RS-232 defines 25 signals for controlling the connection between a Data Terminal Equipment (DTE) and a Data Communication Equipment (DCE). However, in most serial interfaces, whether a 9-pin or 25-pin serial connector is used, only nine of these signals are used. These signal definitions are described below. The pin numbers used in this discussion are for an IS0 2110 DB-25 connector rather than for the 9-pin connector:
Pin 1—Ground (GND).
Pin 2—Transmit data (TD). Sends data from the DTE to the DCE (for example, from the computer to the modem).
Pin 3—Receive data (RD). Sends data from the DCE to the DTE.
The RS-232 flow control signals control the flow of data between the DTE and the DCE. Flow control ensures that the receiving device (the DCE) can forestall a data transmission from the DTE before it is ready to receive data, and can stop the flow of data if its buffers are full.
The following signals are classified as flow control signals:
Pin 4—Request to send (RTS). After carrier detect, the DTE activates this line, asserting request to send when it wishes to send data.
Pin 5—Clear to send (CTS). In response to the request to send signal, the modem asserts clear to send to let the DTE know that it will accept data.
For full duplex transmissions, in which data can be sent in both directions simultaneously, the DTE’s RTS signal is used as a CTS signal. In this scenario, when the DTE asserts RTS, it informs the communicating device that it is ready to receive data.
The following signals are classified as modem control signals:
Pin 6—Data set ready (DSR). This signal tells the DTE that the DCE unit is operational and ready to receive data.
Pin 8—Carrier detect (CD). When two modems connect, they exchange carriers. When this exchange is successfully completed, the DCE tells the DTE that the connection between the two modems is valid.
Pin 20—Data terminal ready (DTR). With this signal, the DTE tells the DCE that the terminal is ready for the modem to accept a connection. Essentially, the terminal uses DTR to control the modem.
Pin 22—Ring indicator (RI). The modem asserts RI to tell the terminal that the remote modem’s telephone is ringing.