• Rs232 Connection Software
• Serial Connection Rs232 Cable

In computing, a serial port is a serial communication 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.

What Is an RS232 9 Pin Pinout? RS232 monitoring hardware establishes a connection between data terminal equipment (DTE) and data communication equipment (DCE). In order to link these devices, an RS232 D9 pinout is essential, as this pinout will allow you to connect two devices successfully. An RS232 pinout 9 pin cable features nine pins: 1. RS-232 Connections That WORK! - Connecting Devices or Converters. Connecting two devices using RS-232 sounds simple, but nearly every day we help a customer get a converter, isolator or other RS-232 device working by helping correct the RS-232 cabling connections. This FAQ will help you troubleshoot and correct such problems. USB to Serial connection solutions are vast and affordable. Link up via CAT5/6 Ethernet cables or direct ports to PCs, DNC RS232 to RJ45, & even with new laptops with only USB connections. We can connect you! The USB-to-Serial converter is the perfect accessory for laptop computers that do not have a traditional 9pin RS232 port. RS232 serial data cable using minimum number of connections More comprehensive systems may utilise the additional lines provided by RS232. According serial data cables for these systems require the additional lines to be incorporated into the serial data cables for them to operate correctly.

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

Modern computers without serial ports may require USB-to-serial 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.

• 1Hardware
• 3Settings

Hardware[edit]

Some computers, such as the IBM PC, use an integrated circuit called a UART. This IC converts characters to and from asynchronous serial form, implementing the timing and framing of data in hardware. 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. Before large-scale integration (LSI) UART integrated circuits were common, a minicomputer 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 FireWire to 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[edit]

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.

Male and female[edit]

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.^[2] 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.^[3]

Connectors[edit]

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. 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 is also used for video, joysticks, and other purposes.

Some miniaturized electronics, particularly graphing calculators and hand-held amateur 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 favor 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 some PowerBook laptops had only one combined port to save space.

Since most devices do not use all of the 20 signals that are defined by the standard, smaller connectors are often used. For example, the 9-pin DE-9 connector is 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, for which the EIA/TIA-561 standard defines a pinout, while the 'Yost Serial Device Wiring Standard'^[4] invented by Dave Yost (and popularized by the Unix System Administration Handbook) is common on Unix computers and newer devices from Cisco Systems. 10P10C connectors can be found on some devices as well. Digital Equipment Corporation defined their own DECconnect connection system which is 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[edit]

The following table lists commonly used RS-232 signals and pin assignments.^[5]

The 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 is intended to be connected by each device to its own frame ground or similar. Connecting this to pin 7 (signal reference ground) is a common practice but not recommended.

Note that EIA/TIA 561 combines DSR and RI,^[10][11] and the Yost standard combines DSR and DCD.

Powered serial port[edit]

Some serial ports on motherboards or add-in cards provide jumpers that select whether pin 1 of the DE-9 connector connects to DCD or a power supply voltage, and whether pin 9 of the DE-9 connector connects to RI or a power supply voltage. The power supply voltage can be +5V, +12V, +9V, or ground. (Selection varies by vendor.) The power is intended for use by point-of-sale equipment. Makers include Dell^[12], HP, and others^[13] (This is not an official standard.)

Hardware abstraction[edit]

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 /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. Ports numbered greater than COM9 should be referred to using the .COM10 syntax.^[14]

Common applications for serial ports[edit]

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.
• Hobbyist programming and debugging MCU's
• Stenography or Stenotype machines
• Software debuggers that run on a second computer
• Industrial field buses
• Computer terminal, teletype
• Older digital cameras
• Networking (Macintosh AppleTalk using RS-422 at 230.4 kbit/s)
• Older GSMmobile phones
• IDEhard drive^[15][16]repair^[17][18]

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, 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. The status bits of the serial port could be sampled very rapidly and at predictable times, making it possible for the software to decipher Morse code.

Settings[edit]

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

Speed[edit]

Serial ports use two-level (binary) signaling, so the data rate in bits per second is equal to the symbol rate in baud. A standard series of rates is based on multiples of the rates for electromechanical teleprinters; some serial ports allow many arbitrary rates to be selected. The port speed and device speed must match. The capability to set a bit rate does not imply that a working connection will result. Not all bit rates are possible with all serial ports. Some special-purpose protocols such as MIDI for musical instrument control, use serial data rates other than the teleprinter series. Some serial port systems can automatically detect the bit rate.

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

Bit rates commonly supported include 75, 110, 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bit/s.^[20]

Crystal oscillators with a frequency of 1.843200 MHz are sold specifically for this purpose. This is 16 times the fastest bit rate and the serial port circuit can easily divide this down to lower frequencies as required.

Data bits[edit]

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

Most serial communications designs send the data bits within each byte LSB (least significant bit) first. This standard is also referred to as 'little endian.' Also possible, but rarely used, is 'big endian' or MSB (most significant bit) first serial communications; this was used, for example, by the IBM 2741 printing terminal. (See Bit numbering for more about bit ordering.) The order of bits is not usually configurable within the serial port interface. To communicate with systems that require a different bit ordering than the local default, local software can re-order the bits within each byte just before sending and just after receiving.

Parity[edit]

Parity is a method of detecting errors in transmission. When parity is used with a serial port, an extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check.

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

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

• None (N) means that no parity bit is sent at all.
• Odd (O) means that parity bit is set so that the number of 'logical ones' must be odd.
• Even (E) means that parity bit is set so that the number of 'logical ones' must be even.
• Mark (M) parity means that the parity bit is always set to the mark signal condition (logical 1).
• Space (S) parity always sends the parity bit in the space signal condition (logical 0).

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, since it ensures that at least one state transition occurs in each character, which makes it more reliable. The most common parity setting, however, is 'none', with error detection handled by a communication protocol.

Stop bits[edit]

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

Conventional notation[edit]

The 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. If a receiver of a 7/E/1 stream is expecting an 8/N/1 stream, half the possible bytes will be interpreted as having the high bit set.

Flow control[edit]

In many circumstances 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 'handshaking' method, usually distinguished between hardware and software handshaking.

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

Software handshaking is done for example with ASCIIcontrol charactersXON/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 circuit starts in the 'sending allowed' state. When the receiver's buffers approach capacity, the receiver sends the XOFF character to tell the sender to stop sending data. Later, after the receiver has emptied its buffers, it sends an XON character to tell the sender to resume transmission. It is an example of 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 doesn't impose any particular meaning such as ASCII on the transferred data; and it is stateless. Its disadvantage is that it requires more hardware and cabling, and these must be compatible at both ends.

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 a) control messages get through even when data messages are blocked, and b) 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) 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 a mechanism to resend data which has been corrupted (e.g. TCP).

'Virtual' serial ports[edit]

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

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

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

See also[edit]

• ITU-T/CCITT V.24 [de]
• ITU-T/CCITT V.28 [de]

References[edit]

1. ^Webopedia (2003-09-03). 'What is serial port? - A Word Definition From the Webopedia Computer Dictionary'. Webopedia.com. Retrieved 2009-08-07.
2. ^'Serial Cable Connection Guide'. CISCO. 2006-08-01. Retrieved 2016-01-31.
3. ^'RS232 - DTE and DCE connectors'. Lantronix. 2006-03-29. Retrieved 2016-01-31.
4. ^Yost Serial Device Wiring Standard
5. ^Ögren, Joakim. 'Serial (PC 9)'.
6. ^ ^abCyclom-Y Installation Manual, page 38, retrieved on 29 November 2008^{[permanent dead link]}
7. ^'RJ-45 8-Pin to Modem (ALTPIN option)'. Digiftp.digi.com. Retrieved 2014-02-08.
8. ^National Instruments Serial Quick Reference Guide, February 2007
9. ^'RJ-45 10-Pin Plug to DB-25 Modem Cable'. Digiftp.digi.com. Retrieved 2014-02-08.
10. ^Hardware Book RS-232D
11. ^RS-232D EIA/TIA-561 RJ45 Pinout
12. ^'OptiPlex XE Powered Serial Port Configuration'(PDF).
13. ^'Powered Serial Cards - Brainboxes'.
14. ^'HOWTO: Specify Serial Ports Larger than COM9'. Microsoft support. Retrieved 2013-10-26.
15. ^'Paul's 8051 Code Library, IDE Hard Drive Interface'. Pjrc.com. 2005-02-24. Retrieved 2014-02-08.
16. ^'IDE Hard Disk experiments'. Hem.passagen.se. 2004-02-15. Archived from the original on 2004-04-15. Retrieved 2014-02-08.
17. ^'The Solution for Seagate 7200.11 HDDs - Hard Drive and Removable Media issues - MSFN Forum'. Msfn.org. Retrieved 2014-02-08.
18. ^'Fixing a Seagate 7200.11 Hard Drive'. Sites.google.com. Retrieved 2014-02-08.
19. ^'SERIAL_COMMPROP structure'. Microsoft. 2018-04-22. Retrieved 2019-09-28.
20. ^ ^ab'DCB Structure'. Windows Dev Center. Microsoft. 2018-12-04. Retrieved 2019-09-28.
21. ^'PC16550D Universal Asynchronous Receiver/Transmitter With FIFOs'(PDF). Texas Instruments. May 2015. Retrieved September 25, 2019.
22. ^'BACnet MS/TP Overview Manual'(PDF). Neptronic. Retrieved September 26, 2019.
23. ^'MultiModem ZBA'(PDF). Multi-Tech Systems, Inc. January 2019. Retrieved September 26, 2019.
24. ^'Courier 56K Business Modem: User Guide: Controlling Data Rates'. USRobotics. 2007. Retrieved September 26, 2019.

Further reading[edit]

• Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems; 2nd Edition; Jan Axelson; Lakeview Research; 380 pages; 2007; ISBN978-1-931-44806-2.

External links[edit]

In telecommunications, RS-232, Recommended Standard 232^[1] refers to a standard originally introduced in 1960^[2] for serial communication transmission of data. It formally defines signals connecting between a DTE (data terminal equipment) such as a computer terminal, and a DCE (data circuit-terminating equipment or data communication equipment), such as a modem. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors. The current version of the standard is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997. The RS-232 standard had been commonly used in computerserial ports and 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.

RS-232, when compared to later interfaces such as RS-422, RS-485 and Ethernet, has lower transmission speed, short maximum cable length, large voltage swing, large 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. Many computers no longer come equipped with RS-232 ports and must use either an external USB-to-RS-232 converter or an internal expansion card with one or more serial ports to connect to RS-232 peripherals. Nevertheless, thanks to their simplicity and past ubiquity, RS-232 interfaces are still used—particularly in industrial machines, networking equipment, and scientific instruments where a short-range, point-to-point, low-speed wired data connection is fully adequate.

• 5Physical interface
  • 5.3Cables
• 6Data and control signals
• 7Seldom-used features

Scope of the standard[edit]

The Electronic Industries Association (EIA) standard RS-232-C^[3] as of 1969 defines:

• Electrical signal characteristics such as voltage levels, signaling rate, timing, and slew-rate of signals, voltage withstand level, short-circuit behavior, and maximum load capacitance.
• Interface mechanical characteristics, pluggable connectors and pin identification.
• Functions of each circuit in the interface connector.
• Standard subsets of interface circuits for selected telecom applications.

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 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.

History[edit]

RS-232 was first introduced in 1960^[2] by the Electronic Industries Association (EIA) as a Recommended Standard.^[4][1] The original DTEs were electromechanical teletypewriters, and 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 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 a reduced voltage. Some manufacturers therefore built transmitters that supplied +5 V and −5 V and labeled them as 'RS-232 compatible'.^{[citation needed]}

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 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 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).^[5] Revision C was issued in a document dated August 1969. Revision D was issued in 1986. The current revision 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 V.24, but equipment built to the current standard will interoperate with older versions.^{[citation needed]}

Related ITU-T standards include V.24 (circuit identification) and V.28 (signal voltage and timing characteristics).^{[citation needed]}

In revision D of EIA-232, 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.^[6]

Overview:

Rs232 Connection Software

• EIA RS-232 (May 1960) 'Interface Between Data Terminal Equipment & Data'^[2]
• EIA RS-232-A (October 1963)^[2]
• EIA RS-232-B (October 1965)^[2]
• EIA RS-232-C (August 1969) 'Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange'^[2]
• EIA EIA-232-D (1986)
• TIA TIA/EIA-232-E (1991) 'Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Data Interchange'
• TIA TIA/EIA-232-F (1997-10-01)
• ANSI/TIA-232-F-1997 (R2002)
• TIA TIA-232-F (R2012)

Limitations of the standard[edit]

Because RS-232 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:^[7]

• The large voltage swings and requirement for positive and negative supplies increases power consumption of the interface and complicates power supply design. The voltage swing requirement also limits the upper speed of a compatible interface.
• Single-ended signaling referred to a common signal ground limits the noise immunity and transmission distance.
• Multi-drop connection among more than two devices is not defined. While multi-drop 'work-arounds' have been devised, they have limitations in speed and compatibility.
• The standard does not address the possibility of connecting a DTE directly to a DTE, or a DCE to a DCE. Null modem cables can be used to achieve these connections, but these are not defined by the standard, and some such cables use different connections than others.
• The definitions of the two ends of the link are asymmetric. This makes the assignment of the role of a newly developed device problematic; the designer must decide on either a DTE-like or DCE-like interface and which connector pin assignments to use.
• The handshaking and control lines of the interface are intended for the setup and takedown of a dial-up communication circuit; in particular, the use of handshake lines for flow control is not reliably implemented in many devices.
• No method is specified for sending power to a device. While a small amount of current can be extracted from the DTR and RTS lines, this is only suitable for low-power devices such as mice.
• The 25-pin D-sub connector recommended in the standard is large compared to current practice.

Role in modern personal computers[edit]

In the book PC 97 Hardware Design Guide,^[8]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 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)^[dubious] and that maximum cable length is much shorter (15 meters for RS-232 v.s. 3 - 5 meters for USB depending on USB speed used).^{[citation needed]}

In fields such as laboratory automation or surveying, RS-232 devices may 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 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 operating system is not running yet 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 a router) as an alternative to monitoring over Ethernet.

Physical interface[edit]

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 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.

Voltage levels[edit]

Serial Connection Rs232 Cable

The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for 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 to +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 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 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. Some RS-232 driver chips have inbuilt 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 circuit to 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 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 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 the differential signaling.^[9]

Unused interface signals terminated to 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 level, for example with a pullup resistor. Some devices provide test voltages on their interface connectors for this purpose.

Connectors[edit]

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 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 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 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.

Cables[edit]

The standard does not define 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).^[10] For longer distances, other signal standards, such as RS-422, are better suited for higher speeds.

Since 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 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 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.

3-wire and 5-wire RS-232[edit]

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 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.

Data and control signals[edit]

The following table lists commonly used RS-232 signals (called 'circuits' in the specifications) and their pin assignments on the recommended DB-25 connectors.^[11] (See Serial port pinouts for other commonly used connectors not defined by 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[edit]

Ring Indicator (RI) 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' 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 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.

RTS, CTS, and RTR[edit]

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 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 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 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.

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A symmetric alternative providing flow control in both directions 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 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.^[12]

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 RTR whenever it is ready to receive data from the DCE, and the DCE asserts CTS whenever it is ready to receive data from 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 the remote system may have begun transmitting just before the local system deasserts RTR.

Seldom-used features[edit]

The EIA-232 standard specifies connections for several features that are not used in most implementations. Their use requires 25-pin connectors and cables.

Signal rate selection[edit]

The DTE or DCE can specify use of 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 pin 23 to ON.

Loopback testing[edit]

Many DCE devices have 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 pin 18 to ON, or the remote DCE (the one the local DCE is connected to) to enter loopback mode by setting pin 21 to ON. The latter tests the communications link, as well as both DCEs. When the DCE is in test mode, it signals the DTE by setting 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 a specialized DTE called a bit error rate tester (or BERT).

Timing signals[edit]

Some synchronous devices provide a clock signal to synchronize data transmission, especially at higher data rates. Two timing signals are provided by the DCE on pins 15 and 17. Pin 15 is the transmitter clock, or send timing (ST); the DTE puts the next bit on the data line (pin 2) when this clock transitions from OFF to ON (so it is stable during the ON to OFF transition when the DCE registers the bit). Pin 17 is the receiver clock, or receive timing (RT); the DTE reads the next bit from the data line (pin 3) when this clock transitions from ON to OFF.

Alternatively, the DTE can provide a clock signal, called transmitter timing (TT), on pin 24 for transmitted data. Data is changed when the clock transitions from OFF to ON, and read during the ON to OFF transition. TT can be used to overcome the issue where 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. Since the relation between 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.

Synchronous clocking is required for such protocols as SDLC, HDLC, and X.25.

Secondary channel[edit]

A secondary data channel, identical in capability to the primary channel, can optionally be implemented by the DTE and DCE devices. Pin assignments are as follows:

Related standards[edit]

Other serial signaling standards may not interoperate with standard-compliant RS-232 ports. For example, using the TTL levels of near +5 and 0 V puts the mark level in the undefined area of the standard. Such levels are sometimes used with NMEA 0183-compliantGPS receivers and depth finders.

A 20 mA current loop uses the absence of 20 mA current for high, and the presence of current in the loop for low; this signaling method is often used for long-distance and optically isolated links. Connection of a current-loop device to a compliant RS-232 port requires a level translator. Current-loop devices can supply voltages in excess of the withstand voltage limits of a compliant device. The original IBM PC serial port card implemented a 20 mA current-loop interface, which was never emulated by other suppliers of plug-compatible equipment.

Other serial interfaces similar to RS-232:

• RS-422 (a high-speed system similar to RS-232 but with differential signaling)
• RS-423 (a high-speed system similar to RS-422 but with unbalanced signaling)
• RS-449 (a functional and mechanical interface that used RS-422 and RS-423 signals - it never caught on like RS-232 and was withdrawn by the EIA)
• RS-485 (a descendant of RS-422 that can be used as a bus in multidrop configurations)
• MIL-STD-188 (a system like RS-232 but with better impedance and rise time control)
• EIA-530 (a high-speed system using RS-422 or RS-423 electrical properties in an EIA-232 pinout configuration, thus combining the best of both; supersedes RS-449)
• EIA/TIA-561 8 Position Non-Synchronous Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange
• EIA/TIA-562 Electrical Characteristics for an Unbalanced Digital Interface (low-voltage version of EIA/TIA-232)
• TIA-574 (standardizes the 9-pin D-subminiature connector pinout for use with EIA-232 electrical signalling, as originated on the IBM PC/AT)

Development tools[edit]

When developing or troubleshooting systems using RS-232, close examination of hardware signals can be important to find problems. A simple indicator device uses LEDs to show the high/low state of data or control pins. Y cables may be used to allow using another serial port to monitor all traffic on one direction. A serial line analyzer is a device similar to a logic analyzer but specialized for RS-232's voltage levels, connectors, and, where used, clock signals. The serial line analyzer can collect, store, and display the data and control signals, allowing developers to view them in detail. Some simply display the signals as waveforms; more elaborate versions include the ability to decode characters in ASCII or other common codes and to interpret common protocols used over RS-232 such as SDLC, HDLC, DDCMP, and X.25. Serial line analyzers are available as standalone units, as software and interface cables for general-purpose logic analyzers and oscilloscopes, and as programs that run on common personal computers and devices.

References[edit]

1. ^ ^abMetering GlossaryArchived 2012-11-29 at the Wayback Machine Landis + Gyr Tutorial (see EIA)
2. ^ ^abcdefEvans, Jr., John M.; O'Neill, Joseph T.; Little, John L.; Albus, James S.; Barbera, Anthony J.; Fife, Dennis W.; Fong, Elizabeth N.; Gilsinn, David E.; Holberton, Frances E.; Lucas, Brian G.; Lyon, Gordon E.; Marron, Beatrice A. S.; Neumann, Albercht J.; Vickers, Mabel V.; Walker, Justin C. (October 1976), Standards for Computer Aided Manufacturing (Second Interim Report ed.), Office of Developmental Automation and Control Technology, Institute for Computer Sciences and Technology, National Bureau of Standards, Washington, DC, USA: Manufacturing Technology Division, Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio 45433, NBSIR 76-1094, retrieved 2017-03-04
3. ^EIA standard RS-232-C: Interface between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange. Washington, USA: Electronic Industries Association, Engineering Department. 1969. OCLC38637094.
4. ^'RS232 Tutorial on Data Interface and cables'. ARC Electronics. 2010. Retrieved 2011-07-28.
5. ^'TIA Facts at a Glance'. About TIA. Telecommunications Industry Association. Retrieved 2011-07-28.
6. ^S. Mackay, E. Wright, D. Reynders, J. Park, Practical Industrial Data Networks: Design, Installation, and Troubleshooting, Newnes, 2004 ISBN07506 5807X, pages 41-42
7. ^Horowitz, Paul; Hill, Winfield (1989). The Art of Electronics (2nd ed.). Cambridge, England: Cambridge University Press. pp. 723–726. ISBN0-521-37095-7.
8. ^PC 97 Hardware Design Guide. Redmond, Washington, USA: Microsoft Press. 1997. ISBN1-57231-381-1.
9. ^Wilson, Michael R. (January 2000). 'TIA/EIA-422-B Overview'(PDF). Application Note 1031. National Semiconductor. Archived from the original(PDF) on 2010-01-06. Retrieved 2011-07-28.
10. ^Lawrence, Tony (1992). 'Serial Wiring'. A. P. Lawrence. Retrieved 2011-07-28.
11. ^Ögren, Joakim (2008-09-18). 'Serial (PC 25)'. Hardware Book. Retrieved 2011-07-28.
12. ^Leedom, Casey (1990-02-20). 'Re: EIA-232 full duplex RTS/CTS flow control standard proposal'. Newsgroup: comp.dcom.modems. Usenet:49249@lll-winken.LLNL.GOV. Retrieved 2014-02-03.

Further reading[edit]

• Axelson, Jan (2007). Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems (2nd ed.). Lakeview Research. ISBN978-1-931-44806-2.
• Interface Circuits for TIA/EIA-232-F: Design Notes(PDF). Mixed-Signal Products. Texas Instruments. September 2002. SLLA037. Archived(PDF) from the original on 2017-03-05. Retrieved 2017-03-05.
• Fundamentals of RS–232 Serial Communications(PDF). Dallas Semiconductor. 1998-03-09. Application Note 83. Archived(PDF) from the original on 2017-03-05. Retrieved 2017-03-05.
• 'RS232C Standard'. Knowledgebase. National Instruments. Archived from the original on 2017-03-05. Retrieved 2017-03-05.
• ITU-T Recommendation V.24 - Data Communication over the telephone network - List of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE). International Telecommunication Union (ITU-T). March 1993. Archived from the original on 2015-08-17. Retrieved 2017-03-05.

External links[edit]

• Media related to RS-232 at Wikimedia Commons
• Serial Programming:RS-232 Connections at Wikibooks