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NTEGRATED
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IRCUITS
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IVISION
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Specification: DS-CPC2400E-R03
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17
CPC2400E
An escape sequence is one or more particular
characters sent from the terminal to the modem during
the On-Line state. This is done to switch the modem
to the command state so modem commands may
be entered during a telephone line connection. The
Escape Sequence characters are typically sent to the
other modems as data.
An escape sequence should not occur accidentally
during an exchange of data between two modems.
Unfortunately, it is impossible to guarantee any escape
sequence will never occur naturally since there are no
restrictions on the data or timing between characters
sent between two modems during the On-Line state.
The only method of switching form the On-Line state
to the Command state that does not ever occur
naturally during an exchange of data is the Data
Terminal Ready signal (the &D1 command). The
terminal has complete control of this signal and it is
not part of the data exchanged between the modems.
The Hayes escape sequence was adopted by many
modem manufacturers and communication programs
before Hayes was granted a patent for the escape
sequence guard times. Now the unpatented Time
Independent Escape Sequence (TIES) has gained
popularity with many modem manufacturers.
Then:
a. If one of the recognized AT commands is received
before the timer expires, the timer is stopped, the
command is executed and its result code is sent to
the terminal.
b. If any other data is received while the timer is
running, the timer is stopped, the modem returns to
the On-Line state and the received data is sent to the
other modem.
c. If the timer expires, a confirming result code is
sent to the terminal, indicating the modem is in
the Command state. The escape character can be
changed through S-Register S2. The prompt delay
timer can be changed (in 1/50 second increments)
through S-Register S12 (see S-Registers section).
Escape Sequences
TIES Escape Sequence
TIES stands for Time Independent Escape Sequence.
TIES was developed by a number of modem
manufacturers in response to Hayes enforcing patent
rights for their escape sequence guard time patent.
The Time Independent Escape Sequence is a
sequence of 3 escape characters (+ characters by
default). Once these characters have been recognized,
the modem enters the Command state without
sending a confirming result code to the terminal and
the modem starts a prompt delay timer.
Carrier Detection
After handshaking, the modem determines if a
telephone line connection exists by detecting the
carrier signal from the other modem. If the carrier is
not detected for a long enough period of time, the
modem assumes the telephone line connection with
the other modem has been broken. The modem uses
S-register S9 to determine how long a carrier must
be present before it is “detected.” The modem uses
S-register S10 to determine how long a carrier may
not be detected before the telephone line is hung up.
