Datasheet

Identifying Purposes and Characteristics of Memory 51
With 84 pins per side, this makes 168 independent pins on each standard SDR module, as
shown with its two keying notches as well as the last pin labeled 84 on the side shown in
Figure 1.30.
FIGURE 1.30 An SDR dual inline memory module (DIMM)
The DIMM used for DDR memory has a total of 184 pins and a single keying notch,
while the DIMM used for DDR2 has a total of 240 pins, one keying notch, and possibly an
aluminum cover for both sides, called a heat spreader, designed like a heat sink to dissipate
heat away from the memory chips and prevent overheating. The DDR3 DIMM is similar
to that of DDR2. It has 240 pins and a single keying notch, but the notch is in a different
location to avoid cross-insertion. Not only is the DDR3 DIMM physically incompatible
with DDR2 DIMM slots, it’s also electrically incompatible.
Figure 1.31 is a photo of a DDR2 module. A matched pair of DDR3 modules with heat
spreaders, suitable for dual-channel use in a high-end graphics adapter or motherboard, is
shown in Figure 1.32.
FIGURE 1.31 A DDR2 SDRAM module
RIMM
Assumed to stand for Rambus inline memory module, but not really an acronym,
RIMM is a trademark of Rambus, Inc. and perhaps a clever play on the acronym
DIMM, a competing form factor. A RIMM is a custom memory module that carries
DRDRAM and varies in physical specification, based on whether it is a 16-bit or 32-bit
module. The 16-bit modules have 184 pins and two keying notches, while 32-bit mod-
ules have 232 pins and only one keying notch, reminiscent of the trend in SDRAM-to-
DDR evolution. Figure 1.33 shows the two sides of a 16-bit RIMM module, including
the aluminum heat spreaders.
86498book.indb 51 7/22/09 5:37:44 AM