12-16-93
                     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) and that external modem will have the address/COMport  of
the  card into which it is plugged. An External modem has no way to  change
it's address other than plugging it into another COMport. 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 because the  driver  for  the
mouse  on  COM 1 is loaded. COMports/serial ports 1, 2, 3 & 4 each  have  a
unique address, but normally share only TWO IRQs between them.

     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, scanner or network  board)  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  on  which you have a device driver, you must change the IRQ  for  that
port.  In  your  modem  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 to use with your modem  on  those
computers. Most Serial ports also have jumpers to change address/IRQs.  And
other devices (scanners ect) usually will allow IRQs like 6,7,8 (I am using
IRQ 7 w/hand scanner).

                                        Don Hinds - Zoom Tech

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