|
About
COM Ports and IRQs
In
general, PCs have from one to four COM (serial) ports. Some
PCs can have more than four COM ports. In
addition, eBeam will function with a USB adapter.
COM
ports are used to connect serial devices, such as a mouse,
modem, PCMCIA sockets, PDAs, and eBeam to the PC.
Each
PC usually has one or two serial sockets on the back which
are generally associated with COM ports 1 and 2. COM ports
3 and 4 are often accessed through one of the slots on the
PC motherboard and don't have an external socket on the
back of the PC.
Devices, such as modems, may be installed "internally"
(i.e., they are not connected to an external
serial socket on the PC). They still make use of a COM port,
even though they don't require a physical external socket
connection. In these cases, the PC "redirects"
the COM port. If the internal device uses a COM port which
is normally associated with one of the external sockets
on the back of the PC, the port is taken away from the external
socket and given to the internal device. The implications
of this will be clear in a moment.
Each COM port gets the attention of the computer's processor
by means of an Interrupt Request, called an IRQ. Think of
these as "channels" by which the COM ports tell the processor
that there is data waiting. There are only 8 IRQ's available
to standard serial devices, and several are already dedicated
internally. Because of this, COM ports 1 and 3 share IRQ4,
and COM ports 2 and 4 share IRQ3.
This
usually means that if there is a serial device, such as
a mouse using COM1, and another device using COM3, such
as a modem, the two devices will experience an "IRQ conflict"
which is likely to disable both. These conflicts must be
resolved before the devices can be used successfully.
Return
to Managing COM Ports
|
