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Chapter 4 - Command Mode
Serial Port Baud Rate $SB
The
$SB
command presets the speed of the modem’s serial (RS-232C) port, in both Originate and
Answer modes. Speed conversion allows you to set this serial port baud rate at a fixed speed of up to
115,200 bps for 1932 and 2834 models, and 57600 bps for 1432 models, regardless of the modem’s
transmission speed setting.
In order for this command to be effective, the modem’s Speed Conversion feature must first be
turned off with the

$BA

command. When Baud Adjust is on, the modem automatically adjusts its
serial port baud rate to match the speed of the computer or terminal it is connected to, as soon as it
receives its first AT command. However, in many applications, such as automatic answer, the
modem may not receive AT commands, in which case it is very useful to be able to preset the serial
port baud rate with this
$SB
command.
In addition to setting the modem’s serial port speed, this command also sets the speed at which the
modem issues Command mode responses.
The modem will accept AT commands at any speed, regardless of the speed preset by the
$SB
command. If the modem receives such a command at a speed that is different than the preset speed,
the modem switches its serial port baud rate to match the new AT command speed, although the
baud rate value stored by the
$SB
command remains the same. This provides you with a convenient
way to switch the serial port speed, and still make it easy to go back to the original speed
automatically the next time the modem is powered up or reset with an

ATZ

command.
The command to set the Serial Port Baud Rate is

AT$SBn

, where n can be:
AT$SB300 = 300 bps
AT$SB1200 = 1200 bps
AT$SB2400 = 2400 bps
AT$SB4800 = 4800 bps
AT$SB9600 = 9600 bps
AT$SB19200 = 19,200 bps
AT$SB38400 = 38,400 bps
AT$SB57600 = 57,600 bps
AT$SB115200 = 115,200 bps
The factory default is for 2834 is 57,600 bps.
Note: Some serial ports, particularly those in older PC-compatible computers, may limit the
performance of a higher speed modem like the MultiModem (see Chapter 3). The limiting factor is a
circuit called a Universal Asynchronous Receiver/Transmitter, or UART. All data from your modem
flows through it. 8250, 16450, and 16550 are UARTs typically used in PC-compatible computers. The
8250 is unreliable above 9,600 bps and the 16450 is unreliable above 19,200 bps. The 16550 UART,
however, is reliable to at least 115,200 bps. With V.42bis data compression enabled, the MT2834BA
can achieve throughputs approaching 115.2K (depending on line quality and file content). If you
presently do not have a 16550 UART in your PC, we recommend that it be replaced with a Multi-Tech
high speed Intelligent Serial Interface (ISI) card. It comes in one, two and eight-port versions for
DOS/Windows and UNIX systems, and includes a 32 K buffer that eliminates data loss even at high
speeds. See your dealer for details.