What is RS-232
RS-232 is a standardized protocol used for serial data communication between a Data terminal Equipment (DTE) and a Data Communication Equipment (DCE). Data Terminal Equipments are the ones responsible for transmission and reception of data such as a Network Terminal or a Personal Computer while Data Communication Equipments are the ones responsible for transferring the data such as a Modem.
RS-232 connector comes in the form of a 25 pin connector used to connect the PC or a terminal to a modem. This connector is known as a DB-25 connector. Not all the 25 pins are used in most of these connections and IBM later developed a DB-9 connector to provide a serial interface between a PC and a Modem.
A process involving a series of handshake signals via different pins is used to establish a serial communication link between a DTE and a DCE in this protocol. This process is elaborated below:-
When the DTE is switched on, it goes in to a self testing mode and after a successful completion of it; it sends a Data terminal ready (DTR) signal to the DCE. This signal is used to indicate to the DCE that the data terminal is ready for serial communication. It is an active low signal. Similarly when the DCE is turned on, it sends a Data Set Ready (DSR) signal to the DTE to indicate its readiness for the serial communication. It is also an active low signal. Now when the Data terminal Equipment has a byte of data to send, it sends a Ready to Send (RTS) signal to the modem. It is also an active low signal. On receiving the RTS signal, if the modem is available for receiving the data, it indicates that to the DTE by sending a Clear to Send (CTS) signal to the DTE. Another signal known as Data Carrier Detect (DCD) is sent from the DCE to the DTE in order to indicate that a valid carrier is available for transmission and the modem is connected to the telephone line. Once this handshaking is established, transmission of the data byte takes place. This protocol uses a pair of pins, one for the transmission of data to DCE and the second for receiving data from DCE.
Another signal known as a Ring Indicator (RI) signal is sent from the modem to the DTE in case of an incoming telephone call. This signal is used only in case when the DTE is in charge of handling telephone calls and not the modem.