GoKawiilEvery Apple Silicon Mac has a port controller chip that handles USB Power Delivery negotiation. When you plug in a cable with an e-marker, the port controller sends a “Discover Identity” message to the chip in the cable and gets back a structured message: vendor ID, speed rating, current rating, voltage limits, whether it’s active or passive, and so on.
macOS writes that response into the IOKit registry. WhatCable reads it using Apple’s public APIs. No root access, no private entitlements. The data isn’t hidden, Apple’s firmware does the negotiation and publishes the result. It’s just not surfaced anywhere in standard macOS tooling. WhatCable reads what’s already there.
The e-marker is one source. WhatCable also reads from the Mac’s own hardware, the actual negotiated connection speed, Thunderbolt link speed, and live voltage and current at each port. The connected device tells us what it is, who made it, and what it supports. Put all three together, cable, device, and Mac, and WhatCable can tell you not just what everything claims to support, but what’s actually happening on the connection right now, and which part is the bottleneck if something isn’t performing as expected.