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IRQS
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1992-12-01
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COMports, addresses & IRQs
Don Hinds
The terms serial port and COMport for all purposes, mean the same
thing. A serial port is a comport built into a computer, onto which you
plug an external device. A modem can be plugged into a serial port
(external modem). An internal modem 'makes' a comport of it's own when you
plug the modem card into the computer internal bus. The comport for which
the internal modem is set depends on jumpers on the modem.
2 devices on a computer CANNOT use the same address. 2 devices on a
computer 'may' use the same IRQ as long as one does not have a device
driver present. A mouse on COM-1 using IRQ-4 is one example, so COM-3
which also uses IRQ-4 cannot be used for anything. Comports/serial ports
1, 2, 3 & 4 each have a unique address.
Think of comports as single family houses. That means one house has
one address. If there were 2 houses (comports/serial ports) with the same
address on the same street (computer bus), then the mail (command) would
go to the wrong house (port). So each comport/serial port must have a
unique address.
Now an IRQ is something like a telephone party-line. You can have 2
parties (devices) on one phone line (IRQ). The party (device) who picks up
the phone can call out. The party (device) who is 'home' (a device waiting
for a call) will answer if it rings. However if one party (device) has the
phone off the hook (a device driver), the other party (device) cannot use
the telephone (IRQ) even if it is another house (comport).
So if you want to use a comport which normally uses the same IRQ as
one comport already in use, you must change the IRQ for that port. In the
manual it shows how jumpers may be set to give additional IRQ of 2 or 5.
This means you could have 4 unique addresses 1, 2, 3, & 4, and each could
have a unique IRQ 4, 3, 5, or 2.