[WriteLog] WriteLog, shared IRQs and COM ports

Dr. Wolf-E. Gruening dl6jz@t-online.de
Thu, 9 Jan 2003 22:00:54 -0000

Hello guys,

I try to summarize my personal experiences with COM-ports, soundcards =
and IRQ's over the past years.=20

There are two kinds of peripheral computer devices:

1. "Legacy" devices: That are all COM- and LPT-ports, which are =
controlled by the conventional Windows-drivers and most of the =
ISA-slot-devices especially COM-port-extension-cards for the ISA-bus. =
All these "legacy" ports or devices do need a dedicated IRQ. And that =
does not depend on your operating system. And that does not depend on =
the message of the windows hardware manager that there are no resource =
conflicts, even in that case when COM1 and COM3 should use the same IRQ =
4. That will not work!

2. PCI-PnP-devices: That are most of the PCI-soundcards, =
PCI-/AGP-graphic-controllers, LAN-controllers and =
COM-port-extension-cards for the PCI-bus. Those devices have their =
special drivers that allow IRQ-sharing. You can imagine this in that way =
 that they are identified not only by their IRQ but also by their =

Attention: A lot of the PCI-soundcards came with an additional legacy =
driver over the last years. That driver has to be used only for =
DOS-mode-games. These legacy drivers are installed automatically and use =
also a dedicated IRQ. You can see that in your hardware configuration as =
"Legacy Audio Device" or so. To save this not needed IRQ (mostly #10) =
for more COM-ports you can delete or deactivate this legacy device from =
your hardware configuration.

IRQ's that are normally used for legacy devices and COM-/LPT-ports are=20

  3 ... COM 2
  4 ... COM1
  5 ... LPT2 (or 1)
  7 ... LPT1 (or 2)
10 ... ISA-device

The IRQ's

9 and

are normally used for PCI-PnP-devices.

The others between 0 and 15 are used for fixed system requirements.

When you install a new Windows on your harddisk and have plugged in all =
devices there will be no problem in most cases. The installation =
programms of Windows and the device drivers will determine all the =
hardware and allocate the IRQ's. Only when you have an =
ISA-COM-port-extension card you will have to change the IRQ's for the =
additional COM-ports manually.

To go sure for 100 per cent most of the BIOS-systems allow to allocate =
manually the several IRQ's either as a PCI-PnP or either as a legacy =
one. So You can make these settings in your BIOS before installing =

3, 4, 5, 7, 10  ... legacy
9, 11             ... PCI-PnP.

I repeat: That are my personal experiences over some years. During the =
last months I built two PC's with new motherboards with 1 ISA-slot for a =
COM-port-card with 2 additional COM-ports. So I have in every PC 4 =
COM-ports, 1 LPT-port, 1 or 2 soundcards, a LAN-controller, ... and Win =

73 + 55
Wolf, DL6JZ

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