GPIB (IEEE-488) cabling in automated test A crash test is monitored from 200 feet away using a GPIB test setup. A custom cable that long is fabricated to save time, and the data becomes erratic. What most likely went wrong?

Introduction / Context:
The IEEE-488/GPIB bus has strict rules on total cable length and device loading to preserve signal integrity and timing margins. Violating these rules leads to reflections, attenuation, and timing errors that manifest as unreliable or “noisy” digital communication.

Given Data / Assumptions:

• Cable length ≈ 200 feet (≈ 61 meters).
• Standard GPIB topology without special extenders/repeaters.
• Erratic, unusable data during the test.

Concept / Approach:
GPIB specifies maximum total length and per-device length constraints. Long, home-made cables far beyond spec cause impedance mismatches and degraded rise times. The correct solution is to shorten the cable within specifications or deploy GPIB extenders/repeaters or fiber/ethernet bridges designed for long-distance operation.

Step-by-Step Solution:
Recognize that 200 ft is well beyond typical GPIB cable length limits.Excess length increases capacitance/inductance and causes reflections.Therefore, a GPIB bus extender or alternative link is required.

Verification / Alternative check:
Vendors specify approximately tens of meters total without extenders; long-distance setups use dedicated extenders to regenerate signals and timing.

Why Other Options Are Wrong:

• Wrong data format: Would typically cause consistent protocol errors, not distance-correlated failures.
• Shielding/grounding: Important, but length remains the dominant out-of-spec factor here.
• Extraneous signals: Possible, yet the deterministic culprit is the extreme cable length.

Common Pitfalls:

• Assuming digital buses are immune to analog transmission line effects.
• Overlooking bus loading limits (number of devices and cable length).

Final Answer:
The GPIB cable is too long; a bus extender should be used.