Z02215
Single Chip Modem with Integrated Controller, Data Pump, and AFE
46
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 modem as data.
An escape sequence must not occur accidentally during an exchange of data
between two modems. Unfortunately, it is impossible to guarantee that any
escape sequence never occurs accidentally, because 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 from the ON-LINE state to the COMMAND state that
never occurs accidentally during an exchange of data is the Data Terminal Ready
signal (the &D1command). The terminal has complete control of this signal, and it
is not part of the data exchanged between the modems.
TIES Escape Sequence
Time Independent Escape Sequence (TIES) is a sequence of three escape char-
acters (+ characters by default). When these characters are recognized, the
modem enters the COMMAND state without sending a confirming result code to
the terminal. The modem then starts a prompt delay timer. During the prompt
delay, the following situations may occur:
•
•
•
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.
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.
If the timer expires, a confirming result code is sent to the terminal, indicating
that the modem is in the COMMAND state.
The escape character and prompt delay timer can be changed by writing new val-
ues to S-registers S2 and S12.
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 specified period of time, the modem presumes the telephone line connection
with the other modem has been broken. The modem uses S-register S9 to deter-
mine 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 tele-
phone line is disconnected.
PS001907-0904
Carrier Detection
ZILOG [ ZILOG, INC. ]
