Field Troubleshooting: Verifying Serial (RS-232) and Parallel (LPT) Ports Which tool is specifically used to check the electrical send/receive paths of serial and parallel interfaces during diagnostics?

Introduction / Context:
PC technicians frequently need to determine whether an I/O port (serial RS-232 or parallel LPT) is functioning. While software can initiate tests, a physical loop is required to confirm the transmit lines correctly return to receive lines. This question asks which tool closes that path reliably.

Given Data / Assumptions:

• Standard legacy PC ports: DB-9/DB-25 serial and DB-25 parallel.
• Use of diagnostic software that sends and expects data back.
• Technician has access to common bench tools.

Concept / Approach:

A loopback plug internally wires the transmit and receive pins (and status lines as needed) so test signals sent by the port are fed back to the same port. Successful round-trip confirms driver, UART, and physical pins. It is the canonical hardware for port self-test together with a diagnostic program.

Step-by-Step Solution:

Verification / Alternative check:

Cross-check with another PC or use a second, known-good I/O card. Signal-level checking with an oscilloscope can supplement but does not replace functional loopback confirmation.

Why Other Options Are Wrong:

Port adapters merely change connector form factors and do not loop signals. Logic probes can detect voltage transitions but do not validate full protocol loopback. “All of the above” is wrong because only the loopback plug performs the required path closure.

Common Pitfalls:

Using a straight-through cable and expecting loopback behavior, or forgetting control-line wiring (for parallel ports). Always use a purpose-built loopback plug and suitable diagnostics.

Final Answer:

Loopback plug