DDR-1 DIMMs
DDR-1 DIMMs require 184 pins instead of the 168 pins used by standard SDRAM DIMMs. DDR-1 is versatile enough to be used in desktop PCs or servers. To vary the cost of DDR-1 DIMMs for these different markets, memory manufacturers provide unbuffered and registered versions. Unbuffered DDR-1 DIMMs place the load of all the DDR modules on the system memory bus, but they can be used in systems that do not require high memory capacity. Registered DDR-1 DIMMs (Figure 11) place only one load per DIMM on the memory bus, regardless of how many SDRAM devices are on the module. Therefore, they are best suited for servers with very high memory capacities.
Figure 11. The 184-pin DRR-1 Registered DIMM. The DDR-1 DIMM has one notch instead of the two notches found on SDRAM DIMMs.
Backward compatibility
Because of their different data strobes, voltage levels, and signaling technologies, it is not possible to mix SDRAM and DDR-1 DIMMS within the same memory subsystem.
DDR-2
DDR-2 SDRAM is the second generation of DDR SDRAM. It offers data rates of up to 6.4 GB/s, lower power consumption, and improvements in packaging. At 400 MHz and 800 Mb/s, DDR-2 increases memory bandwidth to 6.4 GB/s, which is 800 percent more than original SDRAM. DDR-2 SDRAM achieves this higher level of performance and lower power consumption through faster clocks, 1.8-V operation and signaling, and simplification of the command set. The 240-pin connector on DDR-2 is needed to accommodate differential strobes signals (Figure 12).
Figure 12. The DDR-2 DIMM has a 240-pin interface