SCSI bus fundamentals: On a SCSI chain, what is the rule regarding device IDs for all attached devices?

Introduction / Context:
SCSI (Small Computer System Interface) supports multiple devices on a single bus. Proper identification and termination are essential to avoid bus conflicts and ensure reliable operation. Knowing how SCSI IDs work is critical for legacy servers, workstations, and storage arrays.

Given Data / Assumptions:

• SCSI bus may be narrow (IDs 0–7) or wide (IDs 0–15).
• Each device, including the host adapter, occupies one ID.
• We assume single SCSI bus segment with correct termination at ends.

Concept / Approach:

Device arbitration and addressing depend on unique IDs. If two devices share an ID, neither can be reliably addressed, causing bus errors. The host adapter often defaults to a preferred ID (e.g., 7 on narrow SCSI). Thus, each device must have a distinct ID on the chain.

Step-by-Step Solution:

Verification / Alternative check:

SCSI BIOS utilities and OS-level tools list devices by unique ID. Conflicts result in missing or unstable devices.

Why Other Options Are Wrong:

• Shared IDs: cause addressing conflicts.
• Common ID for all: would prevent unique selection.
• IDs irrelevant: false; SCSI depends on unique IDs.

Common Pitfalls:

Forgetting that external enclosures may have their own termination; mixing wide/narrow devices without proper converters and ID planning.

Final Answer:

Each SCSI device must have its own unique ID