Sierra Wireless DART 200 CDPD Modem - Manual (Page 114)

DART 200 CDPD Modem User’s Guide 8 Application Programming
Sierra Wireless, Inc. 8-20
Escape sequence considerations
The escape sequence is one of two ways the MAS forces the DART 200
to leave online or SLIP mode and return to command mode. The other is
to drop the DTR line, as described in Escaping through the control
interface, p. 8-16.
Leaving online or SLIP mode is required for the DART 200 to respond
to AT commands. Commonly, this occurs at the completion of a session
to terminate the connection (hang up) with the ATHn command.
Another common reason is to interrogate modem status for local radio
resource conditions, or for error recovery operations.
The DART 200 escape sequence consists of three escape characters (E)
and a 1-second guard time (G). Unlike standard Hayes-compatible
modems that use a GEEEG escape sequence, the DART 200 uses an
EGEGE sequence. Whether or not the escape characters are passed onto
the network as data (&E1) or filtered from the output data stream (&E2)
can be specified.
Escape code recognition controls whether or not the DART 200 responds
to the escape code. The default is &E1 (recognition enabled and the
escape sequence is passed to the network). For manual operation this
feature must be enabled (&E1 or &E2). For application use it can be
disabled (&E0) if the MAS can control the DTR lead in the RS-232
interface. However; even in this case, the &E0 option is not
recommended as the inability to escape manually it can limit debugging
capability.
The escape character (+) is in S-Register 2 and the guard time (50 in
units of 1/50 second) is in S-Register 12: both can be changed at set up
time or under application control. There is no obvious reason to change
the escape character, but it can be desirable in interactive applications to
reduce the guard time from its 1-second default value to improve
application responsiveness.
Command response options
There are three setup options controlling if, and how result codes are
presented to the MAS:
• Quiet Mode (Qn) - Controls whether or not result codes are returned
to the MAS. The default value is enabled (Q0). There are few
situations where an application can operate without seeing return
codes. However, some non-intelligent devices can be attached to the
DART 200, that do not expect anything but poll characters; in that
situation Quiet Mode enabled (Q1) is appropriate. Use the default
(Q0) except in these special situations
? ?
NOTE:
Terse mode only applies
to command responses
(such as AT). Register
or modem status
inquiries (such as
ATS57?) will still
provide a response
which is bracketed by
both carriage return
<CR> and linefeed
<LF> characters.
CAUTION:
Do not reduce the guard
time to zero, because this
exposes the application to
an unwanted escape if
three successive escape
characters inadvertently
appear in the data
stream. Leave the guard
time as larger than the
expected inter-character
time in the MAS to
modem data transfer.
Each unit of guard time
is approximately 20
milliseconds.