How to Test DisplayPort Cable for Pin 20

A DisplayPort cable that improperly connects Pin 20 can back-feed 3.3V power into your graphics card or motherboard, causing boot failures, sleep issues, or permanent hardware damage. This article provides a straightforward guide on how to test a DisplayPort cable using a digital multimeter to ensure Pin 20 is unpinned, identifies the symptoms of a problematic cable, and explains how to confirm a cable adheres to official VESA standards.

Why Pin 20 Should Be Unpinned

The DisplayPort specification reserves Pin 20 for delivery of 3.3V DC power at up to 500mA. This power is exclusively meant to power active adapters, dongles, or signal boosters directly from the source port. Standard DisplayPort-to-DisplayPort cables intended to connect a computer to a monitor must not wire Pin 20 through from end to end. If wired, power flows between the graphics card and monitor, leading to potential electrical conflicts.

Testing Pin 20 With a Multimeter

The most reliable way to verify Pin 20 is by using a digital multimeter set to test electrical continuity.

Tools Required

  • A digital multimeter with a continuity test or resistance (Ohms) setting.
  • Two thin sewing needles or safety pins (multimeter probes are typically too thick to enter DisplayPort connector pin slots safely).

Pin 20 Location

Looking directly into the DisplayPort connector with the beveled corner on the top left:

  • The bottom row of pins contains the even-numbered pins.
  • Pin 20 is located at the far right of the bottom row.
  • Pin 19 (return line) is directly adjacent to Pin 20. Ensure you do not bridge pins when testing.

Step-by-Step Test Procedure

  1. Prepare the Leads: Insert one fine needle carefully into Pin 20 on the first connector. Insert the second needle into Pin 20 on the second connector. Avoid bending the internal connector contacts.
  2. Configure the Multimeter: Switch the multimeter to the continuity mode (usually marked with a sound wave or diode symbol) or the lowest resistance range (Ohms/\(\Omega\)).
  3. Measure the Connection: Touch the positive multimeter probe to one needle and the negative probe to the other needle.
  4. Interpret the Result:
    • Pass (Correct): The multimeter remains silent or displays "OL" / "1" (open loop/infinite resistance). This confirms that Pin 20 is not connected and the cable is safe to use.
    • Fail (Defective): The multimeter emits a continuous beep or reads near zero Ohms (less than 5 \(\Omega\)). This indicates that Pin 20 is actively wired and the cable should be discarded.

Alternative Verification Methods

Observing System Symptoms

If testing tools are not available, you can observe system behavior for common symptoms of Pin 20 back-feeding:

  • GPU fans or motherboard LEDs stay illuminated after the computer is fully shut down.
  • The computer fails to boot, hangs on post codes, or reboots spontaneously.
  • The monitor displays power-saving loop errors or refuses to wake from sleep.
  • Disconnecting the DisplayPort cable immediately resolves sleep or power state anomalies.

Verifying VESA Certification

VESA strictly mandates that standard DisplayPort cables omit the Pin 20 wire. Check the VESA Certified DisplayPort Products database using the manufacturer's brand and part number. Cables certified by VESA are tested specifically to ensure Pin 20 is left disconnected. Avoid uncertified, non-branded generic cables, as they are the most frequent offenders of incorrectly routing Pin 20 straight through the assembly.