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I had two adapters with me, a Mini DP to HDMI and a HDMI to VGA adapter. I connected my Macbook Air's display out with the Mini DP to HDMI adapter, which was in turn connected to a HDMI to VGA adapter. Sure, I could buy a separate MiniDP to VGA adapter but I was wondering if this would work or not. It didn't.

Can someone shed light into why this combination does not work?

  • Was the HDMI to VGA adapter in question really a HDMI to VGA adapter or perhaps a VGA to HDMI adapter? Did you supply power to the adapter? – Daniel B Jun 16 '16 at 17:28
  • None of the adapters use a power supply. The adapter I have is HDMI (male) <-> VGA (female). – Tanmay Jun 19 '16 at 10:26
  • Because HDMI to/from DVI requires an active conversion, power is required. Sometimes, the power available on a regular connection isn’t enough. Can you perhaps identify the exact make and model of your HDMI-VGA converter? – Daniel B Jun 19 '16 at 10:50
  • @DanielB HDMI <-> DVI does not need active conversion. They both use TMDS. DP <-> HDMI/DVI does. However, DP source ports are often actually DP++, which have TMDS signal output as well. – Tom Yan Jun 19 '16 at 12:37
  • @TomYan You’re right, of course. That was a typo of sorts, meant to write “HDMI to/from VGA”. – Daniel B Jun 19 '16 at 12:55
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VGA is analog port using basically 3 signals. When signal is converted between HDMI, DVI and VGA the same 4 contacts are used (see picture): enter image description here

At the same time these signal are missing in Mini DP pure connector. http://pinouts.ru/Video/mini_displayport.shtml

So, additional circuit is required inside the adapter to convert signal levels from DP to VGA. Like this one: https://en.wikipedia.org/wiki/DisplayPort#/media/File:DP_to_DVI_converter_unmounted.jpg

The chip on the board converts the voltage levels generated by the dual-mode DisplayPort device to be compatible with a DVI monitor. Another is required to convert signals to TTL VGA levels.

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    I’m sorry, but that’s not correct. Those are the DCC pins. They don’t carry image data but rather metadata about the display. – Daniel B Jun 16 '16 at 17:26
  • @ Daniel I know that CLK/SDA do not transfer the image data, RGB analog signals are used for that in VGA and TMDS data used for that in HDMI/DVI/DP. I meant that signaling levels are different because CLK/SDA pins are missing in DP, TMDS clock is used instead. – Hardoman Jun 16 '16 at 17:38
  • They aren’t missing though. DDC is still used all the same. It is entirely (really!) unrelated to the actual image transmission. – Daniel B Jun 16 '16 at 17:40
  • VGA has 5 signals, not just three. What's the purpose of showing a DVI connector, when it's not part of the question? There's no usage of the "same 4 contacts" as you claim between VGA and HDMI; they're completely incompatible interfaces. The conversion between such interfaces involves a lot more than the "convert signal levels" and/or "voltage levels". that you allege. – sawdust Jun 17 '16 at 0:02
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Where @Hardoman description is completely accurate, the shorter answer is simply that you can't magically convert analog signalling to digital. VGA adapters typically only carry the existing analog signals to a display that supports analog inputs. (on the same cable that could do digital) Display-port (and Mini-DP) are digital-only. There is no Analog support. So, you'll never find a passive VGA/Mini-DP adapter. There are some active converters that will essentially take the analog signal, and process the data to produce a digital signal, but these generally are expensive, and only work for a limited set of resolutions/sync rates/etc...

  • The OP has an HDMI to VGA which, presumably has (read: it must have), an active DAC. – Yorik Jun 16 '16 at 16:53
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There are several possible issues.

Instead of "adapters", you need to use converters (with active circuitry).
If either or both "adapters" are bogus adapters (passive devices consisting of wire and connectors), then the connection cannot work.

If the DP connection requires HDCP, and the HDMI-to-VGA converter is of the cheaper variety that doesn't support HDCP, then the connection won't work.

If the DP-to-HDMI converter and/or the HDMI-to-VGA converter do not have external power supplies, then the converters may be starved for power if they try to use line power, and connection won't work.

If you're using a display resolution that the HDMI-to-VGA converter does not support, then the connection won't work. Usually only a subset of 480p, 1024x768, 720p, 1080i and 1080p is supported.

In theory it could work, but you have to use the proper converters and valid resolution.

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