User's Manual

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 = 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.

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.