The nForce 680a SLI is the name of the chipset which NVIDIA has designed for Quad FX. The 680a is based on the nForce 500 series, and so it naturally shares many of the 500 series’ technical characteristics. In reality, the chips are nearly identical save for a few exceptions.

The new Quad FX platform relies on two 680a SLI chips on the same motherboard. With double the number of usual chipsets, as we have said many times before in this article, the two 680a MCPs combine with each other to support four PCI Express 16x electrical slots, 2 of which are only half bandwidth (8 lanes) on a 16x interface. One chipset handles the 32 bit PCI slot while the other chipset handles the PCI Express 1x slot. The coupling of the chipsets also allows for 12 3GB/s Serial ATA channels and up to four Gigabit Ethernet ports.

The nForce 680a chipsets support up to 12 Serial ATA channels. This feature allows the construction of complex RAID configurations. All 12 channels can be used in a single RAID array or alternatively, it is also possible to configure 4 independent RAID arrays.

In 4x4, the Athlon 64 FX processors communicate with each other using a coherent HyperTransport (HT) link. Both chipsets communicate with the processor using two HyperTransport links. The above diagram from NVIDIA is incorrect in the sense that it might lead people into thinking there is a designated processor for which the chipsets communicate with. There is no designated processor that specifically communicates with the chipsets. More realistically, one of the two processors connects directly to one of the two chips using a HyperTransport link.

As AMD is famous for, the memory controller is integrated directly into the CPU. Because of this the 4 memory modules communicate directly with the processor. Using the coherent HyperTransport link between the CPUs, the memory controller of one processor can be accessed by the other processor, thus allowing one CPU to access the other CPU’s data in its DDR2 bank. Data from the L1 and L2 caches can also be requested from the other processor.

The 680i SLI comes with NVIDIA LinkBoost technology. LinkBoost allows the frequency between the SPP and MCP (HyperTransport Link) to be increased by 25%. In addition, this technology also increases the communication speed between the SPP or MCP and the PCI Express bus by 25%.

In order for LinkBoost to properly work, it is required to use a LinkBoost certified video card. Currently, the only video cards that have proper certification are the GeForce 7900 GTX and the GeForce 8800. If these video cards are detected on either the MCP or SPP, then the clock speeds will automatically be increased.

The 680a SLI also supports SLI Memory. SLI Memory is officially known as EPP, or Enhance Performance Profiles. EPP memory was developed by NVIDIA and Corsair in an attempt to make the overclocking of system memory a less arduous task. EPP memory is only for use with certified platforms.

EPP allows users to overclock both their CPU and memory at the same time. In addition, EPP also handles all of the timings and other factors such as voltage. This can be a blessing for relatively new and inexperienced overclockers.

The last three major features in the 680a SLI all deal with networking. These features have been present in later chipset version of the NVIDIA nForce 5 and 6 series. FirstPacket prioritizes packets in order to allow packets to flow smoothly in the upstream bandwidth of a broadband connection. Less latency-sensitive applications are moved down the list in priority while higher latency-sensitive applications are prioritized higher.

Obviously, not every package can be of higher priority. Users are allowed to adjust applications and games and signal what priority they belong under. Networked games are a good example of a latency-sensitive application that would be treated with higher priority. FirstPacket also reduces the amount of dropped packets, helping to prevent stutters and jitters in VoIP transmissions or video conferences.

Using FirstPacket, the networked video game is treated with higher priority over the FTP transmission. Generally, FTP transmissions tend to be much less sensitive compared to video games.

DualNet is the second networking technology that is featured in the 680a SLI. As the name suggests, DualNet allows two Gigabit Ethernet ports to be used placed side by side in order to handle net transmissions.

The last major networking feature included is TCP/IP Acceleration. TCP/IP includes a dedicated network traffic accelerating processor which is built into the Gigabit Ethernet Controllers. Consumers are given full control over this feature via the NVIDIA Network Access Manager.