$MBn
Chapter 3 - AT Commands
3.11 Speed Conversion Commands
Speed conversion allows the MT2834MR6 to communicate at a fixed speed through the serial port while communicating at an independent speed over the online communications link. Speed conversion is necessary for data compression, since data compression requires data to be sent from the computer to the modem at a rate up to four times faster than the modem’s transmission speed.
Speed conversion is also useful for an autoanswer modem connected to a computer that does not have autobaud capability, since the computer must be set at a fixed baud rate regardless of the speed at which the modem communicates over the phone line.
$BAn Baud Adjust n = 0 or 1
Default: $BA0
Turning baud adjust off turns speed conversion on; turning baud adjust on turns speed conversion off.
When baud adjust is on, the modem matches its RS-232 serial port speed to any changes in the computer’s serial port speed in originate mode, or to the speed of the originating modem when it’s in answer mode. With baud adjust on, the speed at which the modem communicates over the phone line is always the same as the speed at which it communicates via its RS-232 serial port.
When speed conversion is on, the modem does not adjust its speed at the serial port to match its transmission speed. Serial port speed and transmission speed can therefore be set independently. Speed conversion is necessary for a high speed modem that is connected to a limited speed serial port.
$BA0 turns baud adjust off and speed conversion on.
$BA1 turns baud adjust on and speed conversion off.
Modem Baud Rate n = speed in bits per second
Default: $MB33600
The $MB command sets a modem’s initial transmission speed for originate mode. In V.32
mode (9600 bps), it also sets the transmission speed for answer mode. With speed conversion on, this transmission speed can be different from the serial port speed.
When the modem answers a call from another modem, it automatically switches its transmission speed to match the calling modem. However, if the MT2834MR6 originates a call to another modem that is unable to connect at the same transmission speed, the
MT2834MR6 automatically drops to a lower speed in an attempt to match the speed of the other modem.
Speed conversion must be on for this command to work. If baud adjust is on ($BA1), speed conversion is off and a modem baud rate command will be ignored. The S48 register also can affect maximum transmission speed. See Chapter 4.
$MB75
$MB1200
$MB4800
$MB9600
$MB14400
$MB19200
$MB33600
ITU-T V.23 mode
1200 bps
4800 bps
9600 bps
14,400 bps
19,200 bps
33,600 bps (default)
$MB300
$MB2400
$MB7200
$MB12000
$MB16800
$MB28800
300 bps
2400 bps
7200 bps
12,000 bps
16,800 bps
28,800 bps
45
$SBn
Chapter 3 - AT Commands
Serial Port Baud Rate n = speed
Default: $SB57600
The $SB command sets the speed of the MT2834MR6’s serial 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.
For this command to work, the modem’s baud adjust feature must first be turned off with the
$BA0 command. When baud adjust is on, as soon as the modem receives its first AT command, it automatically adjusts its serial port baud rate to match the speed of the computer or terminal to which it is connected. However, in applications such as automatic answer, the modem may not receive AT commands. In this case it is helpful to be able to preset the serial port baud rate with the $SB command.
In addition to setting the modem’s serial port speed, this command sets the speed at which the modem issues command mode responses (result codes).
The MT2834MR6 accepts AT commands at any speed, regardless of the speed preset by the $SB command. If the modem receives a command at a speed different from the preset speed, the modem switches its serial port baud rate to match the new command speed, even though 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 while still making it easy to automatically return to the original speed the next time the modem is powered up or reset.
$SB300
$SB2400
$SB9600
$SB38400
$SB115200
300 bps
2400 bps
9600 bps
38,400 bps
115,200 bps ( default)
$SB1200
$SB4800
$SB19200
$SB57600
1200 bps
4800 bps
19,200 bps
57,600 bps
46
