Why does a HDMI cable not work, but a Display-Port cable does work?

Question

I have a Macbook Pro connected to a Dell P2714H IPS 27-Inch monitor. It connects with a mini display port to display port cable. (The monitor has only a large display port connector, it does not have a HDMI connector.) It works fine.

This is a close up of the above mini display port to display port cable. It works fine. I would prefer to connect with HDMI. The reason is so that I can put my HDMI switch in the middle, and be able to switch the monitor to the macbook or windows pc.

So I got a mini display port to HDMI cable + a HDMI to display port adapter. It does not work. It all seems to fit fine, but the monitor is blank.

I wondered if the adapter is bad, so I got another one. It still does not work. Any ideas?

Solution

So I got a mini display port to HDMI cable + a HDMI to display port adapter. It does not work. It all seems to fit fine, but the monitor is blank.

Because it isn't an "HDMI to DP" adapter, but a "DP++ to HDMI" adapter, which picks (ONLY) the TMDS signal (what HDMI uses) output from a DP++ source to an HDMI sink (e.g. monitor).

So I got a mini display port to HDMI cable

And this is pretty much an DP++ to HDMI "adapter" as well.

So as you could have imagine, your connection is something like this:

(Mac) DP++ --> HDMI|HDMI <-- DP++ (Monitor)

And obviously it's only natural that it won't work.

P.S. IF the mini-DP port of your Mac isn't a DP++ port (though it should be), your mini-DP to HDMI cable won't work either even if you connect the HDMI side directly to an HDMI monitor. In that case you will need an active (i.e. involves signal conversion) mini-DP to HDMI adapter.

Ref.: https://en.wikipedia.org/wiki/DisplayPort#Dual-mode

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To convert HDMI/TMDS signal to DP signal "again", you will need an active HDMI to DP adapter (which seems to be rare).

Comments

[Mokubai]

The second HDMI to displayport adaptor probably needs some amount of power as DP to HDMI is going to be an active conversion process (rather than connector conversion as the case would be from certain other connections) and there is likely not enough power being sent out of your first adaptor to power the second one.

[Argonauts]

I would try an HDMI to DVI adapter to see if that worked. There are a few articles related to your issue that resolve the no video problem by forcing the monitor from display port 1.2x to dp 1.1. Your model doesn't appear to provide this setting. Its possible its a power issue but these are very low voltage signals, and I'm skeptical.

[Tom Yan]

Although I posted an answer, I don't really understand what you are trying to do at all. For example, what exactly do you mean by HDMI switch in the middle? I would prefer to connect with HDMI. Then isn't the mini-DP to HDMI cable alone enough? Why are you trying to convert the HDMI to DP again?