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

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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.

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This is a close up of the above mini display port to display port cable. It works fine.
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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.
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I wondered if the adapter is bad, so I got another one. It still does not work. Any ideas?
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Best Answer

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

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

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