How FB-DIMM Memories Work

Motherboard Design

There are several advantages in using FB-DIMM memories on servers. We have already mentioned two of them: greater memory capacity and higher performance. There is also another great advantage: the simplification in motherboard design, since the path between the chipset and the memory sockets uses fewer wires.

Even though FB-DIMM uses standard DDR2-DIMM sockets, which have 240 pins, it actually uses only 69 of these pins, making it easier to make the motherboard routing (design of the wires on the printed circuit board).

In Figure 2, you can see the necessary routing to connect the north bridge to the DDR2 sockets using just one memory channel. A three-layer printed circuit board is needed.

Figure 2: DDR2-DIMM routing, one channel

In Figure 3, you can see the necessary routing to connect the north bridge to the FB-DIMM sockets using two channels. It not only uses fewer wires but also uses only two layers from the printed circuit board, making the motherboard simpler to design. (The motherboard still needs to use a three-layer PCB; the third layer is used for power wires). Notice FB-DIMM using two channels utilizes fewer wires than DDR2-DIMM using just one channel.

Figure 3: FB-DIMM routing, two channels