iMac 27 2020 DIMM memory slots

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iMac 27 2020 DIMM memory slots

I ordered the 2020 iMac 27 which already shipped. It comes with 8gb RAM which I'm fairly certain fills two memory slots with 4gb DIMM modules. I have already purchased two OWC 16gb modules which will give me a total of 40gb, more than enough for my needs.

While I'm pretty sure I can put the 16gb modules in the empty slots, I'm want to be sure the memory is optimized as far as location. Apple's installation instructions - Install memory in an iMac - Apple Support - are not helpful to me. This is what it says:

If your DIMMs are mixed in capacity, minimize the capacity difference

between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when

possible.

What does that mean? Below is the diagram of the memory slots. Can anyone definitely say what is the best location for all four DIMMs, two at 4gb and two at 16gb? Elsewhere I've seen it suggested pairs should go in slots 1 & 3 or 2 & 4 which seems the opposite of Apple's statement if I'm reading it correctly.

Posted on Oct 3, 2020 2:17 PM

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Answer 1

Top-ranking reply

den.thed

Level 10

92,302 points

Oct 11, 2020 12:43 PM in response to Ranger.Jay

Notice how your RAM speed has slowed to 2133 MHz.

This is the reason that rkaufmann87 is telling you to move the stock RAM to 1 & 2 and put the new RAM in 3 & 4.

FWIW all of the previous 27" iMac models from 2013 - 2019 reporter the RAM speed correctly with the configuration that you used and with what OWC is suggesting.

However there appears to be something different about the new 2020 models (the way the slots are orientated on the logic board or the memory controller) that requires using the configuration that rkaufmann87 is suggesting.

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Answer 2

Old Toad

Level 10

216,100 points

Oct 3, 2020 2:33 PM in response to Ranger.Jay

Just put the two OWC modules in the two empty slots, probably 3 and 4. That's all that needed.

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Answer 3

dialabrain

Level 10

136,349 points

Oct 11, 2020 1:56 PM in response to Ranger.Jay

FWIW, Apple's instructions along with rkaufmann87's are correct.

"If your DIMMs are mixed in capacity, minimize the capacity difference

between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when

possible.

The difference in capacity with both 4GB modules in the first two slots is zero.

The difference in capacity with both 16GB modules in the last two slots is zero.

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Answer 4

rkaufmann87

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148,516 points

Nov 2, 2020 6:21 PM in response to MazDuc

Exactly what did OWC say when you phone them?

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Answer 5

rkaufmann87

Level 10

148,516 points

Oct 3, 2020 3:38 PM in response to Ranger.Jay

Please look closely at my 2020 27"'s RAM.

Pay VERY close attention to the slots they are installed in. When you upgrade PLEASE only use www.macsales.com or crucial.com RAM. Do not buy RAM from anywhere else!

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Answer 6

Ranger.Jay Author

Level 1

8 points

Oct 4, 2020 8:47 AM in response to rkaufmann87

Thanks, but now I have two more conflicting responses. Can anyone point to a document from Apple or a bona fide memory supplier that explains this. While my priority is simply to get it right, I'd also like to know why it matters which DIMM goes into which slot.

Also, can anyone explain what it means to: "minimize the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when possible"?

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Answer 7

rkaufmann87

Level 10

148,516 points

Oct 4, 2020 9:56 AM in response to Ranger.Jay

Proof is in the picture my friend. And the document is the https://support.apple.com/en-us/HT201191#27inch2020

iMac (Retina 5K, 27-inch, 2020)

Get memory specifications for iMac (Retina 5K, 27-inch, 2020), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Number of memory slots4Base memory8GB (2 x 4GB DIMMs)Maximum memory128GB (4 x 32GB DIMMs)

For optimal memory performance, DIMMs should be the same capacity, speed, and vendor. Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

PC4-21333
Unbuffered
Nonparity
260-pin
2666MHz DDR4 SDRAM

I have advised you several times and even proved the point by providing photos of my system that indicates how to install and the speed. I don't think there is more I can do to help you. Perhaps you read in some esoteric location that someone tried 3200MHz RAM, the truth is it DOES NOT work.

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Answer 8

Ranger.Jay Author

Level 1

8 points

Oct 11, 2020 1:13 PM in response to den.thed

So I switched the DIMMs around now it shows the faster speed. So the advice given by the seller/manufacturer of the RAM was incorrect. Thanks for the help. It would still be nice to have the explanation, especially since a trusted resource is giving out incorrect information.

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Answer 9

den.thed

Level 10

92,302 points

Oct 11, 2020 1:44 PM in response to Ranger.Jay

Don’t worry you will not be dubbed the Ginny Pig around here, because rkaufmann87 already holds that title. 😉

However, now it is your job to bring this discrepancy to the attention of Apple, so that they can update the support article.

https://www.apple.com/feedback/

Plus it wouldn’t hurt to let OWC know, so that they can update their support teams information as well.

Cheers and enjoy the new iMac.

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Answer 10

Ranger.Jay Author

Level 1

8 points

Oct 11, 2020 2:03 PM in response to den.thed

Apple actually failed to give me any advice so you can't say they were wrong. They weren't right either. My one effort at getting advice via chat found a person who couldn't string together a sentence in English that made any sense at all. My attempt via phone was disconnected while I was on hold awaiting an answer.

I have now responded to OWC with my screenshots hoping to prevent them from offering the same incorrect advice to anyone else. It's not exactly intuitive to have to remove installed memory and put it somewhere else in order to be able to add additional memory.

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Answer 11

Ranger.Jay Author

Level 1

8 points

Oct 11, 2020 2:20 PM in response to Old Toad

Old Toad wrote:

Are you sure you were talking to Apple representatives and not some 3rd party organization?

I called the Apple Support number and chatted via the Apple website. Unless they have completely changed the system, I was communicating with Apple personnel.

Apple support has been excellent throughout the past 30 years I've been a customer and a large reason why I continue to be. My hope is this was a glitch that won't recur.

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Answer 12

MazDuc

Level 1

12 points

Nov 2, 2020 5:26 PM in response to rkaufmann87

Hi rkaufmann87,

I just received my new iMac 27" with 2 - 4 gb of ram. I purchased 2 each 16 gb of new ram from OWC and off I went.

Prior to reading your post, I removed the door, the Apple ram was on the bottom 2 slots. I pulled out the Apple ram and replaced it with new OWC ram, and rebooted. The iMac would not boot, so I replaced with the Apple ram, rebooted the computer and all good, it worked. So I repeated the process of taking the Apple ram out and reinstalled the OWC, rebooted and it worked.

Shut down again and installed the Apple ram in the top 2 slots, rebooted and nothing.

Once again, I shut down and pulled out the Apple ram and rebooted with the OWC new ram with no luck, it won't work.

Now with hours invested for a 10 min job, I pulled the OWC out and replaced it with the Apple ram, and it rebooted and all looks good now.

I finally read your post completely and reinstalled the Apple ram in the top 2 slots, 1 & 2 & the OWC ram in slots 3 & 4. Nothing happened and I am lost.

I pulled all the ram out and replaced the Apple ram in slots 3 & 4 & it booted and life is good with 8 gb of ram, yeah!!!!

So where did I go wrong? Could it be a faulty controller? Do I call Apple, and sit on the phone for ever?

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Answer 13

MazDuc

Level 1

12 points

Nov 3, 2020 8:45 AM in response to rkaufmann87

Today I called OWC and they said that my new ram should work in slots 1 & 2, matter in fact the Apple ram that came with the machine should work to in those slots. The tech thought that I have an issue with slots 1 & 2 and should call Apple and see what they say.

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