The USB-C 20G Loopback Plug
Updated: Sep 9
A simple way to test, benchmark, and diagnose USB-C ports — USB4 compatible, up to 20 Gb/s
USB-C ports may look identical, but their performance can vary significantly. Some ports operate slower than expected, while others may drop data without warning. Occasionally, a port may work well initially but fail under heavy use. The USB-C 20G Loopback Plug is a compact, pocket-sized tool that quickly identifies these issues. Just plug it into any USB-C port, run a test, and receive clear insights on speed, errors, power, and cable quality.
The USB-C 20G Loopback Plug — a compact, all-in-one USB-C port tester.
What This Tool Does
The USB-C 20G Loopback Plug offers several key functions:
Verify that data moving in and out of a USB port is completely error-free.
Measure the actual read and write speed of the port, not just its rated number.
Diagnose faulty ports by showing the exact type of error and how often it occurs.
Key Specifications at a Glance
| Specification | Details |
|---------------------|--------------------------------------------------|
| Maximum Speed | Up to 20 Gb/s (USB 3.2 Gen 2x2) |
| Standard | USB4 compatible |
| Power Delivery | Reads port profiles; can trigger up to 48 V (EPR) |
| Tests Included | Speed, errors, burn-in, power profiling, and cable checks |
| Control | Windows application, programmable API, and firmware updates |
Speed Benchmarking
This test measures the highest read and write speed a USB port can achieve. The device pushes data through as quickly as possible and records both peak and average speeds over time. It allows you to confirm whether a port truly delivers the USB 3.2 Gen 2x2 performance it claims or if it falls short.
Error Detection
A loopback test sends data out of the port and checks that the same data returns correctly. If even a single bit is incorrect, the device catches it. Every error is logged with its type and count, allowing you to see whether a port is fully reliable or silently dropping data. This feature makes it easy to identify weak or failing ports before they cause significant issues.
The results log lists each error by type — physical layer, 8b/10b decode, and CRC — with a running count.
Reliability & Burn-In Testing
Some faults only appear after prolonged, heavy use. This test maintains a steady, continuous data load on the port for an extended period. It helps reveal intermittent glitches and heat-related issues that a short test would miss. This makes it the ideal tool for burn-in testing and proving that a system remains stable under sustained load.
Read speed traced over a long burn-in run, showing how the port holds up over time.
Port Profiling (Power Delivery)
Using USB Power Delivery (PD), the device can read everything a port advertises — its supported voltages, current limits, and maximum power. It can also request specific voltage levels, up to 48 V with Extended Power Range (EPR). This capability allows you to test a port across all its power modes, not just the default one.
Cable Detection
USB-C cables vary significantly in what they support. Through USB Power Delivery, the device reads a cable’s built-in information: its identity, supported data rates, and power rating. This information allows you to quickly determine whether a cable is suitable for high-speed data and high-power charging or if it is the reason a setup is underperforming.
Software, API & Firmware
PC Software
The loopback plug comes with a dedicated Windows app. From a single, clean screen, you can set up tests, choose the speed and voltage, and monitor the results as live logs or graphs. Any result can be saved for later review.
Setting up a loopback test in the PassMark Windows software.
API Programming Support
An API allows other software to communicate with the device directly. You can write your own scripts and automate the entire testing process. This is ideal for a production line or any situation where multiple ports need to be checked at scale.
Updatable Firmware
The firmware can be updated, ensuring the device keeps up with new USB standards and can gain new features over time. This keeps your investment valuable for years to come.
Where It’s Used
Manufacturing & QA — high-volume testing of USB ports on the production line to maintain consistent quality.
Compliance Testing — confirming that ports meet USB specifications for speed, power, and compatibility.
IT Diagnostics & Repair — quickly checking port health and performance during service or repair work.
The Ports on the Device
The loopback plug features three USB-C connections. Here’s what each one does:
The three ports and the optional cables used with the device.
Test Unit Port — the USB-C port you connect to the port being tested. It also powers the device: plug it in, and the loopback plug turns on.
External Load Port — adds extra load to the USB line, allowing you to test under real-world conditions. It includes built-in current protection and is controlled from the software.
Debug Port — an optional port for steady power or for connecting a second computer to log results. It can power the device independently, allowing you to continue testing even if the main port is faulty or unstable. This also enhances cable detection reliability by eliminating power-on delays.
Pre-Testing the USB-C for Over Voltage
Protect your USB-C Loopback plug by pre-testing faulty ports for over voltage with the PassMark USB-C S.O.S Short Circuit Tester.
This durable and simple test plug provides a visual indication of port voltage in seconds, safeguarding your USB-C Loopback unit from potential damage. This is especially useful in production line testing.
USB-C S.O.S Short Circuit Tester.
If your USB S.O.S pre-test returns a Pass, you can proceed to more comprehensive troubleshooting with your USB-C 20G Loopback and accompanying software.
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