Command and Data Modes (modem) - Switching Between Modes

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).