Z02205
Modem Controller
ZiLOG
22 PS001000-MOD0599
ESCAPE SEQUENCES
An escape sequence is defined as one or more particular
characters sent from the terminal to the modem during the
On-Line state. This sequence is activated to switch the mo-
dem to the command state so modem commands may be en-
tered during a telephone line connection. The Escape Se-
quence characters are typically sent to the other modem as
data.
An escape sequence should not occur accidentally during
an exchange of data between two modems. Unfortunately,
it is impossible to guarantee that any escape sequence will
never occur naturally, because there are no restrictions on
the data or timing between characters sent between two mo-
dems during the On-Line state.
The only method of switching from the On-Line state to the
Command state that never occurs naturally during an ex-
change 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 SE-
QUENCE
(TIES) has gained popularity with many modem
manufacturers.
TIES 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). When these
characters are recognized, the modem enters the
COM-
MAND
state without sending a confirming result code to the
terminal. The modem then starts a prompt delay timer. From
that point:
If one of the recognized AT commands is received be-
fore 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 the modem is in the Command
state.
The escape character and prompt delay timer can be
changed by writing new values 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 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 is not detected
before the telephone line is hung up.
BLACKLISTING MANAGEMENT
Blacklisting applies separately to each number dialed, and
indicates the call blocking capability for specific phone
numbers. To save Z02205 RAM, calls are treated as if they
are all to the same number.
The first connection attempt after modem reset or a suc-
cessful connection is always allowed. If a connection at-
tempt is not successful, then each following connection
attempt is checked as follows:
1. If voice answer was detected too many times the
attempt is not allowed, Modem Result Code indicates
the call is BLACKLISTED.
2. If the attempt occurred too quickly (within the inter-
call timing period) then the attempt is not allowed,
Modem Result Code indicates the call is DELAYED.
3. If there is a blacklisting period limiting the number of
attempts that may be made within the period, and too
many attempts have been made, then the attempt is not
allowed, Modem Result Code indicates the call is
either DELAYED (if a call may be made later), or
BLACKLISTED (if no call may be made later).
