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A SO-DIMM, SODIMM, or small outline dual in-line memory module, is a type of computer memory built using integrated circuits. SO-DIMMs are a smaller alternative to a DIMM, being roughly half the size of regular DIMMs.
SO-DIMMs are often used in systems that have limited space, which include laptops, notebooks, small-footprint personal computers such as those based on Nano-ITX motherboards, high-end upgradable office printers, and networking hardware such as routers and NAS devices.
Most types of SO-DIMMs can be recognized visually by the distinctive notches that "key" them for different applications:
- 100-pin SO-DIMMs (SDR SDRAM) have two notches.
- 144-pin SO-DIMMs (SDR SDRAM) have a single notch near the center.
- 200-pin SO-DIMMs (DDR SDRAM or DDR2 SDRAM) have a single notch nearer to one side. The exact location of this notch varies (read below).
- 204-pin SO-DIMMs (DDR3 SDRAM) have a single notch closer to the center than on 200-pin SO-DIMMs.
- 260-pin SO-DIMMs (DDR4 SDRAM), 69.6 mm (2.74 inches) wide and 30 mm (1.2 inches) high, making them 2 mm (0.079 inches) wider than DDR3 SO-DIMMs, with a single notch after pin 144.
- 260-pin SO-DIMMs (UniDIMM), 69.6 mm (2.74 inches) wide and 30 mm (1.2 inches) or 20 mm (0.79 inches) high, with a single notch placed differently than on DDR3 SO-DIMMs.
Variants of 200-pin SO-DIMMs
The 200-pin SO-DIMM can be of the types DDR and DDR2. In both cases the notch is located at one fifth of the board length (20 pins + notch + 80 pins), but in DDR2 the notch is located slightly closer to the center of the board. These two types of memory are not interchangeable. The different notch locations are designed to prevent cross-installation, aiding at the same time in visually identifying 200-pin SO-DIMM modules.
This variance is designed to prevent coupling of memory and controllers of different DDR generations due to electrical incompatibility. DDR SO-DIMMs operate at 2.5 V while DDR2 SO-DIMMs operate at 1.8 V.
Variants of DDR3 SO-DIMMs
These modules, while using the same physical layout, may differ in the specified voltage – 1.5 or 1.35 volts. The lower voltage modules are sometimes, but not always, identified as DDR3L – see also DDR3 SDRAM.
200-pin and 204-pin SO-DIMMs are 67.6 mm (2.66 inches) long, 31.75 mm (1.250 inches) wide, having a maximum total depth of 3.8 mm (0.15 inches).
SO-DIMMs are nearly equal in power and voltage rating to DIMMs; SO-DIMM technology does not mean lower performance compared to larger DIMMs. For example, DDR3 SO-DIMMs provide clock speeds such as 533 MHz (1066 MT/s, PC3-8500), CAS latencies such as 7, and higher capacities such as 4 GB per module.
DDR2 SO-DIMM memory modules commonly have clock speeds from 200 MHz upward (specifications). 204-pin SO-DIMMs can also contain DDR3 SDRAM, with specifications such as PC3-6400, PC3-8500, PC3-10600, PC3-14900 and PC3-17000. 260-pin UniDIMMs contain DDR3 or DDR4 SDRAM depending on configuration. Some SO-DIMMs provide ECC functionality; unbuffered ECC SO-DIMMs are called SO-CDIMMs.
Inserting and removing SO-DIMMs
SO-DIMM modules and slots are keyed, so modules can only fit in one orientation. Simply flip the module over if needed to align. To insert a SO-DIMM module into the slot, gently insert it at approximately a 30-degree angle upwards, until the module's metal contacts disappear into the slot. Then, gently press the module down until it clicks into place. On both sides, the slot's metal arms will spread automatically and click into the side notches on the SO-DIMM module. To remove the SO-DIMM module, gently spread the two metal clips outward (away from the module). The SO-DIMM will be released and "pop up" at the same 30-degree angle and can be pulled out. Inserting and removing SO-DIMMs requires only gentle force, and no tools should be needed.
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