Beyond the electrical specifications, RS-232 includes functional definitions of 24 different signals; however, common usage uses 8 or less of the defined signals. This document covers the eight most commonly-used signals in RS-232. The signals can be broken up into four different categories: Common, Data, Timing, and Control signals; however, most signals fall into the Control category. Timing Signals are rather rare in RS-232 interfaces and have limited use and are not used in standard 8, or less, signal RS-232 interfaces which TI transceivers primarily support. The RS-232 standard defines a Data Terminal Equipment (DTE) which acts as the host and controller and a Data Communication Equipment (DCE) which acts as the peripheral to the host and controller.

The Common category is just the name for the ground wire between two RS-232 devices and has no purpose beyond providing a ground reference.

The Data signals are the lines that receive and transmit data – two data signals are common in RS-232. The TD signal refers to the Transmitted Data signal and the RD signal refers the received data line. The TD signal line is defined as the transmitted data by the DTE to the DCE, where the RD line is defined as the received data from the DCE to the DTE. Even though both nodes are transmitting and receiving data, the TD and RD lines are specific to the host node in the system not the peripheral.

The Control signals are used to control flow of data between the two nodes in an RS-232 system. This gives a higher layer of control beyond what can be done in the physical layer alone and allows for a more directed focus on implementing the firmware for RS-232 systems. There are 2 to 6 common control signals that are used for handshaking between two nodes before data transmission occurs. In applications that require some handshaking, a common use case is to use the two data signals + two control signals. These control signals are Ready to Send (RTS) and Clear to Send (CTS). The RTS signal is asserted when the DTE has data that is ready to transmit to the DCE through the TD signal; while the CTS signal is asserted after the RTS signal has been received and the peripheral is letting the host know it is ready to receive data. However, this simple version of handshaking is not the only common handshaking signal used when using RS-232 and systems such as modems. Modems typically add an additional 4 control signals on top of RTS and CTS. Along with RTS and CTS, common control signals also include Data Terminal Ready (DTR), Data Set Ready (DSR), Data Carrier Detect (DCD), and Ring Indicator (RI). The DTR signal is asserted by the DTE to the DCE that it is ready to transmit or receive data while the DSR signal is asserted by the DCE to the DTE after the DTR signal to let the DTE know that it is connected to the communications line. The DCD signal is a signal from the DCE to DTE to show that there is a valid connection between the DTE and the DCE. Finally, the RI signal is a signal from the DCE sent to the DTE to show that there is a ringing on the communication line – or more simply that the DCE wants to communicate with the DTE itself – by which the DTE responds by asserting the DTR signal.