Multi-Tech Systems MT2834BA Speed Conversion Commands, Speed Conversion $BA, Modem Baud Rate $MB

Chapter 5 - Command Mode

5.4.11Speed 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 modem to communicate at one speed over the phone line, and at another speed at the RS-232C interface. The speed (also referred to as “data rate” or “baud rate”) can be fixed at the RS-232 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 MultiModem 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 the baud rate that the modem is communicating over the phone line at. 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 RS-232C port at a fixed baud rate, which can be pre-selected at 300, 1200, 2400, 4800, 9600, 19,200, 38,400, 57,600 bps 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 RS-232C serial port, even if the modem does adjust its data rate. To turn Speed Conversion ON, enter the command AT$BA0 and hit RETURN.

To turn Speed Conversion OFF, enter AT$BA1 and hit RETURN. The modem will now match its RS- 232C 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 RS-232C serial port.

Modem Baud Rate $MB

The $MB command presets the modem’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 modem receives (answers) a call from another modem, it automatically switches its phone line transmission speed to match the calling modem. However, if the modem originates a call to another modem that is unable to connect at the same baud rate, it automatically drops to a lower baud rate in an attempt to match the speed of that modem. For example, if the modem is set for 14,400 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:

AT$MB75

AT$MB300 AT$MB1200 AT$MB2400 AT$MB4800 AT$MB9600 AT$MB14400 AT$MB19200 AT$MB28800 AT$MB33600

=CCITT V.23

=300 bps

=1200 bps

=2400 bps

=4800 bps

=9600 bps

=14400 bps

=19200 bps

=28800 bps

=33600 bps

The factory default is 33600 bps.

Multi-Tech Systems, Inc. MT2834BA/BL Series User Guide 53