Poor-quality cables can cause false signals by between data and control lines (such as ).If a given cable will not allow a data connection, especially if a is in use, a 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 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 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 for other commonly used connectors not defined by the standard.)CircuitDirectionpinNameTypical purposeAbbreviationDTEDCEDTE is ready to receive, initiate, or continue a call.DTRoutin20DCE is receiving a carrier from a remote DCE.DCDinout8Data Set ReadyDCE is ready to receive and send data.DSRinout6Ring IndicatorDCE has detected an incoming ring signal on the telephone line.RIinout22Request To SendDTE requests the DCE prepare to transmit data.RTSoutin4Ready To ReceiveDTE is ready to receive data from DCE. If in use, RTS is assumed to be always asserted.RTRoutin4Clear To SendDCE is ready to accept data from the DTE.CTSinout5Transmitted DataCarries data from DTE to DCE.TxDoutin2Received DataCarries data from DCE to DTE.RxDinout3Common GroundZero voltage reference for all of the above.GNDcommon7Protective GroundConnected to chassis ground.PGcommon1The signals are named from the standpoint of the DTE.
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 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 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 Many DCE devices have a 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. 1e1e36bf2d