ATI Radeon X1800 XT CrossFire Evaluation
On the surface, the Radeon X1800 XT CrossFire Edition Master card looks just like the standard X1800 XT. The card's share the same dual-slot cooler and red PCB, and are exactly the same size, but underneath the Master Edition's cooling apparatus lies a group of chips that make CrossFire a reality.
Like the standard edition, the Radeon X1800 XT CrossFire Edition Master card is fairly long and may have clearance issues in some systems. Its 16-pipe Radeon X1800 XT GPU and 512MB of GDDR3 memory populate the front side of the PCB, and are adorned with a dual-slot copper/aluminum hybrid cooler. The cooler exhausts heat from the system through vents in the mounting plate, which helps keep internal temperatures down when running a pair of cards in single system.
The GPU on our Master Edition card was clocked at 625MHz and its memory was clocked at 720MHz (1440MHz DDR), just shy of the 625MHz / 1500MHz of the standard Radeon X1800 XT. The 30MHz memory clock disparity should have a minimal effect on performance, so we won't dwell on it here. And CrossFire doesn't require a matched-pair of video cards to function, so the difference in memory clock speed shouldn't pose a compatibility problem either. In fact, the Radeon X1800 XT CrossFire Edition Master card will work alongside a Radeon X1800 XL as well. In this type of configuration though, half of the Master Card's RAM gets disabled, and users are required to re-boot their systems to enabled CrossFire (no re-boot required with X1800 XT CrossFire). We tested an X1800 XL / X1800 XT CrossFire configuration on the proceeding pages, but we question the value of spending $600 on an X1800 XT Master Card, only to have half of its frame buffer memory disabled anytime a user wishes to enabled CrossFire.
To bring CrossFire to the X1800, ATI put together a new compositing engine, that's similar but superior to the one used on the older Radeon X850 XT Master cards. If you remember, because X850 cards were equipped with single-link DVI outputs, X850 CrossFire was limited to a max resolution of 1600x1200 with a lowly refresh rate of 60Hz. Graphics cards in the X1K family of products are equipped with dual-link DVI outputs, however. Having dual-link DVI outputs means more bandwidth, which the new compositing engine capitalizes on to offer higher-resolutions than last-generation's CrossFire implementation.
The compositing engine on the Radeon X1800 XT CrossFire Edition Master card consists of a handful of chips. The biggest chip in the group, in the middle of the picture, is a Xilinx Spartan XC3S400 FPGA. The XC3S400 is a higher-end FPGA (Field Programmable Gate Array) than the one ATI used to enable CrossFire on the X850 XT. The XC3S400 is the chip that's programmed to do the actual compositing work. In total, this chip has ~ 400K logic gates on board, which is fairly low-end by today's standards for an FPGA. And a cost of somewhere south of $7, the overall retail price point of the board isn't affected too adversely. We should note that it's upgradeable via firmware as well, so ATI could theoretically incorporate more features into X1800 CrossFire moving forward. To the left of the Xilinx FPGA is the flash ROM chip, that actually contains the necessary programming and configuration code.
Above and to the left of the Xilinx FPGA in the picture, are a pair of Silicon Image SiI 163B TMDS receivers. These are the chips that receive data from the slave card. The "data" is the information being transmitted from the slave card's dual-link DVI output and on through the custom dongle pictured above. The Silicon Image SiI 163B TMDS receivers are clocked at 165MHz and are capable of processing images at a resolution of up to 1600x1200 @ 60Hz. But because there are two of them and they work in tandem, the maximum resolution is doubled to 2560x1600. The three smaller Silicon Image chips to the right are a pair of SiI PanelLink TDMS transmitters and an Analog Devices RAMDAC, which then power any displays connected to the output on the CrossFire dongle.