RS-232, Recommended Standard 232, DTE, DCE, RS-232 basics, Data communication and networking, What is RS-232

RS-232 (Recommended Standard 232)

It 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.
It can be defined as following:
Electrical signal characteristics such as voltage levels, signaling rate, timing and slew-rate of signals, voltage withstand level, short-circuit behavior, maximum stray capacitance and cable length.
Interface mechanical characteristics, pluggable connectors and pin identification.

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.

Glossary of Abbreviations.