How might two memory modules with equal specs differ? In any ways that matter? This is a matter of curiosity, as I've already made the purchase.

I bought my daughter a laptop (Dell Latitude E5400) for Christmas, and I wanted to give her at least 3 Gb RAM. Dell wanted $110 to go from 2 Gb to 4Gb, way too much, so I opted to pay $10 to get the 2 Gb on 1 stick instead of 2.

I looked for a second 2 Gb module using Dell's memory specs, Crucial's website, and Kingston's. The specs are: 200-pin SODIMM, 2048 Mb, DDR2-800 (PC2-6400), Unbuffered, Non-ECC, 1.8V, CAS Latency 6 (CL=6), 256 Meg x 64 chips.

I should be able to use any memory module that meets those specs, right? NewEgg shows a Kingston and a Crucial with those exact specs, and when I look these up on the Kingston & Crucial sites, they show lots of compatible laptops but no Dells. When I look up the Dell E5400 on either Crucial or Kingston to find their recommended memory, then look up that memory for compatible systems, they show only Dells.

What's going on here? Do Kingston & Crucial have identical memory modules with different part numbers for different markets? Or is there some other factor, not expressed in the specifications listed above, that would affect compatability?

As mentioned before, this is for curiosity sake only. The Crucial (with the website's compatability guarantee) was the same price as NewEgg, so I ordered it. This is not the always the case (I researched memory for my T61 in the same way, found very different prices).

Anything other than perfectly matched memory sticks is asking for it. It might work, it might not. I would never try it unless using memory that could easily be returned if it didn't work.

The issue with mismatched memory is timing -- computers "interleave" memory for storage efficiency. Even identically-specced memory may have memory timing differences that, while within the spec, will cause errors.

In my memory-buying experience, matching the memory technology is sufficient. In this case, you'd want to make sure to get DDR2-800 (PC2-6400) memory. Even something faster won't work because of the interleave mentioned above. I've never had a problem as long as the interface technology (DDR2 in this case) and speed (800 MHz) match. I believe that the PC2-6400 is just a specifier, and I'm not sure what it does, but I've always made sure it matches as well.

Of course, the package has to fit, and that's the 200-pin SODIMM portion.

Cheers.

Also remember that unless you have 64-bit Windows, you won't see the full 4GB showing up. It'll be somewhere around the 3-3.25GB range. A function of 32-bit Windows and RAM capacity.

As long as the major specs match up, i.e. DDR2, SO-DIMM 200 pin and 1.8V, then it should work. You can have mismatches in the minor specs like the frequency and timing. The memory controller will just use the slower module's timing in order to use both memory modules.

Those twin pack matched memory are optimized for dual channel operations, but not required. Today's memory controllers can support asymmetric dual channel mode so you can gain some of the dual channel mode speed boost even with differently sized modules.

Crucial and Kingston used to have different part numbers for each system, even though they ship you a common module that can be used on different systems. It was one of the requirements for their compatibility guarantee.

*USUALLY*.

I've had a mismatch that was driving me nuts trying to diagnose as I didn't have the timing info to realize the problem.

When I finally gave up on the board because it wouldn't run reliably with two sticks and got a new board I found out what was up--the new board immediately yelled about one of the sticks not being quite up to snuff.

The original board perfectly happily went ahead and set things wrong--just far enough out of spec that it made the machine unstable.