How2Day: DDR2 Memory Specifications

The gold stripes at the bottom of DDR2 RAM modules are called pins. Hemera Technologies/AbleStock.com/Getty Images
DDR2 memory was first marketed in 2003. Initially, DDR2 had poor latency speed, and was not immediately embraced by computer manufacturers despite its faster clock speeds. In late 2004, DDR2 began to be used widely in computers and electronic devices. Although DDR3 was released in 2007, DDR2 continued to be utilized frequently in new computer systems.

DDR2 stands for double data rate two. DDR2 is a kind of SDRAM. RAM (random access memory) is the memory a computer uses to open and run programs; it does not store things permanently like a hard drive. DDR runs at double the front side bus (FSB) speed, and DDR2 runs at twice the rate of DDR or quadruple the FSB speed. The FSB is the external data bus that connects the RAM modules to the CPU. DDR2 is packaged in DIMMs or dual inline memory modules.

The DDR2 name for RAM modules is based on the frequency of the modules. DDR2 comes in several frequencies. DDR2-400 combined with a motherboard FSB speed of 100MHz runs at 400MHz. DDR2-533 is used with systems with FSB speeds of 133MHz, and has a frequency of 533MHz. DDR2-667 combined with a motherboard FSB speed of 166MHz has a frequency of 667MHz. DDR2-800 is utilized with a FSB speed of 200MHz, and has a frequency of 800MHz. DDR2-1066 when combined with a FSB speed of 266MHz has a frequency of 1066MHz. DDR2-1200 used with a FSB speed of 300MHz has a frequency of 1200MHz. DDR2-1333 combined with a FSB speed of 333MHz has a frequency of 1333MHz. DDR2-1466 is used with a FSB speed of 366MHz for a frequency of 1466MHz. DDR2-1600 is combined with a FSB speed of 400 giving it a frequency of 1600MHz.

DIMMs also have a "PC2" code number; "PC" stands for "personal computer" or an IBM-compatible system, and the "2" refers to DDR2. This code is based on the RAM module's data transfer rate or DTR. This number is the RAM's frequency multiplied by eight bytes per minute which is the base DTR. The PC2 code is based on megabytes per second transfer rate (rounded to the nearest hundred). Therefore, DDR2-400 is also known as PC2-3200; DDR2-533 is also called PC2-4300; DDR2-667 is also known as PC2-5300; DDR2-800 is also called PC2-6400; DDR2-1066 is known as PC2-8500; DDR2-1200 is known as PC2-9600; DDR2-1333 is also called PC2-10600, DDR2-1466 is also known as PC2-11700; and DDR2-1600 is called PC-12800.

DDR2 modules come in a variety of capacities, including: 8GB, 4GB, 2GB, 1GB, 512MB, 256MB, and 128MB. The density of each module depends upon the number of memory chips per module and the capacity of each chip. Therefore, DDR2 modules can be high or low density. Low density DIMMs require more memory chips and are generally more expensive than high density modules.

Latency is a measure of how long it takes RAM to respond to the CPU memory controller's requests. Column Address Strobe (CAS) latency or CL is rated in clock cycles. A CL3 rating means that the memory controller must wait three clock cycles before the RAM responds to the request. Most DDR2 has latency ratings of CL3, CL4, or CL5.

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