PC interrupts — Which IRQ is traditionally assigned to COM2 (serial port 2) on IBM-PC compatibles?

Introduction / Context:
Legacy serial ports require both an I/O base address and an IRQ line for interrupt-driven communication. Understanding default mappings is essential when diagnosing resource conflicts or configuring legacy hardware.

Given Data / Assumptions:

• Default, historical PC assignments.
• No BIOS remapping or Plug-and-Play overrides.
• COM2 is the target port.

Concept / Approach:

Standard mappings pair COM1 with IRQ4 and COM2 with IRQ3 to avoid conflicts when both ports are present. COM3/COM4 often share or remap IRQs depending on the multi-I/O card or BIOS settings.

Step-by-Step Solution:

Verification / Alternative check:

Old technical references, BIOS setup pages, and OS device settings confirm these defaults. RS-232 troubleshooting guides also cite IRQ3 for COM2.

Why Other Options Are Wrong:

IRQ4 is for COM1, IRQ1 is for the keyboard controller, IRQ2 was historically a cascade to the second PIC and not used for COM2 by default. “None of the above” is not valid here.

Common Pitfalls:

Assuming PCI/USB serial devices use the same legacy IRQs; confusing COM numbers with physical back-panel labels.

Final Answer:

IRQ3.