However, some types of serial port devices support only one-way or half-duplex communications. For these devices, the TD, RD, and GND pins are used. Therefore, separate pins are used for transmitting and receiving data. Most serial port devices support full-duplex communication meaning that they can send and receive data at the same time. The "on" and "off" states for a data signal and for a control signal are shown below. Conversely, to bring the signal to the "off" state, the controlling device asserts the value for data pins and unasserts the value for control pins.
To bring the signal to the "on" state, the controlling device unasserts (or lowers) the value for data pins and asserts (or raises) the value for control pins. The voltage between -3 volts and +3 volts is considered a transition region, and the signal state is undefined. For control signals, the "on" state occurs when the received signal voltage is more positive than 3 volts, while the "off" state occurs for voltages more negative than -3 volts. An inactive signal state is often described as logic 0, off, false, or a space.įor data signals, the "on" state occurs when the received signal voltage is more negative than -3 volts, while the "off" state occurs for voltages more positive than 3 volts. An active signal state is often described as logic 1, on, true, or a mark. An active state corresponds to the binary value 1, while an inactive state corresponds to the binary value 0. Signals can be in either an active state or an inactive state. For example, data is transmitted from the TD pin of the DTE to the RD pin of the DCE. The serial port pin and signal assignments are with respect to the DTE. The term "data set" is synonymous with "modem" or "device," while the term "data terminal" is synonymous with "computer." Table 8-1: Serial Port Pin and Signal Assignments Refer to the RS-232 standard for a description of the signals and pin assignments used for a 25-pin connector. The pins and signals associated with the 9-pin connector are described below. The pin assignment scheme for a 9-pin male connector on a DTE is given below. In fact, only three pins are required for serial port communications: one for receiving data, one for transmitting data, and one for the signal ground. However, most PC's and UNIX platforms use a 9-pin connection. To support these signal types, as well as the signal ground, the RS-232 standard defines a 25-pin connection. Serial ports consist of two signal types: data signals and control signals. Serial Port I/O (External Interfaces/API) External Interfaces/API
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