The difference between PC-6400 and PC2-6400 is in the style of chip clocking. PC 6400 uses the DDR method to clock the chip whereas PC2-6400 uses the DDR2 method. DDR2 requires 4 transfers per clock tick while DDR only allows 2 transfers per clock tick. DDR2 has replaced DDR in new computers. PC-6400 is identical to PC2-6400 except for the timing difference. However, the timing difference dramatically affects the specifications of the memory.
Transfer Rate Differences
Because four transfers occur over the course of a single clock tick, the PC2-6400 theoretically doubles the transfer rate of the PC-6400. The practical data transfer speed does not quite live up to the theoretical promises, but it comes close. Therefore, in a theoretical sense, we can say that PC2-6400 is twice as fast as PC-6400.
Chip Size Differences
The data transfer rate of the PC2-6400 also has implications on the size of chips that can be used. Chip size is limited by the speed at which data can be accessed. When the speed of access doubles (as is the case between PC-6400 and PC2-6400), the amount of memory accessible by the bus increases respectively. Thus, the potential chip size of a PC2-6400 chip verses a PC-6400 chip is nearly doubled as well.
Error Rate Differences
Computers use check bytes at the end of data transfers to ensure that the data is transfered correctly. If the check byte does not match up with the data it is tagged with, the whole data chunk will be recycled. When this happens, the data has to be requested a second time in order to get the corrected data. The increased rate of transfers on the PC2-6400 also increases the fidelity of the data. Thus, the number of errors is decreased and the speed of recycled transfers is faster. These dual forces help PC2-6400 produce far less errors than PC-6400.
- Photo Credit Creative Crop/Digital Vision/Getty Images