Free tool · macOS

Which cable is
the bad one?

A little production magic

Every kit bag has one: the USB-C cable that charges fine and moves no data. BW Dowser tells you what a cable is actually capable of, by measuring the link it negotiates with a device on the other end. Dowse the cable at the bench, not ten minutes before doors. Free, from the people who make BW Logger.

Get it free How it works
  • Free, no account
  • macOS (Apple Silicon)
  • Reads nothing but your own ports

Dowse the cable once, give it a name, and it remembers.

How it works

It measures the link, not the label.

Read this before you download

A USB-C cable is a passive object. With one end in your Mac and nothing on the other, your computer cannot see it at all, and no software on any Mac can tell you what it is. Tools that claim otherwise are guessing.

So BW Dowser works the way a bench tech does: put a known device on the far end and see what the link negotiates. That number is real, it is measured, and it is the number that decides whether your show works.

  1. 1

    Dowse the cable

    Plug a device you know the speed of into your Mac through the suspect cable. An SSD, a dock, an interface. BW Dowser names what it found.

  2. 2

    Read the negotiated link

    USB 2.0 at 480 Mb/s, USB 3.2 at 5, 10 or 20 Gb/s, or USB4 and Thunderbolt at 20 or 40 Gb/s. Plus whether the port is carrying power, and how much.

  3. 3

    Get the verdict on the cable

    The app compares what your Mac can do, what the device can do, and what actually happened. If a 40 Gb/s Mac and a 40 Gb/s SSD only reach 480 Mb/s, the cable is the culprit, and it says so.

  4. 4

    Label the winner, bin the loser

    Save a result per cable with your own name for it. Print the list and tape it inside the flightcase lid.

Straight answers

What it can and cannot tell you.

It can tell you

  • The exact link speed the cable actually carried
  • Charge-only: power flows, no data link at all
  • USB 2.0 only, when you expected 10 Gb/s
  • Whether the link came up as USB or as Thunderbolt / USB4
  • Whether the port is delivering or drawing power, and roughly how much
  • Your Mac's own port ceiling, per port
  • A minimum rating for the cable: it carried 10 Gb/s, so it is at least a 10 Gb/s cable

It cannot tell you

  • Anything at all about a cable with nothing on the far end
  • The cable's printed rating, or its maximum, if your device is slower than the cable
  • The eMarker chip's declared specification. macOS keeps that inside the port controller and offers no public way to read it
  • Whether a cable is genuine, certified, or safe at 240 W
  • The difference between Thunderbolt 4 and Thunderbolt 5 unless you own a device that needs the difference

If you need the cable's declared rating rather than its measured behaviour, you need a hardware tester with its own Power Delivery controller. No amount of software on your laptop can replace one, and we would rather say so than sell you a number we made up.

Get it

Free. Tip only if it saves you.

Give an email address and the download link comes straight back. Pay nothing, or set your own price if it finds the cable that was about to ruin your night.

Send me the download

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