Almost three years ago, I highlighted a terrific $8 cable tester that could quickly indicate whether a USB-C cable was likely to be fast or slow, high-powered or underpowered. Unfortunately, that handy little device was later discontinued, and I haven’t come across anything quite as simple or affordable since. But if you use a Mac powered by Apple Silicon, there is now an even more capable option you can download for free.
The app is called WhatCable, and it works by pulling information your Mac already gathers about connected USB devices — details Apple does not typically present to users. Once installed, it appears as a small menu bar widget at the top of your screen, where it can show every USB-C cable and device currently connected to your computer.
Creator Darryl Morley described the process this way:
Every Apple Silicon Mac includes a port controller chip responsible for handling USB Power Delivery negotiation. When a cable with an e-marker is connected, that controller sends a “Discover Identity” request to the chip inside the cable. In return, it receives structured information such as the vendor ID, supported speed, current rating, voltage limits, and whether the cable is active or passive.
macOS records that response in the IOKit registry, and WhatCable accesses it through Apple’s public APIs. It does not require root access or private entitlements. In other words, the information is not really hidden: Apple’s firmware performs the negotiation and publishes the result, but standard macOS tools do not expose it to users. WhatCable simply reads the data that is already available.
The e-marker is only part of the picture. WhatCable also pulls details from the Mac’s hardware, including the actual negotiated connection speed, Thunderbolt link speed, and live voltage and current at each port. The connected device contributes its own identifying information, including what it is, who made it, and what standards it supports. By combining the cable, device, and Mac data, WhatCable can show not only what each component claims to support, but what is actually happening on the connection in real time — and where the bottleneck may be if performance falls short.
Curious how it performs in practice? I photographed the app while testing several of my favorite cables this week. It is not a flawless solution, since cables can misreport their capabilities, but WhatCable did help me identify a problematic cable during testing.
When I connected the short, lightweight Satechi cable shown above to two ports on my MacBook Pro, this is what appeared:
I know from experience that this info is correct, and that means it’s still a valuable cable. 480Mbps USB 2.0 is very slow, but the cable is self-reporting it can charge at 100 watts, nearly as fast as my Mac can charge.
That’s slightly more useful info than my $8 tester can provide. It, too, shows that the cable only offers USB 2.0 speeds and probably offers 60W or better charging because an e-marker exists. But it can’t read e-marker data to tell that this cable supports 100W charging speeds.
Sure enough, I’m getting more than 60W when I plug a 140W battery into my Mac:
WhatCable can detect that I’m connected to a 100W charger, too:
Now, let’s try one of my five favorite USB-C cables yet — my 10Gbps, 100W Supercalla cable with magnetic winding beads:
That’s strange: The cable’s e-marker does claim it’s 10Gbps and 100W, but the Mac isn’t treating it that way!
When I plug in a fast 10Gbps SSD, I’m not getting that speed with this cord:
And it appears that’s because my daily driver cable is finally wearing out. Guess it’s time to retire this one!
Now let’s try the theoretically latest and greatest cable in my drawer: a 240W USB4 40Gbps cable.
Again, the e-marker seems to validate those speeds, even if the Mac isn’t connecting to itself at that rate.
Once I plug in the drive, WhatCable detects that the Mac has a 10Gbps link:
That’s more like it: This 25GB transfer is measured in seconds instead of minutes:
Here’s a cable that arrived at my home just the other day exclusively for 100W charging. I’m not expecting more than USB 2.0 480Mbps data; on Amazon, the company only advertised USB 2.0 speeds:
But WhatCable says its own e-marker advertises 10Gbps USB 3 data… could it be?
I’m afraid not: This cable’s e-marker wrote checks its body couldn’t cash. Minutes, not seconds, for the same 25GB transfer:
Here, my $8 tester did a better job, immediately detecting that the cable doesn’t support SS (SuperSpeed, aka USB 3).
It delivers on the 5 amp charging speeds, though:
Next, I thought I’d plug in my magnetic accordion USB-A to USB-C cable, which is definitely only capable of 480Mbps USB 2.0 speeds:
Strangely, the Mac insists it’s running at 10Gbps… while connected to my external battery. That seems wrong!
Last but not least, here’s the old faithful cable that came with a LaCie drive I bought way back in 2019, one I’ve always turned to for stability and speed:
It’s reporting as a 20Gbps Thunderbolt cable, even though it says 10Gbps on the end. I don’t have one handy, but I’ll have to try it with a Thunderbolt drive to check!
Morley isn’t the first to realize a MacBook could be a USB-C cable tester. USB Connection Information is a similar paid app that arrived a year ago. But Morley’s version is free, and he tells me it “will always stay free at its core,” though 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 now built an even simpler version of the idea called WhatPort that simply monitors what each of your Mac’s USB-C ports is doing right now, including power, data, and video.
Morley tells me he won’t be able to build a version of WhatCable for Windows because “there’s too much hardware variance and the Windows APIs don’t expose what WhatCable needs,” and says Android and iOS similarly don’t provide enough low-level access.
“If anyone has a workaround, I’d love to hear it,” he says.
But he’s already working on a Linux port and is continuing to update the Mac version. You can follow along with the updates at his GitHub page.
Photos by Sean Hollister / The Verge
