Occam’s Razor and a USB-C to HDMI adapter

The simplest explanation is likely the best, even if its validity isn’t always intuitively obvious in advance.

Speaking of USB-C, and the cables (including splitters) that connect to it to other things…

My long-in-the-tooth Intel-based 2018 Apple Mac mini is still sitting on my desk, humming to my right, even though its M2 Pro Apple Silicon-based successor has been sitting downstairs in storage awaiting its turn in the spotlight for going on three years now. The legacy hardware is no longer a candidate for new Apple operating system releases, but it’s still receiving bug fixes and security patches, until sometime in the second half of next year, if past-history trends remain valid. And as the saying goes, “if it works, don’t touch it” (I’d also be tempted to haul out the “a penny saved is a penny earned” quote, but since I’ve already bought its replacement…).

In conjunction with my transition to it from its 2014-era Mac mini predecessor roughly 2.5 years ago, I upgraded the two-display suite above it to a set of Dell P2415Q 4K LCDs.

The Mac mini leverages the displays’ HDMI inputs; a “Project Volterra” Windows-on-Arm dev kit 2023 stacked above it connects to those same LCDs over their DisplayPort connections, and front panel buttons toggle the displays between the two systems when running concurrently.

Look back at that 2018 Mac mini rear panel “stock” photo a couple of paragraphs ago, however, and you’ll only see one HDMI output, into which I’ve plugged a “straight” HDMI cable running directly to one of the LCDs. How, then, did I connect the system to the other display’s HDMI input? That’s where today’s teardown victim enters the picture.

It’s an Anker A8730 6’ USB-C to HDMI adapter cable; I’d bought and pressed it into immediate service way back in mid-2021. I sourced from Anker’s outlet store on eBay, where it cost $25.90 as a refurb. So, you can say I’ve gotten my money’s worth out of it! Speaking of fiscal topics, there’s a 0.75′′ (19.1 mm) diameter U.S. penny alongside in the second (and non-stock) image that follows, along with subsequent others, for size comparison purposes.

A multimedia-centric repurpose

How it works is (at least to me) an interesting story in and of itself. It fundamentally leverages a DisplayPort technology called “Alt Mode”, which repurposes USB 3.x’s “SuperSpeed” data and Configuration Channel (CC) lines to transport dedicated audio and video streams sourced from the graphics and sound subsystems. The CC leverage explains why, among other implementation reasons, it’s only applicable to USB-C, not also to prior-generation USB physical connector standards.

Macs don’t additionally support DisplayPort’s Multi-stream Transport (MST) mode, either in the operating system (generally) or in hardware (for newer Apple Silicon-based systems). If MST support had existed, it would have conceivable enabled me to extended-mode tether both displays to a single USB-C (or Thunderbolt 3, aka TB3, in this case) system connector, in combination with a separate inter-display connection. But again, per the system back panel photo, I had plenty of spare connector candidates available, not even counting those on the separate expansion hubs (the latter added subsequent to the snap of the earlier “stack” picture).

But what’s with all this DisplayPort talk? I thought we were discussing connecting the computer to a display’s HDMI input! We are, and that’s where the bulge surrounding the adapter cable’s HDMI connector end comes in. Conceptually, and I hope to definitively confirm today, inside is likely a DisplayPort to HDMI protocol-converting bidirectional transceiver; a particularly robust one, it seems, since the Anker A8730 touts 4K 60 Hz specs. HDMI once also had an “Alt” mode for USB-C, at least on specification paper, although actual products never made it to market.

Erratic function = dissection rationalization

Since the adapter cable had been operational in my abode since mid-2021 (and, considering it was a refurb, maybe even earlier than that with its original owner), why’d I decide to take it apart? Curiosity was as always one motivation, although I generally hold that particular urge at bay for as long as the gear remains operational. But this one had eventually failed, in another story-in-and-of-itself, through whose telling I aspire to also rationalize the “Occam’s Razor” title reference.

As previously mentioned, the 2018 Mac mini is still receiving periodic patches for its last-supported MacOS 15 “Sequoia” operating system release. A few updates ago, the left-side display of the pair (the one leveraging the Anker adapter) was no longer recognized by the system after the upgrade and subsequent reboot. Online research revealed that mine wasn’t a unique post-update quirk, and swapping the adapter cable to a different TB3 system connector got the display going again.

So, I figured that an obscure “Alt Mode” bug (specifically, since the first TB3 port still worked fine for generic data) introduced along with the upgrade had made it through Apple’s pre-release QA checks. This purported scenario was unsurprising albeit still disappointing, given that Intel-based Macs in general, and this system in particular, were on their “last legs” and likely no longer receiving their fair share of QA attention. Why my surmised bug only affected one TB3 port but not another wasn’t clear, but… Read Original