Multitech MMH900 User Manual page 181

Appendix J - Internal MMH2834 Composite Link Modem Commands
Serial Port Baud The $SB command presets the speed of the MMH2834's serial
Rate (RS232C) port, in both Originate and Answer modes. Speed
conversion allows you to set this serial port baud rate at a fixed
$SB 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$SB300
AT$SB1200
AT$SB2400
AT$SB4800
AT$SB9600
AT$SB19200
AT$SB38400
AT$SB57600
AT$SB115200
The factory default is 115,200 bps.
*Some serial ports 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
I
speed Intelligent Serial nterface (ISI) card. It comes in 1, 2 and 8-port versions for Windows and UNIX systems, and
includes a 32K buffer that eliminates data loss even at high speeds. See your dealer for details.
= 300 bps
= 1200 bps
= 2400 bps
= 4800 bps
= 9600 bps
= 19,200 bps
= 38,400 bps
= 57,600 bps
= 115,200 bps
181