DDRx incompatibility

Question

My question is - why I can't use ddr2 and ddr3 memory at the same time? What exactly makes them "incompatible"? Thank you.

Answer 1

Give it a read to Wikipedia on DDR3.

In general, modern computers already use different types of memory at the same time, but never, as far as I know, for the same function.

Many modern graphics cards have dedicated (private) DDR modules that differ from system ones. You have memory buffers for network cards, drives, there was in the CD and DVD recorder...

In the specific case DDR2 vs DDR3 there are motherboards that allow you to mount one or the other but not both together. The incompatibilities between those 2 generation of memories are too many.

Speaking loosely, the speeds to perform the different operations are different: not only the access and transfer of data to and from memory, but also the latency times.

Even when the system (the motherboard) allows memory banks of the same generation to work together but with different reaction-times, one ends up working with the slower, if not worse, bank clock.

Theoretically having to work different generations on the same bus, even just because of the different voltages, there would be problems of refractions and additional delays due to the introduction of an active element capable of causing the necessary voltage difference in both directions.

All this to achieve what? A system that at best will perform as the slowest of both types of memory? It has not been implemented.

When slots are both present, it is to reach a larger market share and let the end user choose.

Give it a look e.g.to this page to briefly see differences between successive memory generations.