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I am starting a project to build a DIY monitor. I have a spare lcd panel (model # LP156WF6) taken out of a laptop. I plan on buying a controller board for it and building it into a homemade enclosure. The controller board has an HDMI input and a 12V/4A barrel connector for powering the display. I would like to wire both the HDMI port and the barrel connector into the same USB Type-C plug and use the same USB Type-C cable to power the monitor and send it its signal from the thunderbolt port on my laptop. Is this even possible? I know HDMI over USB Type-C protocols have existed for a while now, so I imagine buying a USB Type-C pinout PCB and soldering a split HDMI cable to it should be relatively simple as long as I can find the wiring diagram. For the power, is it possible to simply wire a small power regulation board in-between the barrel connector and the Type-C pinout? It seems there are many products out there that support power and signal through the same Type-C cable, so this seems possible, but I'm afraid I'm greatly oversimplifying the procedure. I would greatly appreciate any advice anyone out there may have on the subject.

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  • \$\begingroup\$ There aren't dedicated pins for HDMI in USB-C cables. It's sadly not just a case of "relatively simple if I can find a wiring diagram". Additionally you can't feed 12V over USB-C without a proper power management circuit (voltage is negotiated). Laptops also aren't usually designed to source more than 15W over USB-C/Thunderbolt (5V,3A), so won't give you 12V anyway \$\endgroup\$ Mar 26 '20 at 23:19
  • \$\begingroup\$ Thanks for your help! Unless I'm mistaken, there is indeed a way to wire an HDMI straight into USB-C. I think it's called "HDMI Alt Mode for USB-Type C". I've been reading about it here: hdmi.org/spec/typec . I believe the point of the Alt mode is exactly so you can very cheaply make type-c to HDMI adapters without the need of a chip to translate the signals, however it's possible I'm completely misunderstanding the purpose of the alt mode. \$\endgroup\$ Mar 27 '20 at 13:32
  • \$\begingroup\$ the HDMI Alt Mode like many things with USB Type-C connectors is very much dependent on what the computer supports. It's not part of the USB standard, so computers aren't required to support it. It'll work only if your laptop/PC specifically supports it. \$\endgroup\$ Mar 27 '20 at 13:41
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I know HDMI over USB Type-C protocols have existed for a while now, so I imagine buying a USB Type-C pinout PCB and soldering a split HDMI cable to it should be relatively simple as long as I can find the wiring diagram. For the power, is it possible to simply wire a small power regulation board in-between the barrel connector and the Type-C pinout?

so you can very cheaply make type-c to HDMI adapters

In addition to what Sam said about some ports and cables and other things, this would be an enterprise-level effort of two-three engineer-year long, for seasoned engineers. Even if your host laptop does have the Alt-DP/HDMI functionality (most recent laptops with USB Type-C ports do), your monitor must implement full-scale Power Delivery functionality, because engagement into Alt-DP mode can happen exclusively by means of heavy exchange of special class of PD messages, not counting for negotiating for right power contract. So "soldering a split HDMI cable" would be the least of technical concerns (except careful impedance matching of differential transmission line pairs is you want any decent display resolution).

Even if you get a correct interface IC (something from RealTek like RTD2556UT or else) and passes their volume requirement to be able to get any readable documentation, you will need to program these chips with properly formatted VDMs (vendor-defined messages) with right data content. Given the "greatly oversimplifying" prerequisite level of this question, simply forget about this. Your best bet for making a DIY Type-C monitor is to buy a ready-to-go "DIY controller kit, something like this one from Aliexpress:

enter image description here

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  • \$\begingroup\$ Huh? USB-PD is not trivial, but having enough implemented to get it to work in one particular scenario is not super hard. The "monitor" side doesn't need to provide high power. You can bit-bang PD using an Arduino Uno, lol. I didn't know people used specialized ICs for that when a $0.50 MCU will do. It was no big deal IIRC. I made a home-made 100W power supply. I went paragraph-by-paragraph through the PD spec and scripted compliance tests. I didn't want to destroy my Mac. It worked on first try (I was diligent). Are you talking about something else? I'd expect min POC in a week or two max... \$\endgroup\$ Jan 27 at 0:38
  • \$\begingroup\$ "heavy exchange of special class of PD messages" I just checked, seems like the HDMI-specific bits can be pretty much sniffed and mostly replayed and at least one Mac seems to think there's an HDMI monitor attached to it, that also delivers full charging power the Mac can take. That's with a "straight" USB-C cable from Apple, not a USB-C-to-HDMI one. I didn't try particularly hard, just sniffed what went on between the Mac and a USB-C monitor. There was a Wireshark packet dissector plugin for USB-PD available online a year or two ago, maybe it's gone now, it sure helped. \$\endgroup\$ Jan 27 at 0:44
  • \$\begingroup\$ "simply forget about this" If this was for some commercial product then maybe I'd worry about schedule slips when homebrewing it, since scripting compliance test cases is a boring and life sapping job, but otherwise you seem to be making a huge deal out of a whole buncha nothing... The PD part is actually much easier since you don't have to reverse-engineer anything, whereas the HDMI USB-C alternate function is a bloody secret apparently. I wonder what they were thinking... Yes, it'd be a challenging project for a beginner, but for anyone experienced? I'm not particularly clever either. \$\endgroup\$ Jan 27 at 0:51
  • \$\begingroup\$ I've looked some more into it and basically HDMI alt mode is useless. Even my Mac doesn't really support it: the hub I had plugged into it acted as a DP alt mode sink, and then happened to negotiate HDMI alt mode with my monitor. Dratz. Nothing better than fooling myself, ha? :) Even though DP mode spec is not fully public, there's enough stuff out there (e.g. good quality BSD code for DP alt mode PD messages in Chrome EC) that implementing it is less hassle than HDMI alt mode, as long as you have DP signals that you can present over USB-C. The fastest way to that is to use an FPGA with DP IP. \$\endgroup\$ Jan 27 at 23:59
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Well, yes, but not in the way you describe. There are a few things you need to be aware of.

  1. while some USB-C ports have support for external monitors they are either DisplyPort or Thunderbolt, HDMI support comes by way of special (but not unobtainable) chips that can convert DisplayPort to HDMI (these chips are only one-way so check closely which one you have)
  2. while you can theoretically tap into the power of the USB-C PD (Power Delivery) spec, not all computers have full USB PD support in that most can only manage 5V out, while 9V/12V/20V output is usually only seen on USB-C chargers (the PD specs have four official voltages and several current ratings) You'll also need some interface chips so your monitor can tell your PC both which voltage and how much current it needs (NOTE: The PC can always say "NO, that's too much, I can't do that")
  3. Be cautious about Thunderbolt, Thunderbolt only speaks one language, Thunderbolt. If you specifically want TB support, well, tough, 20Gbit PCB design is about as straight forward as a heart transplant... BUT, Thunderbolt controllers have support for the DisplayPort over USB-C protocol (in which case, you're using DisplayPort over a different connector and not using Thunderbolt thus sidestepping the problem).
  4. Be aware, not all USB-C cables have all 4 high-speed differential pairs connected or support higher power loads (USB-C 2.0 only cables exist because... confusion is good right?), even if you had a monitor that natively supported DisplayPort, you'd still need lane switching chips (reversible connectors aren't magic after all) and some chips for negotiating an appropriate amount of power from the PC.

So while it's entirely possible to make a fully fledged USB-C monitor interface adapter, it's probably easier to get an active USB-C to HDMI adapter, rip out the guts and try to hook it into your LCD driver board. Unless your monitor only needs a few watts (then you can boost the default 5V up to whatever you need) you'll probably still have to do some jiggery-pokery to get the 12V you need though (at the end of the day you may have to keep the external power brick after all).

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  • \$\begingroup\$ Thanks a lot! I thought I read about the HDMI Alt Mode for USB-C (hdmi.org/spec/typec) and the whole purpose of it is to natively send an HDMI signal through type-c without the need of a chip to translate, but I could have totally misunderstood. Also, I think the advice to just get an active USB-C HDMI adapter is probably a good idea, but I've only ever seen power/signal sharing adapters send power to the upstream device, so it powers your laptop while plugged into TV/Monitor. Have you ever seen an adapter that powers the downstream device? \$\endgroup\$ Mar 27 '20 at 15:26
  • \$\begingroup\$ @GertBreynolds I've not seen adapters designed for downstream power, but I think the process of negotiating for power is independent of what kind of data you want to send so you'd need to add a new beefier cable and a USB PD interface chip but that's way easier than any of the alternatives. Yeah, the HDMI alt mode doesn't have support everywhere, in fact neither does DisplayPort alt mode or thunderbolt or USB 3.2 so about the only thing you can be certain of with a USB-C port is some kind of USB connection and some amount of 5V power, unfortunately everything else is optional... it's a mess \$\endgroup\$
    – Sam
    Mar 28 '20 at 2:38
  • \$\begingroup\$ The chances of your computer supplying more than 5 V @ 3 A from its USB-C ports is somewhere between slim and none. More than this requires USB-PD output support, USB 3.x and Thunderbolt support up to 15 watts. Getting 12 V from USB-C will be rare as well, this was in the early USB-C and USB-PD spec and is now deprecated. 5, 9, 15, & 20 V are standard. More than 3 A will require a 5 A rated USB-C cable, and more than 45 watts requires 20 V supply. \$\endgroup\$
    – MacGuffin
    Jan 28 at 7:54
  • \$\begingroup\$ Native HDMI is a supported USB-C alt mode but I have yet to see any computer or video card that outputs HDMI on USB-C. DisplayPort is all I see on USB-C. DP to HDMI conversion is certainly an option. Using a Thunderbolt or USB 3.x GPU also an option. Negotiating with the host for which video modes is supported is likely possible but no doubt complicated and messy. It's likely wise to see what your computer supports and build to that. \$\endgroup\$
    – MacGuffin
    Jan 28 at 8:06
  • \$\begingroup\$ Putting power and HDMI on USB-C is possible but would likely require some kind of dock between the computer and monitor to inject power and convert the video signal. That may defeat the purpose of the USB-C cable unless the purpose is to have power and HDMI on USB-C. \$\endgroup\$
    – MacGuffin
    Jan 28 at 8:07

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