Billboard devices explained, and a more honest cable wattage

A small follow-up to the display work in 0.15.0: WhatCable now explains the mystery "Billboard device" that some monitors and docks put on a port, and it stops slightly overstating what a high-end cable can carry.

What's new

• The "Billboard device" finally gets an explanation. Plug in some monitors, adapters, or docks and macOS lists a "Generic Billboard Device" with no hint of what it is or whether it matters. A Billboard device is how a USB-C device reports the alt modes it supports, and it often shows up when something like DisplayPort was offered but didn't fully come up. Every port that has one now says "Billboard device present" plainly, with no alarm: docks park one there quite normally. The Pro Display Diagnostics screen goes further, but only when it actually means something: if a Billboard device is present and your display is running below its best mode, it points at a probable failed alt-mode handshake and suggests a re-plug, a different cable, or a different adapter.

What's fixed

• Cable wattage no longer overstates by 10W. A cable whose e-marker is rated to 50V at 5A was being shown as a 250W cable. USB Power Delivery never actually delivers above 48V, so the real ceiling for such a cable is 48V x 5A = 240W. The 50V figure is the cable's voltage rating, not a delivery voltage. WhatCable now shows the true 240W it can carry, and spells out why (the rating and the deliverable are separate facts) so the numbers make sense. Cables rated to 30V or 40V are unchanged, because those voltages really are delivered.

Install

brew upgrade --cask whatcable, download from Releases, or update in the app.