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

Chainsaw
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  • Maybe a more _limited_ question should be something like... is there some hardware able to handle at the same time ddr2 and ddr3? If hardware allows and you mount e.g. different speed memory module of the same kind, your motherboard should work with a clock accepted by both. Even because there are signals that wait until the slower one answers. Now if you want to mount on the same bus with different voltage you have a problem more... – Hastur Jun 20 '23 at 14:16
  • @Hastur, I've heard, that there are motherboards that can handle parallel working, but overall it is not popular. I am interested in a technical aspect of the limitation. – Chainsaw Jun 20 '23 at 14:18
  • @Hastur, and no, my motherboard doesn't support parallel too. – Chainsaw Jun 20 '23 at 14:19
  • Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. – Community Jun 20 '23 at 14:20
  • @Hastur, is different voltage a problem? I mean, can't just one type "adjust" to the "lower" possible voltage? – Chainsaw Jun 20 '23 at 14:26
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    There are any number of duplicates at https://superuser.com/search?q=ddr2+ddr3 such as https://superuser.com/questions/75402/is-a-ddr3-mb-compatible-with-ddr2-ram and https://superuser.com/questions/802962/ddr2-ddr3-co-mingle and https://superuser.com/questions/132320/using-ddr2-and-ddr3-together – Mokubai Jun 20 '23 at 14:31
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    https://superuser.com/questions/148027/what-will-happen-if-i-use-ddr3-and-ddr2-ram-simultaneously as well as https://superuser.com/questions/1127086/can-i-replace-a-ddr2-ram-module-with-a-ddr3-module and https://superuser.com/questions/431871/can-i-use-ddr2-or-ddr3-memory-on-my-computer – Mokubai Jun 20 '23 at 14:34
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    @Chainsaw regarding voltage the answer is no. While many electronic devices have an operational voltage range there are a lot of devices that have a very tight operating range, especially down at the speeds and voltages modern computers use. DDR2 may operate at 1.8V but it could have a +/- 0.1 (1.7 to 1.9V) range which would be incompatible with a device expecting 1.5V or 1.35V (DDR3) as it could cause damage or overdrive components. – Mokubai Jun 20 '23 at 14:44
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    @Chainsaw no. Different generations also tend to operate differently with regards to how they transfer data, what timings they have and how the work internally. You should read the duplicates. – Mokubai Jun 20 '23 at 16:09
  • Ok, thank you.. – Chainsaw Jun 20 '23 at 16:13

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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.

Hastur
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