Peripheral conflicts – System locks up when parallel-port tape backup is used after installing a sound card What is the most likely cause of the freeze?

Introduction / Context:
Legacy PCs often rely on fixed or limited interrupt (IRQ) and DMA resources. Adding a new sound card can inadvertently overlap with resources used by a parallel-port peripheral (such as a tape backup), leading to hangs or nonresponsive devices.

Given Data / Assumptions:

• System worked before; problem appears when the parallel-port tape unit is active after installing a sound card.
• Operating system uses classic ISA/PCI resource allocation without robust automatic conflict resolution.
• LPT port typically uses IRQ7 (or IRQ5 in some configurations).

Concept / Approach:

Sound cards frequently used IRQ5/7 and DMA channels (e.g., DMA1, DMA5) for legacy compatibility. Parallel-port devices may use IRQ7 for event-driven I/O. If both attempt to use the same IRQ simultaneously, interrupt handling becomes unstable, causing lockups or device failures. Resolving the conflict requires reassigning IRQs or changing port modes.

Step-by-Step Solution:

Verification / Alternative check:

Disable the sound card temporarily; if the tape unit works normally, re-enable sound with a different IRQ/DMA configuration. Event logs or diagnostic tools may confirm shared IRQs in conflict-prone configurations.

Why Other Options Are Wrong:

DMA issues can occur, but parallel-port tape hangs after sound installation most commonly reflect an IRQ clash. Declaring either device defective ignores the timing correlation and classic resource overlap symptoms. Driver faults are possible but less likely than a straightforward interrupt conflict in this scenario.

Common Pitfalls:

Forgetting ECP mode uses a DMA channel; assuming PCI devices can safely share IRQs like modern systems—older OSes and drivers were less tolerant; leaving BIOS on “PnP OS = No” or vice versa causing static allocations that collide.

Final Answer:

IRQ conflict between the sound card and the parallel port/tape device.