SD vs DDR vs DDR2 RAM

SDRAM
DDR
DDR2
DDR3


Voltage
3.3
2.4
1.8
1.5


Pin Count
168
184
240
240



DDR3 is the latest standard of memory, surpassing the aging DDR2 standard.

In the past there was EDO, SD, RDRAM and DDR types of memory. Here we will focus on the most recent 4.

There are a few physical differences involved, the most important is keying.

Keying
Keying refers to the notches on the bottom of the memory module. Both the memory module and the memory slot are keyed a certain way to keep the different types of memory from being put into slots other than the intended one.

In this image you can see that DDR2 memory has a key that is slightly offset from DDR.


Top: DDR, Bottom: DDR2

Here you can see the slot difference between DDR on the left and SD on the right:



Here is DDR2 are the leftmost slots followed by DDR. Note the slot order here is: DDR2, DDR, DDR, DDR2, DDR DDR



Voltage
Along with the pin count, the voltage is different. DDR2 is 1.8 volts, DDR is 2.5 volts, and the voltage for SDRAM is a whopping 3.3 volts.

Looking very closely at the memory slots th voltage is stated on each type of slot:


From Top Down: DDR, DDR2, SDRAM