The best way to differentiate one type of modem from another is through the docu-
mentation that comes with the system or to visit the manufacturer’s Web site.Internal versus External Modems
Aninternalmodemisinstalledlikeanyotherexpansioncardintoacompatibleexpan-
sion slot. Most of the internal modems currently sold do not require much physical
configuration, but there are some that still require DIP switches or jumpers to be set to
select the transmission speed and to designate the COM port to be used. It is common for
most of the configuration of an internal modem to be done through the operating system.
For example, Windows has the Modem Wizard that can be used to install a non-Plug and
Play (PnP) modem. PnP modems are typically installed and configured automatically by
the operating system and BIOS.
An external modem is attached to the PC through one of its COM ports. About the
only configuration issue involved with installing and configuring an external modem is
possible system resource conflicts, especially conflicts with the IRQ (interrupt request).
External modems are connected to the PC through a cable called a null modem cable.
Manyexternalmodemkitsincludethiscable;ifitdoesn’t,allcomputerstoressellthem.AT Commands
Virtually all PC-compatible modems are also Hayes Standard AT command set–compatible.
This command set provides you with the ability to control the functions and settings of
the modem directly through a modem interface or from a scripted set of commands.
AT does not mean Advanced Technology, as it would with a motherboard or power
supply. On a modem, AT refers to “attention,” which is used to precede each command
given the modem from the AT command set. Table 20-4 lists some of the more commonly
used AT commands. Remember that each command is preceded with AT.(^530) PC Hardware: A Beginner’s Guide
Command Action
A0 Answer incoming call
A/ Repeat last command
DT XXX-XXX Dial the telephone number using touch-tone dialing
H On hook (hang up)
L Speaker loudness (volume)
M Mute (speaker off)
Z Reset the modem to default settings
&X Advanced configuration commands, where X is a
command letter
Table 20-4. Sample Commands in the Modem AT Command Set