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Data Terminal Ready (DTR) is a control signal in RS-232 serial communications, transmitted from data terminal equipment (DTE), such as a computer, to data communications equipment (DCE), for example a modem, to indicate that the terminal is ready for communications and the modem may initiate a communications channel. The DTR signal is present on pin 20 of the 22-wire RS-232 interface using a DB-25 connector, and on pin 4 of a newer DE-9 serial port. The signal is asserted (logic "1") by raising the voltage of the pin from negative to positive. Dropping the signal back to its negative state indicates to the modem that the communications session shall be terminated. ==Signaling for modems== The DTR signal is an important call control signal for a data modem. According to the RS232 standard, dropping DTR from active to inactive for at least two seconds tells the modem to disconnect (end) a call or data connection. When a modem is being used for automatic answering (such as with the command ATS0=1), the DTR signal confirms to the modem that the computer is available to accept a call. When a computer wants to place a call, it asserts the DTR signal before sending commands. If the DTR signal is not asserted and the modem receives a ''dial'' command, modems either refuse to place the call, or they silently disable DTR support for the duration of that call; the actual behavior depends on the modem software. Other aspects of responses to changes in DTR can be manually overridden or configured on most newer modems. and higher values are used by some vendors. Many external modems have LED indicators on the front, one of which is TR (''"terminal ready"''). This light follows the state of the DTR pin. The light is on when DTR is high, and off when it is low. Modems will typically keep the TR light illuminated when the AT&D0 command is used to force the modem to ignore the DTR signal, regardless of the pin's actual state. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Data Terminal Ready」の詳細全文を読む スポンサード リンク
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