551: 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. 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

11880-407: The transmitted bit and TT can be fixed in the DTE design, and since both signals traverse the same cable length, using TT eliminates the issue. TT may be generated by looping ST back with an appropriate phase change to align it with the transmitted data. ST loop back to TT lets the DTE use the DCE as the frequency reference, and correct the clock to data timing. Serial port A serial port

11990-637: The work of standardization, ITU-T cooperates with other SDOs, e.g., the International Organization for Standardization (ISO) and the Internet Engineering Task Force (IETF). Most of the work of ITU-T is carried out by its Sector Members and Associates, while the Telecommunication Standardization Bureau (TSB) is the executive arm of ITU-T and coordinator for a number of workshops and seminars to progress existing work areas and explore new ones. The events cover

12100-463: Was developed and marketed in the late 1980s by various equipment manufacturers. It redefined the RTS signal to mean that the DTE is ready to receive data from the DCE. This scheme was eventually codified in version RS-232-E (actually TIA-232-E by that time) by defining a new signal, "RTR (Ready to Receive)", which is CCITT V.24 circuit 133. TIA-232-E and the corresponding international standards were updated to show that circuit 133, when implemented, shares

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