In telecommunications, RS-232 (Recommended Standard 232) is a standard for serial binary data signals connecting between a DTE (Data terminal equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports. A similar ITU-T standard is V.24.'
In RS-232, 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. Since 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
The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels. Valid signals are plus or minus 3 to 15 volts. The range near zero volts is not a valid RS-232 level; logic one is defined as a negative voltage, the signal condition is called marking, and has the functional significance of OFF. Logic zero is positive, the signal condition is spacing, and has the function ON. 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 power supplies available within a device. 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 used by integrated circuits, special intervening circuits are required to translate logic levels, and to protect circuitry internal to the device from short circuits or transients that may appear on the RS-232 interface.
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.
Because the voltage levels are higher than logic levels used by integrated circuits, special intervening circuits are required to translate logic levels, and to protect circuitry internal to the device from short circuits or transients that may appear on the RS-232 interface.
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.
Connectors
RS-232 devices may be classified as Data Terminal Equipment (DTE) or Data Circuit termination Equipment (DCE); this defines at each device which wires will be sending and receiving each signal. The standard recommended but did not make mandatory the D-subminiature 25 pin connector. In general, terminals have male connectors with DTE pin functions, and modems have female connectors with DCE pin functions. Other devices may have any combination of connector gender and pin definitions.
Presence of a 25 pin D-sub connector does not necessarily indicate an RS-232C compliant interface. For example, on the original IBM PC, a male D-sub was an RS-232C DTE port (with a non-standard current loop interface on reserved pins), but the female D-sub connector was used for a parallel Centronics printer port. Some personal computers put non-standard voltages or signals on their serial ports.
Presence of a 25 pin D-sub connector does not necessarily indicate an RS-232C compliant interface. For example, on the original IBM PC, a male D-sub was an RS-232C DTE port (with a non-standard current loop interface on reserved pins), but the female D-sub connector was used for a parallel Centronics printer port. Some personal computers put non-standard voltages or signals on their serial ports.
The standard specifies 20 different signal connections. Since most devices use only a few signals, smaller connectors can be used. For example, the 9 pin DE-9 connector was used by most IBM-compatible PCs since the IBM PC AT, and has been standardized as TIA-574. More recently, modular connectors have been used. Most common are 8 pin RJ-45 connectors. Standard EIA/TIA 561 specifies a pin assignment, but the "Yost Serial Device Wiring Standard" invented by Dave Yost is common on Unix computers and newer devices from Cisco Systems. Many devices don't use either of these standards. 10 pin RJ-50 connectors can be found on some devices as well. Digital Equipment Corporation defined their own DECconnect connection system which was based on the Modified Modular Jack 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).
Signals
Commonly-used signals are:
Transmitted Data (TxD)
Data sent from DTE to DCE.
Received Data (RxD)
Data sent from DCE to DTE.
Request To Send (RTS)
Asserted (set to 0) by DTE to prepare DCE to receive data. This may require action on the part of the DCE, e.g. transmitting a carrier or reversing the direction
Clear To Send (CTS)
Asserted by DCE to acknowledge RTS and allow DTE to transmit.
Data Terminal Ready (DTR)
Asserted by DTE to indicate that it is ready to be connected. If the DCE is a modem, this may "wake up" the modem, bringing it out of a power saving mode. This behaviour is seen quite often in modern PSTN and GSM modems. When this signal is de-asserted, the modem may return to its standby mode, immediately hanging up any calls in progress.
Data Set Ready (DSR)
Asserted by DCE to indicate an active connection. If DCE is not a modem (e.g. a null modem cable or other equipment), this signal should be permanently asserted (set to 0), possibly by a jumper to another signal.
Data Carrier Detect (DCD)
Asserted by DCE when a connection has been established with remote equipment.
Ring Indicator (RI)
Asserted by DCE when it detects a ring signal from the telephone line.
NOTE: The standard defines RTS/CTS as the signaling protocol for flow control for data transmitted from DTE to DCE. The standard has no provision for flow control in the other direction. In practice, most hardware seems to have repurposed the RTS signal for this function.
Commonly-used signals are:
Transmitted Data (TxD)
Data sent from DTE to DCE.
Received Data (RxD)
Data sent from DCE to DTE.
Request To Send (RTS)
Asserted (set to 0) by DTE to prepare DCE to receive data. This may require action on the part of the DCE, e.g. transmitting a carrier or reversing the direction
Clear To Send (CTS)
Asserted by DCE to acknowledge RTS and allow DTE to transmit.
Data Terminal Ready (DTR)
Asserted by DTE to indicate that it is ready to be connected. If the DCE is a modem, this may "wake up" the modem, bringing it out of a power saving mode. This behaviour is seen quite often in modern PSTN and GSM modems. When this signal is de-asserted, the modem may return to its standby mode, immediately hanging up any calls in progress.
Data Set Ready (DSR)
Asserted by DCE to indicate an active connection. If DCE is not a modem (e.g. a null modem cable or other equipment), this signal should be permanently asserted (set to 0), possibly by a jumper to another signal.
Data Carrier Detect (DCD)
Asserted by DCE when a connection has been established with remote equipment.
Ring Indicator (RI)
Asserted by DCE when it detects a ring signal from the telephone line.
NOTE: The standard defines RTS/CTS as the signaling protocol for flow control for data transmitted from DTE to DCE. The standard has no provision for flow control in the other direction. In practice, most hardware seems to have repurposed the RTS signal for this function.
Cables
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 above. Cables with 9 pins on one end and 25 on the other are common, and manufacturers of equipment with RJ-45 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).
Connecting two DTE devices together requires a null modem that acts as a DCE between the devices by swapping the corresponding signals (TD-RD, DTR-DSR, and RTS-CTS). This can be done with a separate device and two cables, or using a cable wired to do this. If devices require Carrier Detect, it can be simulated by connecting DSR and DCD internally in the connector, thus obtaining CD from the remote DTR signal. One feature of the Yost standard is that a null modem cable is a "rollover cable" that just reverses pins 1 through 8 on one end to 8 through 1 on the other end.
For configuring and diagnosing problems with RS-232 cables, a "breakout box" may be used. This device normally has a female and male RS-232 connector and is meant to attach in-line; it then has lights for each pin and provisions for interconnecting pins in different configurations.
RS-232 cables may be built with connectors commonly available at electronics stores. The cables may be between 3 and 25 conductors; typically 4 or 6 wires are used. Flat RJ (phone-style) cables may be used with special RJ-RS232 connectors, which are the easiest to configure. Cables are often unshielded, although shielding cables will help reduce electrical noise radiated by the cable.
The reason that a minimal two-way interface can be created with only 3 wires is that all the RS-232 signals share a common ground return. The use of unbalanced circuits makes RS-232 susceptible to problems due to ground potential shifts between the two devices. RS-232 also has relatively poor control of signal rise and fall times, leading to potential crosstalk problems. RS-232 was recommended for short connections (15 meters or less), however the limit is actually defined by total capacitance and low capacitance cables allow reliable communications over longer distances exceeding 50 m. RS-232 interface cables are not usually constructed with twisted pair because of the unbalanced circuits.
While the control lines of the RS 232 interface were originally intended for call setup and takedown, other "handshakes" may be required by one or the other device. These are used for flow control, for example, to prevent loss of data sent to a serial printer. For example, DTR is commonly used to indicate "device ready". Pins may also be "jumpered" or routed back within the connector; a pin saying "are you ready?" from device A might be wired to the pin saying "I'm ready" on device A, if device B did not transmit such a signal. Common handshake pins are DTR, DSR, DCD, and RTS/CTS.
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 above. Cables with 9 pins on one end and 25 on the other are common, and manufacturers of equipment with RJ-45 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).
Connecting two DTE devices together requires a null modem that acts as a DCE between the devices by swapping the corresponding signals (TD-RD, DTR-DSR, and RTS-CTS). This can be done with a separate device and two cables, or using a cable wired to do this. If devices require Carrier Detect, it can be simulated by connecting DSR and DCD internally in the connector, thus obtaining CD from the remote DTR signal. One feature of the Yost standard is that a null modem cable is a "rollover cable" that just reverses pins 1 through 8 on one end to 8 through 1 on the other end.
For configuring and diagnosing problems with RS-232 cables, a "breakout box" may be used. This device normally has a female and male RS-232 connector and is meant to attach in-line; it then has lights for each pin and provisions for interconnecting pins in different configurations.
RS-232 cables may be built with connectors commonly available at electronics stores. The cables may be between 3 and 25 conductors; typically 4 or 6 wires are used. Flat RJ (phone-style) cables may be used with special RJ-RS232 connectors, which are the easiest to configure. Cables are often unshielded, although shielding cables will help reduce electrical noise radiated by the cable.
The reason that a minimal two-way interface can be created with only 3 wires is that all the RS-232 signals share a common ground return. The use of unbalanced circuits makes RS-232 susceptible to problems due to ground potential shifts between the two devices. RS-232 also has relatively poor control of signal rise and fall times, leading to potential crosstalk problems. RS-232 was recommended for short connections (15 meters or less), however the limit is actually defined by total capacitance and low capacitance cables allow reliable communications over longer distances exceeding 50 m. RS-232 interface cables are not usually constructed with twisted pair because of the unbalanced circuits.
While the control lines of the RS 232 interface were originally intended for call setup and takedown, other "handshakes" may be required by one or the other device. These are used for flow control, for example, to prevent loss of data sent to a serial printer. For example, DTR is commonly used to indicate "device ready". Pins may also be "jumpered" or routed back within the connector; a pin saying "are you ready?" from device A might be wired to the pin saying "I'm ready" on device A, if device B did not transmit such a signal. Common handshake pins are DTR, DSR, DCD, and RTS/CTS.
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