Question Memory upgrade with different capacity dimms

[robburne]

I currently have 8gb in my Acer XC-605 desktop, configured as follows:

1. 4gb 12800 DDR3 1.35v
2. 4gb 12800 DDR3 1.35v

I want to add 8gb to give me 12gb in total. I purcahsed some Crucial branded memory with the same spec, I tried the 8gb in both slots with the exisitng 2 4gb dimms plus another 4gb and in some combinations it would POST, other times it begun to boot then reset.

So I ordered another brand of 8gb with the aame spec and I have the same problem. The only combination that will POST and boot are the 2 new 8gb dimm together.

Does this suggest that I can not mix the capacity of the dimms?

2 x 4gb DDR3 1600 memory in my desktop. I want to swap one of the 4gb module for an 8gb. I purchase some new memory of the same type (including voltage of 1.35v but the machine will not boot with the new memory combined with either of the 4gb slots. I then purchased a 2nd 8gb and this also will not work in any combination - however the 2 x 8gb work together.

Does the the memory capacity have to match in each slot?

Thanks in advance.

 

[Calvin7]

Memory is guaranteed in the form sold. Other combinations you decide to make have no guarantee to be compatible together. I suggest buying a kit of two.

 

[Karadjgne]

Ram and carpet have 1 thing in common. Lots. If you buy carpet, you always buy what you need in 1 shot, it's guaranteed to be the same Dye lot, same color. If you buy 2 different Dye lots, even if the carpet says it's the exact same color, it isn't and you won't see that until to lay it, then it gets really obvious where the seem is. Ram is exactly the same. You can have 2x identical kits, bought on same day, same shelf on same store, 1 on the hook right after the other, and they don't work together. Because the silicon that makes up the ram is from a different batch, with different properties and different impurities.

You only see the 5 primary timings, the 16-16-16-36 2T. What you don't see is the 50ish secondary and tertiary timings, those differ in every batch and its not hard to have enough difference to be incompatible.

If you want 8Gb total, buy 8Gb in 1 kit. If you want 16Gb total, buy 16Gb in 1 kit. If you insist on 12Gb, that's 2 kits and take your chances.

 

[robburne]

Thanks for the replies. The existing 2 x 4gb RAM is not a matched pair, one is Crucial the other is unbranded as far as I can see. So perhaps I just got lucky with those two? That being said there is no reason that two different capacity DIMM should work right?

 

[Karadjgne]

I've had 2 DIMMs with different brands, speeds, timings, capacity, IC counts and worked just fine.

I've had 2 DIMMs of exactly the same ram, they were only 9 numbers apart on manufacturing, so literally identical and made seconds apart, and total failure. 2 different kits.

When you buy ram in a package, kit, it's factory tested. The factory will have thousands of sticks to mix-n-match with until it gets a pair or quad to work. But it's guaranteed to work. When you add ram, you become the tester, so you could get lucky, or unlucky, could work directly or rma 100 times and still no joy.