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CHAPTER 5: Command Mode
computer must be set at a fixed baud rate, regardless of whether the
modem is communicating over the phone line at 300, 1200, 2400, 9600,
14,400,19,200, 24,000, 28,800 or 33,600 bps. In this application, speed
conversion allows the modem to match its speed to that of the calling
modem, while at the same time communicating with the attached
computer through its RS232C port at a fixed baud rate, which can be
pre-selected at 300, 1200, 2400, 4800, 9600, 19,200, 38,400, 57,600 or
115,200 bps.
Speed Conversion $BA
When using speed conversion, you must set the modem so that it does
not adjust its speed at the RS232C serial port, even if the modem does
adjust its data rate. To turn Speed Conversion ON, enter the command
AT$BA0&W0 and hit RETURN.
To turn Speed Conversion OFF, enter AT$BA1&W0 and hit RETURN.
The modem will now match its RS232C speed to that of the computer,
and will adjust its speed to any changes in the computer’s speed in
Originate mode, or to the speed of the originating modem in Answer
mode. The speed at which the modem communicates over the phone
line will always be the same as the speed at which it communicates via its
RS232C serial port.
Modem Baud Rate $MB
The $MB command presets the Series II Modem’s transmission baud
rate for originate operations, (i.e., the speed of the modem’s transmis-
sions over the telephone lines when originating a call). With speed con-
version, this transmission speed can be a different baud rate than the
serial port speed.
When the Series II Modem receives (answers) a call from another
modem, it automatically switches its phone line transmission speed to
match the calling modem. However, if the Modem originates a call to
another modem that is unable to connect at the baud rate of the
Modem, it automatically drops to a lower baud rate in an attempt to
match the speed of that modem. For example, if the Modem is set for
19,200 baud and calls another modem that has a top speed of
2400 baud, it drops to 2400 baud.