Editing RS-232 (section)

==Data and control signals==
[[File:DB-25 male.svg|thumb|Male pinout of a 25-pin serial port (D-subminiature, DB-25) commonly found on 1980s computers]]
The following table lists commonly used RS-232 signals (called "circuits" in the specifications) and their pin assignments on the recommended DB-25 connectors<ref>{{cite web |url=https://pinoutguide.com/SerialPorts/RS232_pinout.shtml |title=Serial (PC 25) |work=Pinout Guide}}</ref> (see [[Serial port pinout]]s for other commonly used connectors not defined by the standard).
{| class="wikitable sortable" style="margin:auto; text-align:center;"
! colspan="3" | Circuit !! colspan="2" | Direction !! rowspan="2" | [[DB-25]] pin
|-
! Name !! width="400px" | Typical purpose !! Abbreviation !! DTE !! DCE
|-
| align="left" | [[Data Terminal Ready]] || align="left" | DTE is ready to receive, initiate, or continue a call. || DTR || Out || In || 20
|-
| align="left" | [[Data Carrier Detect]] || align="left" | DCE is receiving a carrier from a remote DCE. || DCD || In || Out || 8
|-
| align="left" | Data Set Ready          || align="left" | DCE is ready to receive and send data. || DSR || In || Out || 6
|-
| align="left" | Ring Indicator      || align="left" | DCE has detected an incoming ring signal on the telephone line. || RI  || In  || Out || 22
|-
| align="left" | Request To Send         || align="left" | DTE requests the DCE prepare to transmit data. || RTS || Out || In || 4
|-
| align="left" | Ready To Receive        || align="left" | DTE is ready to receive data from DCE. If in use, RTS is assumed to be always asserted. || RTR || Out || In || 4
|-
| align="left" | Clear To Send           || align="left" | DCE is ready to accept data from the DTE. || CTS || In || Out || 5
|-
| align="left" | Transmitted Data        || align="left" | Carries data from DTE to DCE. || TxD || Out || In || 2
|-
| align="left" | Received Data           || align="left" | Carries data from DCE to DTE. || RxD || In || Out || 3
|-
| align="left" | Common Ground           || align="left" | Zero voltage reference for all of the above. || GND || colspan="2" | <small>Common</small>|| 7
|-
| align="left" | Protective Ground       || align="left" | Connected to chassis ground. || PG || colspan="2" | <small>Common</small>|| 1
|}

The signals are named from the standpoint of the DTE. [[Single-ended signaling|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.

==={{anchor|RI}}Ring Indicator===
[[File:Modem US Robotics Courier Dual Standard.jpg|thumb|right|[[USRobotics]] Courier external modem had a [[DB-25]] connector that used the Ring Indicator signal to notify the host computer when the connected [[telephone line]] was ringing]]
''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===
{{further|Flow control (data)#Hardware flow control}}

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 (data)|flow control]].

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.<ref>{{cite newsgroup |title=Re: EIA-232 full duplex RTS/CTS flow control standard proposal |author-first=Casey |author-last=Leedom |date=1990-02-20 |newsgroup=comp.dcom.modems |message-id=49249@lll-winken.LLNL.GOV |url=http://groups.google.com/group/comp.dcom.modems/msg/39042605325cc765?dmode=source |access-date=2014-02-03}}</ref>

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 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 de-asserts RTR.

===3-wire and 5-wire RS-232===
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.