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Command and Data modes refer to the two modes in which a computer modem may operate. These modes are defined in the Hayes command set, which is the de facto standard for all modems. These modes exist because there is only one channel of communication between the modem and the computer, which must carry both the computer's commands to the modem, as well as the data that the modem is enlisted to transmit to the remote party over the telephone line. When a modem is in command mode, any characters sent to it are interpreted as commands for the modem to execute, per the Hayes command set. A command is preceded by the letters 'AT', which stand for 'Attention'. For example, if a modem receives 'ATDT5551212' while in the command mode, it interprets that as an instruction to dial the numbers 5551212 on the telephone, using touch-tone dialing. While in command mode, the modem may send responses back to the computer indicating the outcome of the command. For example, the modem may respond with the word "BUSY" in response to the ATDT command, if it hears a busy signal after dialing and is configured to listen for busy signals. Any communication in command mode (in both directions) is terminated by a carriage return. When a modem is in data mode, any characters sent to the modem are intended to be transmitted to the remote party. The modem enters data mode immediately after it makes a connection. For example, if ATDT5551212 resulted in a phone call that was answered by another computer modem, the modem would report the word "CONNECT" and then switch to data mode. Any further characters received over the serial link are deemed to be from the remote party, and any characters sent are transmitted to the remote party. When a voice-capable modem is in "voice data" mode, any data sent to the modem is interpreted as audio data to be played over the phone line, rather than character bytes to be transmitted digitally to the other party. ==Switching between modes== Modems always start out in command mode when powered up. Here are the ways a modem can switch to data mode: *After a successful dial-out connection in response to an "ATD" dial command in which the modem reaches another modem. *After answering the phone with the "ATA" answer command, if another modem is on the other end. *After answering the phone automatically for some pre-configured reason (such as auto-answer), and connecting to another modem. (Almost all modems support auto-answering when given the command "ATS0=1".) *After being given the "ATO" (that's three letters A-T-Oh, not A-T-zero) command after being put back in the command mode with an escape sequence (see below). *In response to similar dialing or connecting commands for fax or voice communications. Modems switch back into command mode from data mode for the following reasons: *The connection got broken (for example, the other party hung up). *The computer issued an escape command, which is usually a 1-second pause, then the three characters "+++", then another 1-second pause. The connection remains, but the modem can accept commands, such as "ATH" for hangup. The computer can issue the "ATO" command to return to data mode. *The computer instructed the modem to terminate the call by setting the Data Terminal Ready (DTR) pin to an "off" state. (This is usually how computers invoke the termination of a modem call nowadays - the +++ escape sequence is ''rarely'' used, and usually disabled to avoid malfunction in case these characters are legitimately a part of the data stream). 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「Command and Data modes (modem)」の詳細全文を読む スポンサード リンク
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