Discover the Secrets of Your USB-C Cables with This Free Mac App

Almost three years ago, I introduced you to a fantastic $8 cable tester that quickly indicated whether your USB-C cable was fast, slow, powerful, or weak. Unfortunately, that handy tool has been discontinued, and I haven’t found anything as user-friendly or affordable since. However, if you’re using a Mac with Apple Silicon, there’s an even better option available for free.

Meet WhatCable, a tool that leverages the data your Mac already gathers about connected USB devices—information that Apple typically keeps under wraps. With a simple click on a widget in your menu bar, you can view all USB-C cables and devices connected to your Mac.

Darryl Morley, the creator, explained it this way:
> Every Apple Silicon Mac features a port controller chip that manages USB Power Delivery negotiations. When you connect a cable with an e-marker, the port controller sends a “Discover Identity” message to the cable’s chip and receives a structured response: vendor ID, speed rating, current rating, voltage limits, and whether it’s active or passive.
> macOS logs that response in the IOKit registry, and WhatCable reads it using Apple’s public APIs. No root access or special permissions are needed. The data isn’t hidden; Apple’s firmware handles the negotiation and publishes the results, but it’s not accessible through standard macOS tools. WhatCable simply taps into what’s already there.
> The e-marker is one source of information. WhatCable also pulls data from the Mac’s hardware, including the actual negotiated connection speed, Thunderbolt link speed, and real-time voltage and current at each port. The connected device provides details about its identity, manufacturer, and capabilities. By combining information from the cable, device, and Mac, WhatCable reveals not just what each component claims to support, but what’s actually happening in real-time, including identifying any bottlenecks if performance isn’t up to par.

Curious to see it in action? I snapped some photos while testing a few of my favorite cables this week. While it’s not a foolproof solution—cables can sometimes misrepresent their capabilities—WhatCable helped me identify a faulty cable during my tests.

When I connected the lightweight Satechi cable to two ports on my MacBook Pro, I received this information:
The key takeaway here is that it’s “Cable rated for 5 A at up to 20V (~100W).” This confirms it’s a solid charging cable. While 480Mbps USB 2.0 is quite slow, the cable accurately reports that it can charge at 100 watts, which is nearly the maximum charging speed for my Mac.

This is slightly more detailed than what my $8 tester can provide. It also indicates that the cable only supports USB 2.0 speeds and likely offers 60W or better charging due to the presence of an e-marker. However, it can’t read the e-marker data to confirm that this cable supports 100W charging speeds.

Sure enough, when I plugged in a 140W battery, I was getting more than 60W:
WhatCable also confirmed that I was connected to a 100W charger.

Next, I decided to test one of my favorite USB-C cables: a 10Gbps, 100W Supercalla cable with magnetic winding beads. Oddly, while the cable’s e-marker claims it supports 10Gbps and 100W, my Mac isn’t recognizing it as such!

When I connected a fast 10Gbps SSD, I didn’t achieve that speed with this cable. It seems my go-to cable is finally wearing out—time to retire it!

Now, let’s check out the latest addition to my collection: a 240W USB4 40Gbps cable. The e-marker appears to validate those speeds, but my Mac isn’t connecting at that rate.

Once I plugged in the drive, WhatCable detected a 10Gbps link. This is more like it: transferring a 25GB file now takes seconds instead of minutes.

I also tested a new cable that arrived recently, designed solely for 100W charging. I expected it to only support USB 2.0 480Mbps data, as that’s all the company advertised. However, WhatCable indicated that its e-marker claims 10Gbps USB 3 data. Could it be true?

Unfortunately, it wasn’t: this cable’s e-marker promised more than it could deliver, resulting in a lengthy transfer time for the same 25GB file. My $8 tester did a better job here, immediately indicating that the cable doesn’t support SuperSpeed (USB 3).

It does deliver on the 5 amp charging speeds, though.

Next, I plugged in my magnetic accordion USB-A to USB-C cable, which is definitely only capable of 480Mbps USB 2.0 speeds. Strangely, my Mac reported it was running at 10Gbps while connected to my external battery. That seems off!

Lastly, I tested the reliable cable that came with a LaCie drive I purchased back in 2019, which I’ve always relied on for stability and speed. It’s reporting as a 20Gbps Thunderbolt cable, even though it’s labeled as 10Gbps. I’ll need to try it with a Thunderbolt drive to verify!

Morley isn’t the first to realize that a MacBook could serve as a USB-C cable tester. There’s a similar paid app that launched a year ago. However, Morley’s version remains free at its core, with an optional Pro version available for a fee that includes a real-time power monitor, diagnostics, and a terminal view.

He’s also developed a simpler version called WhatPort, which monitors the activity of each USB-C port on your Mac, including power, data, and video.

Morley mentioned that he won’t be able to create a version of WhatCable for Windows due to hardware variability and limitations in the Windows APIs. He also noted that Android and iOS don’t provide sufficient low-level access.

“If anyone has a workaround, I’d love to hear it,” he said.

However, he is working on a Linux version and continues to update the Mac version. You can keep track of the updates on his GitHub page.