Multitech MMH900, User Manual

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Appendix J - Internal MMH2834 Composite Link Modem Commands
J.1.10 Speed Conversion Commands
Speed conversion is a necessary part of data compression since data
must be presented to the modem faster than it can handle data, if data
compression is to be effective.
Speed conversion allows the MMH2834 to communicate at one speed
over the phone line, and at another speed at the RS232C interface. The
speed (also referred to as “data rate” or “baud rate”) can be fixed at the
RS232 interface independently of the baud rate of the on-line
transmissions.
In addition to data compression, another popular application for speed
conversion involves an auto-answer MMH2834 connected to a computer
that does not have autobaud capability. This means that the 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 or 28,800 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
preselected at 300, 1200, 2400, 4800, 9600, 19,200, 38,400, 57,600 or
115,200 bps.
Speed Conversion 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
$BA modem does adjust its data rate. To turn Speed Conversion ON,
enter the command AT$BA0.
To turn Speed Conversion OFF, enter AT$BA1 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 The $MB command presets the MMH2834’s transmission
baud rate for originate operations, (i.e., the speed of the modem's
$MB transmissions over the telephone lines when originating a call).
With speed conversion, this transmission speed can be a different
baud rate than the serial port speed.
When the MMH2834 receives (answers) a call from another
modem, it automatically switches its phone line transmission
speed to match the calling modem. However, if the MMH2834