Hi Peter,

Thanks for your reply. If that's the case i.e. "normal operation" then that's fine however I would like to see it documented somewhere for the OptiPlex 755 as right now I don't know if I have a defective machine or incompatible memory. If there was such a limitation in the case of the OptiPlex 755 I would have expected to see it referenced somewhere in the documentation for this particular machine.

Regard's

It seems as though some machines throttle back the performance with 4G installed (or maybe 4 modules).  You can test this by putting in only the new memory and see if it shows 667 or 800.

You should also note that 667M RAM in dual channel mode provides the same bandwidth as your FSB (1333M).  800M RAM has more bandwidth that the FSB so there should be little gain to running at 800M.

Peter

Try the new memory by itself and see if it runs at 800M.  If so, then clearly the memory and machine are both capable of running it at 800M.  If both sets run at 800M individually but not together, it either means the memories are not compatible at 800M (i.e. they have no common timing parameters) or the machine is purposely cranking the speed down.

Dell doesn't appear to document details such as these.  They even often state that 4G is the max when a machine will take 8G.  Their strategy seems to be "keep it simple".

Peter

You are also mixing single Rank (8 chips) and Double Rank (16 chips, "double density") memory. There can be compatibility problems because of this as well as the issues presented by Peter345.

 

Dimension 9100, Dual-Boot Win XP / Vista Home Premium, 3.0 GHz P4, 3 GB DDR2 533 MHz RAM, 160 GB SATA II Samsung (XP), 300 GB SATA II Seagate (Vista), 250 GB SimpleTech USB (WD Drive), Nvidia Go 6800 (425/825 MHz - XP, 400/800 MHz - Vista, Vista Driver - 169.25), Dell 1901 UltraSharp FP

Inspiron E1705, Win Vista Premium, T7200 Core 2 Duo (4MB, 2.0 GHz 667MHz), 2 GB DDR2 677 MHz RAM, 120 GB Samsung HD, Nvidia Go 7900 GS - 156.69 Driver, 17” Sharp UltraSharp TrueLife Wide-Screen WUXGA

 

 

Hi KirkD,

Thanks for your input. I did notice this and I suspect it could indeed be a factor. I thought the whole purpose of having a designation system such as PC2-6400U was to avoid the end user having to deal with these varances in that a PC2-6400U no matter who manufacturer it should always function correctly in a compliant mother board.

I checked the Dell brochure for this machine and it does indeed support 800 MHz speeds up to 8GB of installed RAM so it does liik like I might have a compatability problem. Looks like I need to re-visit the theory and principles surrounding DDR2 memory.

Regards.

Have you tried what Peter345 suggested and run the original pair alone to confirm you get 800 Mhz and then test the new pair to see what they give? This is to determine if both sets are 800 Mhz. You should also download CPU-z, a free program that lists system parameters, including RAM type and speeds.

As Peter said there are lots of problems with 4GB RAM resulting in the RAM speed being dialed back from the maximum. No concensus as to the reason, but those with a BIOS with adjustable parameters ("Real PCs !!!) have fixed it by a slight increase in RAM voltage.

You could also add only 1GB more as 2x512 MB for 3GB total. This is close to the amount you will get with 4GB installed using a 32-bit OS; ~3.1GB vs. ~3.2GB. You only get 4GB with a 64-bit OS. A 64-bit S is also necessary for 8GB RAM. #GB is the sweet spot for Windows, as a single application is limited to a max of 2GB.

Dimension 9100, Dual-Boot Win XP / Vista Home Premium, 3.0 GHz P4, 3 GB DDR2 533 MHz RAM, 160 GB SATA II Samsung (XP), 300 GB SATA II Seagate (Vista), 250 GB SimpleTech USB (WD Drive), Nvidia Go 6800 (425/825 MHz - XP, 400/800 MHz - Vista, Vista Driver - 169.25), Dell 1901 UltraSharp FP

Inspiron E1705, Win Vista Premium, T7200 Core 2 Duo (4MB, 2.0 GHz 667MHz), 2 GB DDR2 677 MHz RAM, 120 GB Samsung HD, Nvidia Go 7900 GS - 156.69 Driver, 17” Sharp UltraSharp TrueLife Wide-Screen WUXGA

I finally got an opportunity to try both your suggestion's and both sets of memory DIMMS perform at their rated
speed of 800MHz when inserted as a pair in to Slot 1 and 2 so no problems with the rated speed of the individual matched pair.

BIOS screen Info after each test is as follows :-

For the original DELL supplied ProMos DIMM's (DDR2-80MHz-CL6)

Memory Info

Installed Memory = 2.0 GB
Memory Speed = 800 MHz
Memory Channel Mode = Dual Symmetric

Memory Slot Size ECC Rank Type             Org
DIMM 1        1GB NO    2     Un-Buffered   X8
DIMM 3 Empty
DIMM 2        1GB NO    2     Un-Buffered   X8
DIMM 4 Empty

And for the Crucial DIMM's (DDR2-80MHz-CL5)

Memory Info

Installed Memory = 2.0 GB
Memory Speed = 800 MHz
Memory Channel Mode = Dual Symmetric

Memory Slot Size ECC Rank Type            Org
DIMM 1       1GB  NO  1      Un-Buffered   X8
DIMM 3 Empty
DIMM 2        1GB NO  1      Un-Buffered   X8
DIMM 4 Empty


But when both sets are installed they only operate at 667 Mhz ! ! !

The supplied literature (specifically page 319 of the User Manual)  for Dell OptiPlex 755
only refers to PC2-6400 ( DDR2 800 MHz ) and PC2-5300 ( DDR2 667 MHz )
DIMM's

Could be either the 4GB voltage drop problem or the Rank compatibility issue. Trying a second pair of Rank 1 modules would settle it, but becomes a chore of buying and returning RAM. You might want to search the web for others with the problem. Someone may have figured it out.

Dimension 9100, Dual-Boot Win XP / Vista Home Premium, 3.0 GHz P4, 3 GB DDR2 533 MHz RAM, 160 GB SATA II Samsung (XP), 300 GB SATA II Seagate (Vista), 250 GB SimpleTech USB (WD Drive), Nvidia Go 6800 (425/825 MHz - XP, 400/800 MHz - Vista, Vista Driver - 169.25), Dell 1901 UltraSharp FP

Inspiron E1705, Win Vista Premium, T7200 Core 2 Duo (4MB, 2.0 GHz 667MHz), 2 GB DDR2 677 MHz RAM, 120 GB Samsung HD, Nvidia Go 7900 GS - 156.69 Driver, 17” Sharp UltraSharp TrueLife Wide-Screen WUXGA

My viewpoint is that it's Dell's problem  ! !   If there is a sensitivity to Rank order or a variance in Voltage then it should be documented in their supplied literature. In the absence of any guidence from Dell it is reasonable to assume that all DIMM's that meet the industry standard PC2-6400 spec are suitable.

I added DIMM's that were IMHO superior in performance to the ones that DELL's supplied  and somehow it manages to cause the PC to operate at less than optimum performance.

In any computer, memory will run at the speed of the lowest speed module. If it ran at the highest speed, there would be lots of memory errors. Some lower speed RAM will work at the higher speeds, but not many. When speed demons overclock RAM, usually a voltage increase is necessary.

I agree that the Rank issue seems not to be appreciated by many.  I have found posts indicating Rank should not be mixed. Memory in Dells appears to be very sensitive to variations that really should not be. However, posts by others with non-Dell machines also have these problems. I do not think Rank should be mixed.

The voltage situation is also something not well studied. The underclocking with high RAM levels has occurred many times and can be corrected by a voltage increase. If circuitry is borderlline, many issues like this can occur.

Dimension 9100, Dual-Boot Win XP / Vista Home Premium, 3.0 GHz P4, 3 GB DDR2 533 MHz RAM, 160 GB SATA II Samsung (XP), 300 GB SATA II Seagate (Vista), 250 GB SimpleTech USB (WD Drive), Nvidia Go 6800 (425/825 MHz - XP, 400/800 MHz - Vista, Vista Driver - 169.25), Dell 1901 UltraSharp FP

Inspiron E1705, Win Vista Premium, T7200 Core 2 Duo (4MB, 2.0 GHz 667MHz), 2 GB DDR2 677 MHz RAM, 120 GB Samsung HD, Nvidia Go 7900 GS - 156.69 Driver, 17” Sharp UltraSharp TrueLife Wide-Screen WUXGA

I guess what I am really struggling with here is the complete absence of any detailed specification for the memory requirements for the Dell OptiPlex 755 machine in the supplied User Manual.

In the absence of any specification there at least should be a list of compliant and Dell qualified parts.

And I really don't want to pay Dell exorbitant prices for a "standard "  memory upgrade.Even then if I did decide to buy the upgrade from them I would have no idea what I would receive ! (I guess I would get two more pieces of what I have already.

Right now I have no idea if I need Rank 1 or Rank 2 parts of if I need CAS 5 or CAS 6 or what the Mother board is designed and guaranteed to work with.

My reason for buying the higher spec. CAS 5 Range 1 part (for what is a couple of bucks more) is to provide the maximum re-use potential at a future date when I want to upgrade or re-use the parts elsewhere.

 

 

 

 

One of the problems with Rank 2 is that the OS sees it as TWO modules. Each system can address a maximum number of modules. Might be 4, might be 6, or ........

Your system appears to see at least 6 (2 from Rank 1 plus 4 from Rank 2). Some posters have systems that only saw 4. If you install 4x1GB Rank 2, it might not be recognized.

I wouldn't worry about CAS timings. Only avid system benchmarkers go for the lowest timings. Will not really make any difference for normal work.

Dimension 9100, Dual-Boot Win XP / Vista Home Premium, 3.0 GHz P4, 3 GB DDR2 533 MHz RAM, 160 GB SATA II Samsung (XP), 300 GB SATA II Seagate (Vista), 250 GB SimpleTech USB (WD Drive), Nvidia Go 6800 (425/825 MHz - XP, 400/800 MHz - Vista, Vista Driver - 169.25), Dell 1901 UltraSharp FP

Inspiron E1705, Win Vista Premium, T7200 Core 2 Duo (4MB, 2.0 GHz 667MHz), 2 GB DDR2 677 MHz RAM, 120 GB Samsung HD, Nvidia Go 7900 GS - 156.69 Driver, 17” Sharp UltraSharp TrueLife Wide-Screen WUXGA 

I think what I will do is remove the DELL supplied RANK 2 DIMM's and buy a second pair of the 800 MHz Crucial RANK 1 DIMM's to match the pair that I already have. This should be fine.

However I have sent an e-mail to DELL Customer Support asking if they can confirm that this is a valid configuration (use case) for this motherboard and also to  request a list of compliant memory modules that they have actually tested at 800MHz with 4 DIMM slots fully loaded. (Just in case this motherboard does not support 4 slots @ 800 MHz )

I am still waiting for an informed response (not the scripted stuff with no information !).

 

Just wondering if you've found any answers from Dell or decided which way you gonna go? I'd appreciate if you share your resolution (or lack of thereof).

 

I'm in the same exact boat. Here's my post on it:

http://www.dellcommunity.com/supportforums/board/message?board.id=Desktops_Workstation_Optiplex&thread.id=6228

 

Thanks.

Hi,

No updates yet ! I have just purchased another matched pair of the Crucial DIMM's today so I will now have 4 identical DIMM's.

I will post my findings once I have received them.

I also chechkec the BIOS revisions and there is no mention of a Memory speed issue

 

BTW This is the latest response that I have received from Dell Tech Support

Thank you for your reply.

Mr. xxxx, I would like to inform you that Dell does not support third party devices and to resolve the issue, I would suggest you to return the memory modules to the manufacturer from where you have purchased the same and then purchase the memory modules from our part sales department. Our part sales department will provide you all the options for the compatible memory modules for your system.

The Part Sales Department can be reached at 0870 9073689 (UK) and (01) 2860500 (Ireland).

Do write back to me if you have any other issue or query. I will be happy to assist you.

Thank you again for contacting Dell Hardware E-Support.

Respectfully,
Tech Support :-

 

This was my question :-

Hello,
I have checked  the compatability of the third party memory with the vendor
and everything is fine.
I am now going to ask a very specific question answer that I need an
accurate response for.
Can Rank 2 pairs and Rank 1 pairs be mixed on this motherboard  say DIMM 1,2
Rank 2 and DIMM 3,4 Rank1. ? If so what are the conditions or constraints if
any .Have Dell tested this Use Case or condition.(This is a typical user
scenario by the way)
If I replace all of the memor with Rank 1 devices will it work at the full
800MHz speed.
Both the OptiPlex 755 User Manual and the Dell OptiPlex 755 Technical guide
book only say that the memory should be PC2-6400 (800MHz ) compliant. If
there are limitations or restrictions , THEN IT SHOULD BE CLEARLY STATED IN
THE DAOCEMTATION ! ! !
Under what conditions can I get all of the memory to operate at 800Mhz ?
Is there a list of compliant memory modules and manufactuters that are
guaranteed to work at correctly and and at ful speed in this system ?
I await your response .
Regards.