Mixed CrossFire

When reading through the documentation it was clear that the card has much in common with the 2900 series. The cards look a little similar with their dual slot coolers, they have the same amount of memory, use the same driver and feature the same CrossFire connector type... now surely that is just asking for someone to try and CrossFire up a 2900XT and 3870... Isn’t it?

Initially we installed the 3870 as the card in the primary PCIe connector and the 2900 XT in the second 16x PCIe slot (CrossFire bridge also installed). In this configuration both cards were seen by Windows and the ATI installer reported successful install of the Catalyst drivers. After rebooting we checked device manager and although the cards were both listed the 3870 had a warning message beside it and was not fully working.

Before we gave up on this idea we decided to have one last attempt and switched round the cards so that the 2900 XT was the primary and the 3870 was the secondary. Once again both cards were recognised by Windows and once again the ATI installer reported success on driver install. We rebooted and to our surprise device manager reported both cards as working.

A quick look in Catalyst Control Centre confirmed that it would not let us configure the cards individually but what would happen if we tried some gaming?

As shown in the graphs above, we did see performance increases over single card mode when using the mixed card CrossFire. In the case of Call of Duty 4 the gain was nowhere near that of true 3870 configuration but it did make our tested settings playable. World of conflict was much more impressive with a doubling of minimum framerates over a single 2900 XT.

We would imagine that this loophole will be locked down in future drivers but it was certainly interesting to see what was possible.