Unraveling the Secrets of Your USB-C Cables with a Free App
It's been years since I introduced you to a handy $8 tool that could tell you the speed and power of your USB-C cable. Unfortunately, that gadget is no longer available, and I haven't found anything as simple or cheap since. However, if you have a computer using the latest technology, there's an even better tester that you can download for free.
This app is named "CableDex", and it works by analysing data that your computer already collects about connected USB devices - information that isn't usually visible to you. All you need to do is click a small widget in your computer's menu bar, and you'll be able to see every USB-C cable and device connected to your machine.
How Does CableDex Work?
The creator of CableDex, Darryl Morley, explained the process to me. Every computer with the latest technology has a port controller chip that manages USB Power Delivery negotiation. When you connect a cable with an e-marker, the port controller sends a "Discover Identity" message to the chip in the cable. The response contains structured information like the vendor ID, speed rating, current rating, voltage limits, and whether it's active or passive.
Your operating system records this response in the IOKit registry, and CableDex reads it using public APIs, without requiring root access or private entitlements. The data isn't hidden; the system's firmware negotiates and publishes the result. It's just not usually visible in standard operating system tools. CableDex simply reads what's already there.
Along with the e-marker, CableDex also reads from the computer's own hardware, including the actual negotiated connection speed, Thunderbolt link speed, and live voltage and current at each port. The connected device also provides information about what it is, who manufactured it, and what it supports. Combining all this data, CableDex can not only tell you what everything claims to support, but also what's happening on the connection right now, and which part is causing a bottleneck if something isn't performing as expected.
A Closer Look at CableDex in Action
Want to see CableDex in action? I took some pictures while testing a few of my favorite cables. It's not a foolproof solution, as cables can misrepresent their capabilities, but CableDex genuinely helped me identify a faulty cable.
When I connected a short, lightweight cable into two ports on my laptop, CableDex showed me that this cable could charge at 100 watts – almost as fast as my laptop can charge. This is slightly more useful information than my old $8 tester could provide. It, too, showed that the cable only offers USB 2.0 speeds and probably offers 60W or better charging because an e-marker exists. But it couldn't read the e-marker data to tell that this cable supports 100W charging speeds.
Now, let’s test one of my top five favorite USB-C cables – a 10Gbps, 100W cable with magnetic winding beads. The cable's e-marker claims it's 10Gbps and 100W, but the laptop isn't treating it that way! When I connect a fast 10Gbps SSD, I'm not getting that speed with this cord. It seems that my daily use cable is finally wearing out. Time to replace this one!
Testing More Cables with CableDex
Next, I tested the newest and greatest cable in my drawer: a 240W USB4 40Gbps cable. Again, the e-marker seems to validate those speeds, even if the laptop isn't connecting at that rate. Once I connect the drive, CableDex detects that the laptop has a 10Gbps link. This 25GB transfer is measured in seconds instead of minutes.
Here’s another cable that arrived at my house recently, intended solely for 100W charging. I wasn't expecting more than USB 2.0 480Mbps data; the manufacturer only advertised USB 2.0 speeds. But CableDex says its own e-marker advertises 10Gbps USB 3 data. Unfortunately, it seems this cable's e-marker made promises its body couldn't keep. The same 25GB transfer took minutes, not seconds.
Future Developments for CableDex
Morley isn't the first to realize a laptop could be a USB-C cable tester. There are similar paid apps available. However, Morley’s version is free, and he assures me it “will always stay free at its core,” although you can pay £9.99 to get the Pro version that offers a real-time power monitor, diagnostics, and a terminal view.
He’s also developed an even simpler version of the app called WhatPort that simply monitors what each of your laptop’s USB-C ports is doing right now, including power, data, and video. Morley won't be able to build a version of CableDex for Windows due to "too much hardware variance and the Windows APIs don’t expose what CableDex needs," and says Android and iOS similarly don’t provide enough low-level access.
However, he's already working on a Linux version and continues to update the laptop version. You can follow along with the updates on his GitHub page.
It's been years since I introduced you to a handy $8 tool that could tell you the speed and power of your USB-C cable. Unfortunately, that gadget is no longer available, and I haven't found anything as simple or cheap since. However, if you have a computer using the latest technology, there's an even better tester that you can download for free.
This app is named "CableDex", and it works by analysing data that your computer already collects about connected USB devices - information that isn't usually visible to you. All you need to do is click a small widget in your computer's menu bar, and you'll be able to see every USB-C cable and device connected to your machine.
How Does CableDex Work?
The creator of CableDex, Darryl Morley, explained the process to me. Every computer with the latest technology has a port controller chip that manages USB Power Delivery negotiation. When you connect a cable with an e-marker, the port controller sends a "Discover Identity" message to the chip in the cable. The response contains structured information like the vendor ID, speed rating, current rating, voltage limits, and whether it's active or passive.
Your operating system records this response in the IOKit registry, and CableDex reads it using public APIs, without requiring root access or private entitlements. The data isn't hidden; the system's firmware negotiates and publishes the result. It's just not usually visible in standard operating system tools. CableDex simply reads what's already there.
Along with the e-marker, CableDex also reads from the computer's own hardware, including the actual negotiated connection speed, Thunderbolt link speed, and live voltage and current at each port. The connected device also provides information about what it is, who manufactured it, and what it supports. Combining all this data, CableDex can not only tell you what everything claims to support, but also what's happening on the connection right now, and which part is causing a bottleneck if something isn't performing as expected.
A Closer Look at CableDex in Action
Want to see CableDex in action? I took some pictures while testing a few of my favorite cables. It's not a foolproof solution, as cables can misrepresent their capabilities, but CableDex genuinely helped me identify a faulty cable.
When I connected a short, lightweight cable into two ports on my laptop, CableDex showed me that this cable could charge at 100 watts – almost as fast as my laptop can charge. This is slightly more useful information than my old $8 tester could provide. It, too, showed that the cable only offers USB 2.0 speeds and probably offers 60W or better charging because an e-marker exists. But it couldn't read the e-marker data to tell that this cable supports 100W charging speeds.
Now, let’s test one of my top five favorite USB-C cables – a 10Gbps, 100W cable with magnetic winding beads. The cable's e-marker claims it's 10Gbps and 100W, but the laptop isn't treating it that way! When I connect a fast 10Gbps SSD, I'm not getting that speed with this cord. It seems that my daily use cable is finally wearing out. Time to replace this one!
Testing More Cables with CableDex
Next, I tested the newest and greatest cable in my drawer: a 240W USB4 40Gbps cable. Again, the e-marker seems to validate those speeds, even if the laptop isn't connecting at that rate. Once I connect the drive, CableDex detects that the laptop has a 10Gbps link. This 25GB transfer is measured in seconds instead of minutes.
Here’s another cable that arrived at my house recently, intended solely for 100W charging. I wasn't expecting more than USB 2.0 480Mbps data; the manufacturer only advertised USB 2.0 speeds. But CableDex says its own e-marker advertises 10Gbps USB 3 data. Unfortunately, it seems this cable's e-marker made promises its body couldn't keep. The same 25GB transfer took minutes, not seconds.
Future Developments for CableDex
Morley isn't the first to realize a laptop could be a USB-C cable tester. There are similar paid apps available. However, Morley’s version is free, and he assures me it “will always stay free at its core,” although you can pay £9.99 to get the Pro version that offers a real-time power monitor, diagnostics, and a terminal view.
He’s also developed an even simpler version of the app called WhatPort that simply monitors what each of your laptop’s USB-C ports is doing right now, including power, data, and video. Morley won't be able to build a version of CableDex for Windows due to "too much hardware variance and the Windows APIs don’t expose what CableDex needs," and says Android and iOS similarly don’t provide enough low-level access.
However, he's already working on a Linux version and continues to update the laptop version. You can follow along with the updates on his GitHub page.