Practical Contemporary Use
Today, most modems are configured with the characters "&C1&D2" in the initialization string, or otherwise behave this way by default. This causes the following behavior:
- The modem uses the Data Carrier Detect (DCD) pin to signal whether it's connected to a host. The computer can generally just read the DCD pin at any time and always know if the modem is in command or data mode. DCD high means data mode, and low means command mode. DCD is a signal sent from the modem to the computer. (Note that DCD also remains high if in command mode after a +++ escape sequence, but in practice, software uses this so rarely, if at all, so it's not really an issue.)
- The modem interprets the Data Terminal Ready (DTR) pin as a signal from the computer to know when it wants to terminate a call. DTR is a signal from the computer to the modem. The computer keeps DTR high at all times until it wants to terminate a call, at which time the computer lowers DTR for a second or two. The computer also keeps DTR low when no programs are running that want to use the modem - this keeps the modem from answering calls due to auto-answer or otherwise doing something unexpected or undesired. The computer may safely assume that after DTR has been brought low for a couple seconds, that the modem will be in command mode.
Read more about this topic: Command And Data Modes (modem)
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