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ATI Radeon HD 4870 X2: PCB Design

The Radeon HD 4870 X2 graphics card is a very sophisticated thing as it has to carry two GPUs with accompanying memory chips and power systems. A PCI Express switch must also be accommodated on the PCB. Designing such a graphics card is a daunting task but ATI’s engineers drew upon their earlier experience with the Radeon HD 3870 X2 and solved the problem brilliantly. Although the new card carries two RV770 cores, it is no larger than the single-processor GeForce GTX 280. Both are 27 centimeters long.


Of course, 27 centimeters is quite a lot. The new card won’t fit into all system cases. On the other hand, such top-performance solutions are not meant for compact cases actually. The Radeon HD 4870 X2 doesn’t look as massive as the GeForce GTX 280 because it doesn’t have a metallic casing as the latter. Interestingly, ATI’s dual-processor flagship is painted black rather than the company’s traditional red. This must have been done to make it look more impressive, an indication that this card belongs to the elite of the 3D hardware world.

As opposed to the GeForce GTX 280, the cooler of the Radeon HD 4870 X2 can be removed easily, giving you the access to the exciting internals of the new card. The overall component layout resembles the Radeon HD 3870 X2. The new card is in fact the result of the evolution of the latter.

Although the back part of the PCB is still empty, the power circuit has been revised and reinforced because the RV770 chip needs more power than the RV670. It incorporates two three-phase voltage regulators based on Volterra VT1165MF controllers that can often be seen in high-frequency voltage regulators of modern graphics cards. Each set of memory chips is powered by a separate regulator. As opposed to the 3870 X2, there is no place to reinforce the power circuit further.

The single Radeon HD 4870 is known to have a peak power consumption of 130 watts. So, the Radeon HD 4870 X2, which in fact combines two such cards on one PCB, should require two times as much, i.e. 260 watts. Therefore the 8-pin PCI Express 2.0 connector is a must. A high load is expected on this connector because the Radeon HD 4870 X2 won’t even start up, reporting power problems, unless you connect the appropriate PSU plug to it. The card can be made to work with two 6-pin power connectors but we wouldn’t recommend you to do that for the reasons explained below. The connectors are placed facing downward which makes it inconvenient to plug the power cables into the card when it is already installed into the system.

A PEX8647 chip from PLX Technology is responsible for data transfers between the two RV770 cores. It is a second-generation 3-port PCI Express switch supporting PCI Express 2.0 which means a double data-transfer rate. Like its predecessor PEX8547, the new switch supports peer-to-peer mode for direct data transfers between the GPUs, bypassing the root PCI Express controller in the mainboard’s chipset. Notwithstanding the improved specs, the new switch has a very low level of power consumption, only 3.8W, whereas its predecessor required about 5W at lower data-transfer rates. The new chip is also considerably smaller.

Besides using the switch, the GPUs can communicate via the CrossFire interface or the additional data-transfer channel called Sideport which provides a bandwidth of 5GBps in each direction (the Sideport is currently disabled on the software level). The Radeon HD 4870 X2 being in fact a mix of two Radeon HD 4870 cards, the GPUs are linked with a traditional CrossFire channel. There is also one CrossFire connector on the PCB to support quad-processor configurations. You can’t use more than two Radeon HD 4870 X2 cards in a single graphics subsystem due to technical reasons. And such a subsystem, if possible, would have tremendous power requirements!

The RV770 cores are placed on both sides of the switch. As opposed to the Radeon HD 3870 X2, the developer didn’t increase their clock rates relative to the single-chip card, obviously due to power consumption reasons. Thus, the GPUs are clocked at 750MHz like on the ordinary Radeon HD 4870. They have a standard configuration with 800 ALUs (grouped into 160 superscalar execution modules), 40 texture processors and 16 raster back-ends. The Radeon HD 4870 X2 thus has a total of 1600 ALUs, 80 texture processors and 32 raster back-ends. This should be enough to beat any single graphics card from Nvidia including the GeForce GTX 280 and GeForce 9800 GX2. Unlike the Radeon HD 3870 X2, the 4870 X2 doesn’t seem to have any theoretical bottlenecks. The only problem it may suffer from is the lack of CrossFire support in the drivers, but that’s the common problem of any modern homogeneous multi-GPU system from both ATI and Nvidia. In the worst case, the card will have only one of its GPUs working, but the RV770 is quite competitive to Nvidia’s G200-based solutions even alone.

The titanic computing capacity of the Radeon HD 4870 X2 is only one constituent of its expected triumph in games. The other constituent is the memory subsystem that should affect the card’s performance greatly in at high resolutions. The new card carries two sets of eight GDDR5 chips with a total capacity of 2 gigabytes, i.e. 1 gigabyte for each graphics core. The ordinary Radeon HD 4870 comes with Qimonda IDGV51-05A1F1C-40X chips. The dual-core Radeon HD 4870 X2 is equipped with Hynix H5GQ1H24MJR-T0C memory (1Gb chips, 32Mb x 32, 1.5V). The chips are rated for a frequency of 1000 (4000) MHz but are clocked at 900 (3600) MHz by the card. 3D applications can access 1 gigabyte of graphics memory since this is a homogeneous dual-processor solution. The Radeon HD 4870 X2 should not feel a lack of graphics memory especially as ATI’s solutions feature more efficient memory management than their Nvidia counterparts. The new card has a total memory bandwidth of 230.4GBps which is far higher than what Nvidia’s single graphics cards can offer. So, the Radeon HD 4870 X2 is an armed and dangerous rival to any opponent. It is virtually free from weaknesses.

The card has a standard configuration of connectors. It has two DVI-I ports with support for resolutions up to 2560x1600 pixels. Contrary to Nvidia’s SLI systems, the Radeon HD 4870 X2 provides full support for multi-monitor configurations. It also offers a universal 7-pin mini-DIN connector which is hardly necessary today, in the era of HDMI. As mentioned above, there is one CrossFire connector on the PCB so that you could use the card in a quad-GPU subsystem together with another Radeon HD 4870 X2.

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