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Appendix F - MMH2834 Modem Commands
F.11 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 $BA
When using speed conversion, you must set the modem so that it does not adjust its speed at
the RS232C serial port, even if the modem does adjust its data rate. To turn Speed Conversion
ON, enter the command AT$BA0.
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 $MB
Modem Baud Rate The $MB command presets the MMH2834’s transmission baud rate for
originate operations, (i.e., the speed of the modem's transmissions over the telephone lines when
originating a call). With speed conversion, this transmission speed can be a different baud rate
than the serial port speed.
When the MMH2834 receives (answers) a call from another modem, it automatically switches its
phone line transmission speed to match the calling modem. However, if the MMH2834 originates
a call to another modem that is unable to connect at the baud rate of the MMH2834, it
automatically drops to a lower baud rate in an attempt to match the speed of that modem. For
example, if the MMH2834 is set for 19,200 baud and calls another modem that has a top speed
of 2400 baud, it drops to 2400 baud.
The command to set the Modem Baud Rate is AT$MBn, where n can be 75 (in ITU-TSS V.23
mode) 300, 1200, 2400, 4800, 9600, 14,400,19,200 or 28,800 as listed below:
AT$MB75 = CCITT V.23
AT$MB300 = 300 bps
AT$MB1200 = 1200 bps
AT$MB2400 = 2400 bps
AT$MB4800 = 4800 bps
AT$MB9600 = 9600 bps
AT$MB14400 = 14400 bps
AT$MB19200 = 19200 bps
AT$MB24000 = 24000 bps
AT$MB28800 = 28800 bps
The factory default is 28800 bps.