Black Box MD1640A, MD1641A manual Immediate Action Commands, Help Screens $H

CHAPTER 5: Command Mode

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 automati- cally 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

=300 bps

=1200 bps

=2400 bps

=4800 bps

=9600 bps

=19,200 bps

=38,400 bps

=57,600 bps

=115,200 bps

The factory default is 115,200 bps.

Note: Some serial ports, particularly those in older PC-compatible computers, may limit the performance of a higher speed modem like the Series II Modem (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 Modem can achieve throughputs approaching 115.2K (depending on line quality and file content).

5.4.12 Immediate Action Commands

Help Screens $H

The Help command is designed to give you short explanations on how to use each Series II Modem command. The Help command can be quite useful if your manual is not handy and you are in the middle of a communications session. Although the explanations are quite abbreviat- ed compared to those in this manual, they should prove to be helpful reminders when needed.

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