Kingston or Crucial 256 MB sodimm for T21

[gss]

Will this Kingston module work on the T21?

http://www.newegg.com/Product/Product.a ... 6820172003

I read here on another post that this Crucial brand module works for the T21 2648 - http://www.crucial.com/store/mpartspecs ... E0A5CA7304

Only difference that I see is the cas latency, 3 for the Kingston and 2 for the Crucial.

Thanks for any help.

[Neil]

One of the customer reviews, right there on the newegg page you linked, says "Does not work with Thinkpad T22". If it won't work in a T22 it won't work in a T21. Which is exactly what I would expect from high density PC133.

[rkawakami]

The short piece of advice: When looking for 256MB modules for A20-A22 and T20-T22 systems, make sure that they have a total of 16 memory "chips" on them. Those are what's normally referred to as "low density" and it's what those laptops require.

The long explanation: You will see a lot of discussion about "low" and "high" density memory modules. This basically is only a real problem with PC100 systems like the T21. During the time that these computers were designed and marketed, a change was taking place in the DRAM memories that were being manufactured. 128Mbit (Megabit, not megabyte or MB) memories were giving way to 256MBit devices. One important change that was made during this transition is that the 128Mbit, or what we are now calling "low density" memory, required a refresh spec of 4096 (4K) cycles, while the larger 256Mbit (or "high density") part required a refresh spec of 8192 (8K) cycles. Since the chipset inside the T21 (and T20 and T22) was designed with 4K refresh parts in mind, you need to install modules built with 128Mbit devices. If you consider that a 128Mbit memory chip is really the same thing as 16MB (megabyte), then you can see that 16 of those parts will total 256MB (16 x 16MB). By the same token, a 256Mbit chip is actually 32MB so you would only need 8 of those to make a 256MB module.

The Crucial module you linked to may work but it would have to contain 16 memory components on the board. Out of the quoted specs:

Specs: SDRAM, PC133 • CL=2 • Non-parity • 133MHz • 3.3V • 32Meg x 64

"32Meg x 64" can be a little misleading sometimes. My understanding is that it's supposed to mean 32 Million (Meg) addresses by 64 bits of data. Since a byte is represented by 8 bits, we're talking about 32Meg addresses of 8 bytes (64 bits / 8 bits to a byte), or 256MB. Crucial says that it's the Component Configuration; "the size of the memory chip components on the module". However, that could mean it's built with a single bank of 32Meg address devices (if there's eight of them) OR two banks of 16Meg, if there are 16 devices on the module. Rather than rely on this potentially ambiguous definition, it's usually easier to ask the seller "how many memory parts are there on the module?" and rely on the following rules of thumb:

A 256MB module with 16 devices is "low density" and will work in systems like a 600X, T20-T22 and A20-A22
A 256MB module with 8 devices is "high density" and will NOT work in the systems mentioned above

ref: http://www.crucial.com/support/mem_type ... lue=choose