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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.