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DataTalker Owner’s Manual
Serial Port Baud Rate $SB
The $SB command presets the speed of the MMH2834's serial (RS232C) 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, 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 MMH2834 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 MMH2834’s serial port speed, this command also sets the speed at
which the modem issues Command mode responses.
The MMH2834 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 300, 1200, 2400,
4800, 9600, 19,200, 38,400, 57,600 or 115200 bps as listed below:
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 115,200 bps.
*Some serial ports, particularly those in older PC-compatible computers, may limit the
performance of a higher speed modem like the MMH2834. 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
MMH2834 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.