Let’s say we’re dealing with an adapter able to keep three display outputs well-fed, such as a Radeon HD 5870. In this configuration grid, you can see the connectivity options for a four-port Eyefinity card. The card integrates two DVI ports, one HDMI, and one DisplayPort. Despite having four physical ports, the Eyefinity implementation allows for three actual output streams. One port is always disabled.
Under the hood, we can see the reason why only three displays are supported. The 5000-series designs host two internal clocks and one external clock. Legacy displays—now defined by AMD as VGA, DVI, and HDMI—each require their own internal clock. The magic of DisplayPort in this context is that it allows users to drive multiple displays from only one external clock. This external clock is what allows Eyefinity to fuel up to six monitors from a single card. If the card design accommodates it, all six monitors could be based from the same clock. However, given the profusion of legacy displays today and the relative dearth of DisplayPort screens, Eyefinity implementations currently provide for two legacy displays.
From this diagram of the display engine behind Eyefinity, you can see how a four-port card could be alternatively configured. You could have one VGA port, one DVI, and two DisplayPorts, for example. With the proper adapters, your configuration options expand considerably. Converting from legacy to legacy, as with the DVI to VGA example in configuration #5 above, requires a passive adapter. Similarly, according to AMD, “if only one or two DVI/HDMI monitors are attached through a DisplayPort connector, then only simple passive adapters are required.” A passive adapter performs no signal conversion; it merely acts as a pass-through. However, going from DisplayPort to legacy Dual-Link DVI or VGA requires an active adapter.