Active vs Passive DisplayPort Adapter: Whats the Difference? Why does Eyefinity care?!

Recently the idea of having my third monitor back, inspired from my days with a dual output AGP working alongside a PCI video card, had me piqued. I am currently running a Radeon HD 5770, which like most current cards, come with multiple outputs of varying kinds. In my case I have 2x DVI, 1x HDMI and 1x DisplayPort for a total of four outputs. None of my monitors support DisplayPort. Simple, I’ll grab an HDMI connector and be done with it. Wrong. “No? What about those cheap passive adap-“ Nope.

There are limits imposed on most of the Radeon video cards. Lets take a look at what AMD Eyefinity FAQ states as basic rules:

1. The first two monitors can connect to the graphics card with any display output on your product: HDMI, VGA, DVI or DisplayPort.
2. The third (or greater) display must be connected to the graphics card via DisplayPort.
3. If your monitor does not have a DisplayPort connection, you will need an inexpensive active DisplayPort adapter for it. DVI to DP adapters can be had for less than $30 USD.
4. Every family of GPUs supports a different maximum number of displays.

Using my video card for an example. Why can’t I just add a third monitor with the HDMI, while the first two work with the two DVI ports? That comes down to “Transition-minimized differential signaling”… Here’s a blurb from Wikipedia to save the trip your curiosity is pushing to create:

Transition-minimized differential signaling (TMDS) is a technology for transmitting high-speed serial data and is used by the DVI and HDMI video interfaces, as well as other digital communication interfaces.

‘That’s nice, why does it matter” you say? Well you only have TWO* TMDS streams on your card. Running two monitors? Use the HDMI and DVI port? You’re good. Use the DVI and DVI port? You’re good. Use the HDMI and DisplayPort (True connection/Active or Passive adapter)… You’re good. In each case you would be using both streams up – except from using a true DisplayPort connection via a DisplayPort cable/device or an active adapter. When you toss on a PASSIVE adapter to the DisplayPort connector it is merely moving wires around with no signal type conversion (12/23/2012: after continued research this appears to only be the case with devices having the DisplayPort logo with two plus signs next to it). The video card adjusts the signal and occupies a TMDS stream to do so. Now you likely already understand what the Active adapter does, it mimics a TRUE DisplayPort connection by use of a internal chip – thus keeping both TMDS streams available to the HDMI/DVI/VGA ports. A passive adapter would only be useful in a two monitor setup where there are limited output types.

*One thing to note, those four commandments are not always the case! Video card specs will always have to be checked out to see what exactly is possible. Sapphire makes a Eyefinity enabled video card with three DVI ports which can all be used at once. Handy.

Do note that there is often confusion around Active adapters requiring a power source (external power or USB power) however not all active adapters require this. Though having those cables is likely a great indication that is not a passive adapter. As well, understand that resolution is limited to what DVI is normally limited to (1920×1200 @ 60HZ) when using a DisplayPort adapter!

Resources

This article was a mashup of several message boards and most importantly the Eyefinity FAQ. I did not find all this information in one location! If I am incorrect of anything above please let me know.

AMD Eyefinity FAQ
DisplayPort – Wikipedia
HDMI VS DVI
Displayport to DVI adapter
TomsHardware – General Dongle Education: Part 2