FB-DIMM 667 MHz in Mac Pro 2008 ?

Discussion in 'Mac Pro' started by Spacedust, Nov 15, 2009.

  1. Spacedust macrumors 6502a

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    May 24, 2009
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    Poland
    #1
    Is it possible to put slower memory to Mac Pro 2008 ?

    Will they be automatically overclocked to 800 MHz if you mix some 667 and 800 modules ?
     
  2. Cindori macrumors 68040

    Cindori

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    Jan 17, 2008
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    Sweden
    #2
    no, I believe your 800 mhz modules will be downclocked to 667 if you do that.
     
  3. Spacedust thread starter macrumors 6502a

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    May 24, 2009
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    #3
    What about FSB ? Will it work at 1600 MHz or 1333 MHz then ?
     
  4. dlewis23 macrumors 6502a

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    Oct 23, 2007
    #4
    If you mix and matched 667 Mhz ram in with 800 Mhz ram it will work, but it will slow all the 800 Mhz ram down to 667. Depending on what your doing the difference between 800 Mhz and 667 Mhz ram could be negligible, and having more ram might cancel out the speed difference.
     
  5. nanofrog macrumors G4

    Joined:
    May 6, 2008
    #5
    If you mix 667 and 800MHz memory, it will downclock all of it to 667MHz. That translates to an FSB of 1333MHz in practical use, as the memory's not running fast enough to use the remaining bandwidth.

    So if you need/want the FSB to remain at 1600, you need to get the 800MHz RDIMM's for your system.
     
  6. icyderguru macrumors member

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    Apr 20, 2008
    #6
    Do you guys know that from experience or is it a guess?
     
  7. nanofrog macrumors G4

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    May 6, 2008
    #7
    Experience. That's just how Intel's parts work together for that system (MP or PC variant with the same components on the board).
     
  8. Spacedust thread starter macrumors 6502a

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    May 24, 2009
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    Poland
    #8
    Thanks for anwers. I'm will get my MP 2008 next week with 2 GB RAM, so I ordered another 6 GB RAM 800 MHz.

    My current 12 GB seems to be unusable. 8 GB is more than enough.
     
  9. J&JPolangin macrumors 68030

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    Thule GL @ the TOW
    #9
    ...would the speed difference in the modules be offset by the much larger capacity of RAM being used as the OP has 12 GB of the 667 Mhz RAM available vs the 6 GB on order?
     
  10. nanofrog macrumors G4

    Joined:
    May 6, 2008
    #10
    Generally speaking, it's not a good idea to mix memory like that. Though it will downclock, the actuall position even matters, as the SPD (timings) are read on one of the DIMM's per channel, not all of them. So you can actually confuse the system (i.e. attempt to OC the memory in some cases). It gets down to the firmware.

    So to prevent issues, stick with the same memory. Now if you have an '08, you could go ahead and use all 667MHz memory. The throughput is slower, but there are cases where the speed is less important than capacity. It's up to the user to determine that. But the cost difference between the two types isn't that bad last I checked, so might as well get what the system can actually utilize fully. ;)
     
  11. Pressure macrumors 68040

    Pressure

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    Denmark
    #11
    I have mixed matched my Mac Pro (early 2008) with 800Mhz and 667Mhz DIMMs with no problems before. It just downclocks the ram to 667Mhz. Not the Front Side Bus as someone seemed to suggest.
     
  12. nanofrog macrumors G4

    Joined:
    May 6, 2008
    #12
    It's just easier to prevent issues by keeping it the same if possible. I've seen issues with some systems (in general), as the SPD was determined by one of the DIMM slots per channel (specific location). So if that location was filled with the faster sticks, problems came about (unstable).

    I didn't have the differing clocked memory to test in an MP, but knew it's technically possible to mix it, as others such as yourself have (posted some time ago).

    As per the FSB, it's not that it actually slows down, but the memory can't exceed 1333. Other aspects of the system (i.e.chipsets for example), will remain the same.
     
  13. akadmon macrumors 68010

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    Aug 30, 2006
    Location:
    New England
    #13
    Huh? Since when less is more?:confused:
     
  14. 4God macrumors 68020

    4God

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    Apr 5, 2005
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    My Mac
    #14
    So, how 'bout 1066MHz ram in an early '08 8 core 2.8 Mac Pro?
     
  15. CaptainChunk macrumors 68020

    CaptainChunk

    Joined:
    Apr 16, 2008
    Location:
    Phoenix, AZ
    #15
    1066MHz DDR2 RDIMMs would probably work, but...

    1. It would downclock to 800MHz on a 2008 MP.
    2. I don't think anyone makes 1066MHz DDR2 RDIMMs with Apple-approved heat spreaders. Most of the ones I've seen don't have heat spreaders.

    ...so, it's kind of pointless.
     
  16. 4God macrumors 68020

    4God

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    Apr 5, 2005
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    My Mac
    #16
    That sucks. Thanks for the reply though. :)
     

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