I have a laptop PC with an HDMI port on it, and an external monitor with a DisplayPort, and would like to connect the two. I am finding that some of the cables that feature the right connectors for the job, like this one, will not actually work because the cable only moves data in one direction, and the HDMI and DisplayPort plugs are are on the wrong ends of the cable for my needs.
What’s going on inside cables like this that only allows them to work in one direction and not the other?
Unidirectional cables have built-in amplifiers, filters, or other devices that mitigate signal loss over long distances. Note that one end of the cable is larger than the other.
DisplayPorts and HDMI use different signals and different standards.
The cable you linked to relies on the device with the Display Port to support DisplayPort dual-mode (DP++). What that means is the device actually detects that an HDMI (or DVI) adapter is attached and sends the appropriate signals rather than DisplayPort signals.
Better explained in this DP++ section of this wikipedia page.
As far as I am aware you cannot go from HDMI to DisplayPort without an external converter (and they are not cheap…like $300). At that price you are well within thinking of getting a different monitor or video card.
The HDMI specification does not support the DisplayPort LVDS signal type, and if the HDMI TMDS is plugged into a DP monitor, it will not work. The DisplayPort receptacle on a monitor or display will only accept the LVDS 3.3v DisplayPort signal type. SOURCE
Here’s one for about $30 - but as you correctly state, cables with HDMI in, DisplayPort out, require circuitry to convert the signal.