Windows NT Does Not Provide IRQ Sharing

• Microsoft Windows NT operating system, version 3.1
• Microsoft Windows NT Advanced Server, version 3.1

Windows NT does not provide for interrupt request line (IRQ) sharing unless the machine's hardware supports it, such as machines that use Micro Channel Architecture (MCA). Unless it is supported by the machine, you cannot use two IRQs simultaneously, such as devices on ports COM1 and COM3, which both use IRQ 4.

The ability to share IRQs is hardware dependent. MCA machines are normally able to share IRQs by default. Although Extended Industry Standard Architecture (EISA) hardware does have IRQ sharing as part of its specification, not all EISA machines implement IRQ sharing. Also, on some implementations of EISA, IRQ sharing must be configured in order to work. An example of this is Compaq's EISA machines, which are not configured to share IRQs by default.

Industry Standard Architecture (ISA) machines running Windows NT may have two devices set up to use the same IRQ with different COM ports, but only one can have the IRQ or port open. Depending on the devices, it is possible that either one or both will not operate, or there may be intermittent problems with devices on the same IRQ.