XPS 8900, won't accept 64GB RAM, only up to 56GB

Update:

Also Works:  16-16-Empty-Empty

Doesn't Work:  16-16-16-Empty

With 16-16-16-Empty, the power button blinks 7 times then 2 times, both in Amber color. I believe this means the power supply is having an issue? 

Which now makes me wonder - do DDR4-2666 DIMM's draw significantly more DC current than DDR4-2133 DIMMs ?

But if this were an issue, doesn't make sense that 8-16-16-16 would work fine, which it does.

Also re: Power Supply, system components:

460W stock power supply

GTX1660ti graphics card

Evo 970Plus 1Tb running as boot drive on PCIe x4 with NVMe-PCIx adapter card

Evo 970Plus 1Tb running on motherboard NVMe slot (PCIx x1 speed limited)

Original 1Tb HDD

Enermax ETS-N31 CPU Cooler w/ fan

Intel dual 10GbE NIC card X550-T2

See the post here by DellXPS8910, he is having the same problem only with a Dell XPS 8910: https://www.dell.com/community/XPS-Desktops/Need-Memory-for-my-XPS-8910/td-p/6054903

Bump...   Anyone? Hoping there are other Dell XPS 8900 users who've tried installing 64GB of RAM...

install the ram with all power , removed,  AC cord pulled, push power button for 10 seconds

to get all caps discharged,

then insert you sticks of ram , carefully.

then in BIOS , reset bios (to defaults in the menu),   many a BIOS will not fully adjust to new ram, if not reset.

ok>? this is first.

It may help if you could run CPU-Z of the 56GB configuration and post the results here of the SPD tab for each of the four memory slots.

Thanks all for the great suggestions. As I was up against a return policy deadline, I returned the 4x16GB RAM kit and re-ordered 2x16GB DDR4-2133 MHz sticks to go with my 2x8GB sticks. So, I'm at 48GB now, and this install went smoothly as expected.

Like the other post referenced, I found that at least 1 8GB stick had to be in place for any upgrade to work - with one exception. If I only installed 2 16GB sticks, this actually worked (as noted in my first posts). So whatever the BIOS limitation is, it's not simply a matter of needing 1 stick at 2133 MHz. Otherwise my 2x16GB (2666 MHz) install would have failed I guess.

FYI, each installation was done with power cord pulled. Also tried the long power button push, that didn't seem to matter. One thing I did not do, however, was to reset the BIOS. That suggestion crossed my mind, but since I have modified the box with a new boot drive (SSD NVMe over PCIex4 slot), added GTI 1660 Ti graphics, a 10GB Ethernet card, and a second SSD drive in the M.2 SATA slot, I was worried that resetting the BIOS would cause the entire system to just go kaput and not boot at all - leaving me with a chicken-and-egg question (was it the RAM install, or the BIOS reset, causing the problem?). So I chickened out on that option...

Now I am considering to purchase a second set of 2x16GB 2133 MHz sticks, to try one more time to get to 64GB. My logic, likely flawed, is that if all 4 sticks are 2133 MHz, perhaps whatever BIOS issue existed with the higher speed RAM will not come into play here. We shall see!

I think you are correct to think it is an issue with the higher speed RAM. Whether it is a BIOS issue or some other compatibility issue is difficult to determine. You never indicated what RAM you had purchase, but if you had purchased it from Crucial it is guaranteed compatible if you used their memory selector app.

Actually, I specified exactly which ram I purchased in the first post. Crucial, and yes the higher speed ram is supposed to be compatible. And it is, just not when all 4 slots are taken up with it. Has to be a BIOS related issue.

I can't argue with what Crucial sent you but if you search Crucial.com today neither CT4K16G4DFD8266 or CT16G4DFD4266 results in any matches. Also no 64GB kits are found if you enter your computer model.

Thanks, but I did check this. If you click "view more" at the bottom of the Crucial.com list of compatible memories for the XPS 8900, you'll find this one:  CT2K16G4DFD8266    This is 2x16GB of DDR4-2666. The 4x16GB kit I bought is simply 2 of these kits, which made the price just a little cheaper. Also, the site specifically says "up to 64GB supported" and "4 slots"... so obviously that's 4x16GB to max out the board. And finally, you'll observe from their compatibility list, if you choose "Filter Results", that they only list out kits of size 1 or 2, period. No 4-kits are listed, for any speed or size. (I've been running 4x8GB DDR4-2133's for months for example). So even for the DDR4-2133 MHz speed that came populated with the system, they don't show any kits of 4 even though they exist; the same is true for the 4-kits of DDR4-2400 and DDR4-2666. So basically, the fact that 4-kits aren't listed means nothing as far as I can tell. Appears the compatibility checker isn't quite perfect.

I agree with everything you wrote, but if you do a search for the kit you brought on the site, the search results are now finding nothing. I don't know what this means, but it could mean that it has been dropped from the compatibility list or that it is not that popular an item or that it is equivalent to two 32GB kits so that there is no need for a separate listing.

those sites if no certified memories are listed, then shows a link to alternates, well do NOT go there.

those are generics ok? (risky for sure those ,like seen on ebay dirt cheap, and no brand top names)

first off memories are not simple, like lightbulbs.

there are 2 areas that even that any full load of 64GB can fail.

first off is density, some chips are of a non supported density, do you want to learn this?  it is complex.

but there are single side sticks double and tall and short sticks, this tells you (a hint) that all those sticks are using different density RAM chip,  and not all are supported by dell , Intel or BIOS.,

that is why top makers of MOBO like say AUSU have this HUGE LIST OF supported sticks, just for  that very reason.,. ever look  there? huge.

you also think crucial is and authority over dell, sorry that will never be true ever.

dell designed the PC and BIOS and intel the DDR, engine is there's and there limits.

not crucial,  learn that. and dell DOES NOT TEST or upgraded ram lists over time like wonderful Asus does.

see? so in vacuum you go to CRUCIAL I get that.

chip density and bus capacitance can stop them from working or even simply BIOS.

then the crystal ball effect, (or reverse)

the maker of the PC, did not have his./her ball to see what will be made 5 and 10 years in the future.

sure the G.Moore  effect (density shrink) but no crystal bell is sold for seening actual chips and fitted to actual DDR chips, so it can not be tested nor certified.

that is why ASUS keeps there list up to date, (in some set of year span rules IDK)

dell does not even publish any list even close to that Asus list (or MSI, nor like Gigabytes) nada.

so what  is the cure, well, IDK but you can older all sticks of all brands and try them you sure  can.

there is no other way, unless very lucky and someone did on any forum on earth, for 8900.

good luck to you and your saga.

the manual for you PC shows this. 2015 max old.

i could not find a per slot limit so must guess, at  8GB each?  but gee, dell did not test the big ones.

or this list would  be different. OK?> cut and pasted out of product spec. see that ? yet?

or does CRUICAL have Red phone hot line to DELL engineering, IDK but I BET NOT.

or does that mean each slot can have 32BG (no that be wrong!) it fails to say slot total memory config.

the word total is missing. dell docs leave lots to be desired, nor density rules. AND MORE.

see density and ranks here,  and you can be sure there ARE LIMITS.

read this, learn this first. the when they fail, you know why. (and other reasons too, ask)

Ok thanks... think we’re getting off track a bit. My conclusion is, this is a simple BIOS bug. The faster DDR4-2666 memory works perfectly when installed as 2x16GB, which means the BIOS knows how to clock this memory. All other factors are identical to the original memory (dual rank, CL, etc.), so makes sense this memory would work. And it does. Just not when all 4 slots are populated with it. In a couple of weeks I’ll have two more 16GB DDR4-2133 sticks, will confirm if 4x16GB works for that combo. Will post an update. 

I don't know if you ran CPU-Z like I recommended with the DDR4-2666 RAM. You say the BIOS knew how to clock the memory, but at what speed was it clocked? Did you also check the BIOS to see what memory speed it detected?

I don't know about the XPS 8900, but with the XPS 8910 the RAM speed is set by using the SPD (Serial Presence Detect) information in the memory module. In the XPS 8910, it does not read the XMP (Extreme Memory Profile) in the memory module and there is no capability to use the XMP by setting it in the BIOS. So even with higher speed RAM a computer like the XPS 8910 will run at the lower speed. With Crucial RAM, when they specify DDR4-2666 you cannot tell from the specs whether that is the SPD or XMP speed. There have been cases on this forum when someone brought DDR4-2666 and it only ran at 2133 MHz because that was the SPD speed.

You may be correct that it is a simple BIOS bug but the system spec says that it supports 64GB of memory so I don't think you can rule out a memory compatibility issue.