Compaq BL10e - HP ProLiant - 512 MB RAM Overview page 20

Each stub-bus connection creates an impedance discontinuity that negatively affects signal integrity. In
addition, each DIMM creates an electrical load on the bus. The electrical load accumulates as DIMMs
are added. These factors decrease the number DIMMs per channel that can be supported as the bus
speed increases. For example, Figure 16 shows the number of loads supported per channel at data
rates ranging from PC 100 to DDR-3 1600. Note that the number of supported loads drops from eight
to two as data rates increase to DDR2 800.
Figure 16. Maximum number of loads per channel based on DRAM data rate
Increasing the number of channels to compensate for the drop in capacity per channel was not a
viable option due to increased cost and board complexity. System designers had two options: limit
memory capacity so that fewer errors occur at higher speeds, or use slower bus speeds and increase
the DRAM density. For future generations of high-performance servers, neither option was acceptable.
Future generations of servers require improved memory architecture to achieve higher memory
bandwidth and capacity. Consequently, JEDEC developed the Fully-Buffered DIMM (FB-DIMM)
specification, a serial interface that eliminates the parallel stub-bus topology and allows higher
memory bandwidth while maintaining or increasing memory capacity.
The FB-DIMM architecture has serial links between the memory controller and the FB-DIMMs, which
are connected in a daisy chain configuration (Figure 17). Relative to the memory controller, there are
ten outbound links and fourteen inbound links, also known as southbound and northbound links,
respectively. These serial links connect the memory controller to an advanced memory buffer (AMB)
chip that resides on each FB-DIMM, creating a point-to-point architecture. The outbound links transmit
commands and write data to the FB-DIMMs while the inbound links transmit read data back to the
memory controller.
The clock signal is distributed over a different set of pins. In addition to communicating over the
Outbound lanes, the memory controller communicates configuration information with each AMB over
the System Management bus (SMBus). The AMB is an intelligent chip that manages serial
communication with the memory controller and parallel communication with local DRAM devices.
Each AMB receives signals (address, write data, and command information) through the outbound
links and re-transmits the signal to the next FB-DIMM on the channel. Each AMB decodes the
command data and ignores the commands that are targeted for a different DIMM. The targeted AMB
performs a read or write operation to local DRAM devices through a parallel interface. In the case of
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