PCB Design and Cooling System
All Radeon HD 4870 X2 cards are copies of the reference samples because they are all made under control and according to the specifications of ATI. The HIS HD 4870 X2 makes no exception as its only distinguishing trait is the sticker on the cooler’s fan.
The PCB is 27 centimeters long just as you could expect from a graphics card that has two graphics cores, two sets of memory chips, a PCI Express switch, and an advanced power circuit. Developing a similar product, Nvidia even used a dual-PCB design whereas ATI’s engineers limited themselves to a single PCB. The HIS HD 4870 X2 is meant for top-end gaming systems that cannot be compact, so there shouldn’t be any installation-related problems. Each GPU is powered by a dedicated three-phase voltage regulator based on a Volterra VT1165MF controller. The card has two power connectors, a 6-pin and an 8-pin one. An appropriate cable must be attached to the latter because its load is almost two times as high as the maximum load of the 6-pin connector (75W). Therefore any experiments with “cheating” the power circuit are not recommended.
A PLX Technology PEX8647 chip serves as a PCI Express switch here. It offers full support for PCI Express 2.0 and can establish direct communication between the GPUs in peer-to-peer mode. Two RV770 chips are installed on both sides of it. Each GPU is clocked at 750MHz and incorporates 800 ALUs, 40 texture processors and 16 raster back-ends. There is not much elbowroom for overclocking because the Radeon HD 4870 X2 generates as much as 260W of heat even at its default frequencies, its cooler barely coping with that.
Each graphics core has a dedicated 1GB bank of local graphics memory consisting of eight GDDR5 chips from Hynix (H5GQ1H24MJR-T0C, 1Gb capacity, 32Mb x 32). The chips have a rated frequency of 900 (3600) MHz and are indeed clocked at it on this card. The total amount of graphics memory that applications can access is not 2GB but only 1GB due to the specifics of the card’s architecture. 1 gigabyte is quite enough even for the most demanding of today’s games, though. But since the contents of each memory bank are duplicated, the total bandwidth of the memory subsystem is two times as high as that of the Radeon HD 4870, namely 230.4GBps against 115.2GBps. This is far higher than the memory bandwidth of any solution from Nvidia, including the GeForce GTX 280 with its 512-bit memory bus.
The card is equipped with a standard set of interfaces including two dual-link DVI-I ports, a universal 7-pin port for analog video output, and a CrossFire connector. The latter allows combining two Radeon HD 4870 X2 cards into a single graphics subsystem with a total of four GPUs. Such a subsystem should deliver unprecedented performance and we will check this out when we transition to 64-bit Windows Vista that removes all memory-related limitations. The RV770 incorporates an audio core, so you don’t have to connect to your sound card via S/PDIF. A DVI-I → HDMI adapter is enough.
The cooling system of the Radeon HD 4870 X2 is surprisingly simple for a card with a heat dissipation of 260 watts. It consists of two separate copper heatsinks, one of which has an evaporating chamber. These heatsinks cool the graphics cores whereas the other components such as the PCI Express switch, memory chips and load-bearing components of the power circuit are cooled with the aluminum frame of the cooler that has juts and elastic thermal pads at the appropriate places. A separate metallic plate cools the memory chips located on the reverse side of the PCB.
The heatsinks are cooled by a 12W fan from NTK Technologies (CF1275-B30H-C004). It doesn’t work at its full speed as the noise would be just unbearable then. The hot air is exhausted through the slits in the card’s mounting bracket. The plastic casing of the cooler has a sticker warning you about the high temperature of the metallic parts. Indeed, this precaution is necessary as those parts become scorching hot at work.
Generally speaking, the cooler of the HIS HD 4870 X2 (and of any other Radeon HD 4870 X2) illustrates the dead end the graphics card makers have found themselves in when they need to dissipate over 200 watts of heat while staying within the constraints of the dual-slot form-factor. We guess a single copper heatsink reinforced with heat pipes would be more efficient, but its weight would be too high. Alternative coolers are unlikely to appear in mass quantities due to the dual-chip design of the card although some makers, such as ASUS and CoolIT, have already announced liquid cooling systems for Radeon HD 4870 X2.