RS-232 Signalling

The RS-232 standard defines 25 signal lines in its interface, although in practice PCs rarely use more than nine of these lines. In fact, with just three of these lines—receive data (RD), transmit data (TD), and ground (GND)—bi-directional RS-232 communication can occur.

The other lines are designated for a variety of control purposes. These include the remainder of the basic nine lines: data carrier detect (DCD), data terminal ready (DTR), data set ready (DSR), request to send (RTS), clear to send (CTS), and ring indicator (RI). These main nine serial signals are what are typically used between a PC and a serial device such as a modem.

RS-232 SIGNAL DESCRIPTIONS

DTR: Data Terminal Ready
Used by a piece of Data Terminal Equipment to signal that it is available for communication.
DSR: Data Set Ready
The companion signal to DTR, it is used by a piece of Data Circuit-Terminating Equipment to signal that it is available for communication.
CTS: Clear to Send
Used by a piece of Data Circuit-Terminating Equipment to signal it is available to send data, and also used in response to an RTS request for data.
RTS: Request to Send
Used by a piece of Data Terminal Equipment to indicate that it has data to send.
DCD: Data Carrier Detect
Used by a piece of Data Circuit-Terminating Equipment to indicate to the Data Terminal Equipment that it has received a carrier signal from the modem and that real data is being transmitted. Sometimes abbreviated as CD.
RI: Ring Indicator
Used by a Data Circuit-Terminating Equipment modem to tell the Data Terminal Equipment that the phone is ringing and that data will be forthcoming.
TD: Transmit Data
This wire is used for sending data. Sometimes abbreviated as TXD. This wire will also carry flow control information if software flow control is enabled.
RD: Receive Data
This wire is used for receiving data. Sometimes abbreviated as RXD. This wire will also carry flow control information if software flow control is enabled.
GND: Ground
This pin is the same for Data Circuit-Terminating Equipment and Data Terminal Equipment, and it provides the return path for both data and handshake signals.

If you're thinking I glossed over a lot earlier when I said "The RS-232 standard defines 25 signal lines in its interface, although in practice PCs rarely use more than nine of these lines," you're right. The full complement of RS-232 signalling lines are used for synchronous RS-232 — a version of the protocol in which the two sides of the serial link are coordinated by a common clock — and to this point we have been focusing on asynchonous RS-232, which is by far the most common. We'll get along to synchonous RS-232 eventually, but for now we will stick with the basics.

The next article will look at the electrical characteristics of the signals that are carried on these wires, and I think things will start developing onto a complete picture as we do.