Escaping the Modem to Command State
To escape the modem, the controller delays the escape sequence guard
time (specified by S12), then issues the escape
sequence character three times (specified by S2 -
Hayes Escape Character ); then waits for an OK result. Waiting for the
result also enforces the required guard time after the escape sequence.
Once the OK result is received, the modem enters command state. The controller
then hangs up and restores the modem.
The controller software must wait the required guard time before sending
the escape sequence characters. Your controller may have been called just
after data was transmitted and, without the delay, your characters are sent
without triggering the escape sequence recognition process. It is important
that the serial transmitter be permitted to be idle for the escape sequence
guard time, plus a few extra milliseconds to allow for error, before sending
characters.
For example, if S12=10, a delay of at least 200 milliseconds is required
before sending the escape sequence characters. After sending the escape
sequence characters, the OK result is received after another 200 millisecond
wait. This completes the escape sequence process in slightly over 400 milliseconds.
If S12=50 (factory setting) is used, 1 s must pass before the characters
can be sent, then another 1 s delay must pass prior to the OK result. This
completes the process in slightly over 2 s. Hayes recommends that S12=10
be issued to speed up this process.
A delay slightly longer than that stored in S12 allows for errors in the
system clock as well as in the modem clock. 100 ms is an adequate safety
margin.
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