So... ARE SDRAM and SODIMM ram interchangeable?

No.

Their architecture, voltage, and other parameters are different.

As are the computers that were designed for their use. Check

specifications against known databases before upgrading...

Good luck & happy computing! 🙂

According to what I can tell, using Mactracker.ca (current version download database of Mac specs) the 27" computer uses upt to (4) 204-pin SO-DIMMs of the specification shown in their information, not both kinds. And the specifications have to match. This means, in example, you have a buffered or non-buffered, it should match the correct nomenclature for the hardware as a minimum, and physically be also able to fit the logicboard. The chips are a different shape, size and have dissimilar details; they don't interchange.

•Example: iMac (27-inch, Mid 2011)

RAM information from MacTracker v. 7.2.1...

4 - 204-pin PC3-10600 (1333 MHz) DDR3 SO-DIMM

Total RAM capacity: 32 GB (Actual) 16 GB (Apple)

{While the iMac (21.5-inch, Mid 2011) can only use a total of 8GB of same type RAM in 4 chips}

•iMac How to remove or install memory:

http://support.apple.com/kb/HT1423

If you do an 'identify by serial number' search using online database, you can get very close to the build model year & any change in specification would be verified by the Serial Number. Here's one of three (links to active site) that I use: http://www.powerbookmedic.com/identify-mac-serial.php

Also http://everymac.com has a ton of information on their site. Takes a bit of time to check.

Some resellers have configuration pages but they aren't always correct. OWC - other world computing - is a mac specialist but I've never used them. A call to them about a specific machine is only as good as what you tell them. And these last three posts should've been in a new category, not in an 'answered' thread.

Or check with an Apple product specialist or genius in an authorized reseller near you. A copy of the build information from the retail box can be of help, it shows the kind of RAM the unit was supplied with.

Hopefully this helps somewhat.

Good luck & happy computing! 🙂

{PS: i've asked the hosts to move this into a new thread}

If you do not have Mactracker (see http://mactracker.ca )

can get it free from their site, download, etc, and look.

The pin count and voltage appear similar, and the build specs

should match. But as time goes on, a 204-pin chip may be

another 204-pin chip by function and by another name.

Yours is a 21.5 - inch iMac, or a 27 - inch? -edited to add.

I have computers that use both kinds in different slots, &

those are not interchangable; in some they apparently are.

Didn't use to be that way so very long ago. But then

times and economies have changed a little bit.

{What does the identify by serial number say your mac IS?}

The late 2011 model 21.5" does use 8GB

Total, according to MacTracker... Sorry.

Bet the GPU is a little different too.

Good luck & happy computing! 🙂

Without your serial number to run though databases online,

I figured the SO-DIMM would be good to go, and until I

re-searched the SDRAM chip specs, doubted the numbers.

The pin count is usally a little different, but these are kissin cousins

of a sort. There is a company in LA I've used for RAM and often

get samsung and other top brands in 1st class post for much less.

Hope that works out for your 27" iMac buddy... unless thats not it.

Good luck & happy computing! 🙂