A tale of four adapters
I don’t ask for a lot. I just want to connect a screen to my laptop.
The two devices are pretty far apart in terms of age but they can connect just fine. An employer-provided Apple MacBookPro 16” M3 laptop and an Apple 30” Cinema Display HD. That’s almost 20 years gap between the two which, apart from really making me feel old, is impressive in terms of backwards compatibility and somewhat upsets the ‘planned obsolescence’ narrative you hear a lot about. However I need some technology to bridge the age gap…
The 30” Cinema Display takes its signal through a DVI-D connection. Specifically a Dual Link DVI-D connection. We’ll unpack what this name means at the end, it’s a small odyssey in itself and interrupts the narrative.
Now my Mac doesn't have a DVI connection to we’ll need to turn it into something else. There’s a few ways to do this (probably) but here’s what worked for me: First we need an Apple Mini DisplayPort to Dual-Link DVI Adapter.
Apple Mini DisplayPort to Dual-Link DVI Adapter. Image from https://support.apple.com/en-us/111808
This gets me halfway, as my Mac also does not have a DisplayPort connector, Mini or otherwise. Part two, therefore, is a Mini DisplayPort to USB-C adapter.
Adapter one
A generic Mini DisplayPort to USB-C adapter from Amazon UK. Image is my own capture.
Great success!
Display to Adapter to Adapter to Mac > Done!*
However…
Then I got given another 30” Apple Cinema Display HD. Having two matte-finish screens side by side at eye level is ergonomically better than switching to the hi-res XDR laptop screen and back so I’d like to use the MBP in clamshell mode and run the two screens. Will my Mac support this?
All we have to do is repeat the above trick with the adapters, right? Wrong! Obviously, because we haven’t talked about 4 adapters yet. How frustrating.
Troubleshooting
I had received the Apple adapter with the screen and sourced another MDP to USB-C adapter from Amazon. The one I had originally bought was out of stock so I bought one using the name of the original as the search term. Plugged it all in and nothing. I did get a white power light on the screen but no image, no recognition from the Mac that a display had arrived. So, on to troubleshooting. I tried the following:
Just the new display, disconnecting the original. FAIL, so it would appear to reside in the chain of adapters and screen, not in the Mac. Probably.
Tried the new display on the original adapter chain. SUCCESS. OK so the new screen works, it’s in the adapter chain.
Tried the original display with the new adapter chain. FAIL. Validates the conclusion above.
Tried the original display, new Apple Adapter, original USB-C adapter. SUCCESS. So we know the Apple adapter that came with the new screen also works. This points directly to the new USB-C adapter. Let’s confirm.
Either screen, with either Apple adapter and the original USB-C adapter. SUCCESS. Validates above.
Either screen, with either Apple adapter and the new USB-C adapter. FAIL. OK that new USB-C adapter IS the culprit. But let’s do just one more check…
I attached both screens with just the Apple adapters to an older 2015 MBP model that has two Thunderbolt 2 compatible ports in the Mini DisplayPort form factor. SUCCESS. Both screens work connected to one older Mac.
matrix of the testing configurations and results.
Adapter two
Here’s the suspect USB-C adapter I bought and subsequently returned…
Unavailable now, huh? What a surprise…
Note the description. USB-C (Thunderbolt 3 compatible) to Mini DisplayPort (not for Thunderbolt 1/2). Does this not work because it needs to be Thunderbolt something compatible on the MDP side? That seems unlikely as the 30” Display predates Thunderbolt 1.
Next, let’s compare to what I can find out about adapter one…
Adapter one. No longer available.
No mention of Thunderbolt here, however it does specifically call out MacBook computers. I can’t get any more detail is it’s no longer available and the link is dead.
Could it be that there’s incompatibility between third party adapters? I decided to go to the source…
Adapter three
Apple themselves sell an adapter that might work! I ordered that from the online Apple Store…
Nice packaging.
Nope. Nothing. I worked through the same list of troubleshooting steps as before but no joy. This makes me think that Thunderbolt is the issue and I need to focus on finding something non Thunderbolt-y. As it turned out a simple exercise of turning the packaging over would provide clarity.
Descriptive, if frustrating.
Adapter four
I’d discovered that not all adapters are created equal. Also that Thunderbolt compatibility is, at best, a distraction and, at worst, an incompatibility. My goal with adapter four was to find a brand with some reliability and a clearer indication of compatibility.
Amazon UK description of adapter four, the StarTech model.
Details from the packaging of adapter four
This looks much better! None of the Thunderbolt references. Let’s run through that same testing matrix for the sake of due diligence.
Nope. Doesn’t work.
Suffice to say at this point I am… despondent.
I’m thinking my options at this point are to try yet another adapter from Amazon (there’s a newer StarTech one that references Macs specifically and a couple that look to reference closer to adapter one), or look more into what is going on here. The latter seems wise before spending more money even if it is refundable.
Next steps
My first port of call is to go back to the stated supported external display support for the Mac. Since we have one chain of things working fine, it must be compatible with that I am thinking. Here’s the text from the support page linked previously…
One external display
Supports one display in any one of these configurations:
One display up to 6K at 60Hz over Thunderbolt
One display up to 8K at 60Hz (or 4K at 240Hz) over HDMI
Two external displays
Supports two displays in any one of these configurations:
Two displays up to 6K at 60Hz over Thunderbolt
One display up to 6K at 60Hz over Thunderbolt, and one display up to 4K at 144Hz over HDMI
So my one working display does not use the open HDMI port, it uses the USB-C port. Therefore we must be sending video over Thunderbolt since that’s what the Mac spec states and we have the evidence of one display working. How is this possible when the display predates Thunderbolt? The mention of Thunderbolt on the Mac side seems to indicate that it is a requirement despite the deductions made from the adapter product descriptions.
Side bar: Now you are probably shouting at me right now for ignoring the HDMI output option. I did not. I simply spared you the tedious journey I went on trying to find a male HDMI to female Mini DisplayPort adapter with no success. The closest I got was a male HDMI to male Mini DisplayPort cable and a female to female Mini DisplayPort converter. Which also didn’t work.
Thunderbolt as the mystery again. What IS Thunderbolt?
Thanks Wikipedia: It’s a standard introduced by Intel and Apple that combines three signals: Power, PCI Express and DisplayPort. Ah! So Thunderbolt CONTAINS DisplayPort! This explains why a display that accepts DVI-D adapted from DisplayPort adapted from Thunderbolt over USB-C works.
It mighty also explain why the adapters three and four didn’t work. Three had Thunderbolt but clearly no support for the DisplayPort within it (maybe just PCI-Express and power?). Four is pure DisplayPort without the wrapping Thunderbolt compatibility. As for adapter two maybe it’s just not very good?
I think the way forward is one of two options.
Try other adapters and try ensure they either mention Mac-compatibility in the description.
Take adapter one apart and see what’s inside. if it’s straight rewire between pinouts on different connectors then I could just hack one together from bits. If it’s got some specific magic going on inside it then return to option 1.
I’ll report on progress.
Update: working
I went down a bit of a rabbit hole, opening up the working adapter and looking at the chip that it uses. Turns out it’s from Analogix but is no longer listed on their website. However similar chips are listed and all are to do with conversion of DisplayPort signals. So at least we’re on the right track sticking with DisplayPort and not Thunderbolt.
Sorry about chewing up the plastic but it wasn’t built to be disassembled.
Since it’s clear I won’t be soldering anything to resolve this, I decided to spread-bet with some different adapter choices.
First there’s the previously mentioned updated version of the StarTech one. This is more expensive but has more reviews that are likely not all written by the rising singularity of AI bots. We shall call this Adapter Five.
Second there’s a random choice among a bunch of adapters all claiming to do the job for the least amount of money. I’m not rich so let’s see what the bottom of the barrel of regrettably poorly paid labour output can do. This is Adapter Six.
I repeated all of the testing with both adapters and finally got the second screen working.
Wallpapers courtesy of Louie Mantia. Both inspired by the Stanley Kubrick movie of The Shining.
Conclusion
I am sure you’re waiting to hear which of the new adapters made this world of dual screens possible?
It was the cheap one, Adapter Six. At the end of the day the lesson is it’s hard to be rigorous and deterministic to solve problems like this and it’s better to fuel the society-spiralling practice of ordering lot’s of things from Amazon and sending back the things that don’t work. Not the outcome I would have hoped for, however now that I am basking in the LCD-driven light of this screens, maybe it’s a moot point.
Oh, one more ironic thing. A third one of these monsters has made it;’s way into my life. Thankfully I am out of patience, desk space and USB-C ports to try to add it to the mix.
Addendum: DVI-A/D & Dual Link
Now let’s track back and explain that Dual Link DVI-D name, and you’ll see why the Apple adapter part of the setup is important and how it needs to be used. Firstly DVI expands to Digital Video Interface.
Technically this should be Digital* Video Interface with the subscript of *other options available. This is because the connection has pins to transfer analog video, from VGA, S-Video etc.. as well as a set of pins for a digital connection. If you’re using the analog part you’re technically DVI-A (for Analogue) as opposed to DVI-D (Digital).
Even more pedantically, it should be Digital* Video Interfaces (plural) because it has two sets of pins for Digital. This is the Dual Link part of the name. The 30” Cinema Display HD has a resolution of 2560 x 1600. Now that’s not a huge amount by today’s standards, but to these old eyes it’s fine. However it’s also above the ability of a single DVI-D link to carry, so the display uses one link for the top half of the display and the other for the bottom half if you want to support the highest resolution. If you stick with a single DVI-D link you get the complete screen but at a lower resolution.
Finally there’s DVI-I which is a connector where all three options are available…
DVI (female) connector. Image from https://www.the-home-cinema-guide.com/dvi-connector.html, annotations my own.,
All of this brings us to the aforementioned Apple Mini DisplayPort to Dual-Link DVI Adapter. As you can see in the picture there’s a USB-A port socket in the adapter body and a USB-A plug on a fly-lead with the Mini DisplayPort connection.
There are three reasons for the presence of these USB-A connections.
There’s a USB Hub in the display (as well as a FireWire 400 one) and so to enable these ports there is a USB-A fly lead on the display cable which you would plug into the Mac to carry power and data to the Hub. USB 2.0 in all it’s glory.
The display needs this USB-A connection to provide power for the display to work in addition to the 120/240 VAC from the mains. Like a switching voltage maybe?
The Apple adapter is clearly doing something to combine the dual link DVI signals into one that Mini DisplayPort can send onwards and needs power to do so.
Now I don’t use the in-display hub in this wireless device world we live in so I just plug the displays fly-lead into the adapter and the adapters fly-lead into a USB power adapter. This works perfectly well for the 2 cases that need power.
