                     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 like  an
answering  machine  or modem waiting for a call) will answer if  it  rings.
However if one party (device like a mouse or scanner) has the phone off the
hook  (a device driver loaded), the other party (device modem)  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  which has a device driver, 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. Some VIDEO cards use the address  of
COM4  leaving COM3 for use. 386/486 computers use IRQ2 to  access  extended
IRQs (above 7, sort of like call-forwarding). So you usually are left  with
one alternate IRQ5 on those computers.

     