Setting SCSI IDs with jumpers: SCSI IDs start at 0. What jumper pattern selects the fourth SCSI ID on a 3-bit ID block (weights 4-2-1)?

Introduction / Context:
SCSI devices use binary ID selection. Understanding how to set jumpers avoids bus conflicts and ensures proper device ordering on the chain.

Given Data / Assumptions:

• SCSI IDs are zero-based: 0, 1, 2, 3, ...
• Three jumpers represent binary weights 4, 2, and 1 (from MSB to LSB).
• We want the fourth ID counting from 0.

Concept / Approach:

Counting from 0: the fourth ID is 3 in decimal (0,1,2,3). Convert 3 to binary using weights 4-2-1: 3 = 2 + 1 = 011. Therefore, enable the 2 and 1 jumpers, leave the 4 jumper off.

Step-by-Step Solution:

Verification / Alternative check:

Most SCSI devices print a truth table on the chassis. Selecting ID 3 aligns with pattern 011 on 3-bit selectors.

Why Other Options Are Wrong:

• 110: equals 6 (4 + 2), not 3.
• 010: equals 2 only.
• 101: equals 5 (4 + 1).
• 001: equals 1.

Common Pitfalls:

Misreading bit order (left-to-right vs. right-to-left), forgetting the host adapter also consumes one ID, and neglecting termination rules.

Final Answer:

011