Using faster RAM in older PC

My customers typically have Win7 PCs in their offices, from various manufacturers.  Recently I ran into problems at two sites where they wanted to add 4GB RAM.  They both already had a single 4GB stick of PC3-10600 RAM, but all I had with me was generic 4GB PC3-12800--and I could not get that to work.  I believe it was an Acer that did BIOS error beeps and nothing else.  And it was a Lenovo that did nothing, i.e. no beeps, no video, no nothing other than powering up.  I am not absolutely sure, but I feel confident that both were capable of accepting 8GB total.  In fact, with the Lenovo, when I finally found another PC3-10600 stick, I was able to boot it up like normal, showing the full 8GB.

I thought faster RAM was backward compatible, and should have worked--but clearly I was wrong.  I'm hoping someone can explain "the rules" to me so I will know how to deal with such situations.  TIA
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sasllcAsked:
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rindiCommented:
Not all types of RAM will work on the same board. You should always check the manual, it should normally tell you which modules can be used on that board. Sometimes it will also depend on the CPU and BIOS, and a BIOS upgrade may help.

Usually slower RAM will be more likely to work than faster ones.
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Gary CaseRetiredCommented:
If may not have been the speed that was the issue -- but the internal structure of the RAM.    Some systems are sensitive to whether these are 1R or 2R modules ... i.e. whether the modules are single or dual rank.

Normally you're correct -- the higher rated speed shouldn't have caused an issue ... but if there was a rank difference, that could easily explain why they wouldn't work.
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andyalderCommented:
Hit problems upgrading my own PC, bought x4 DIMMs but mobo only works with x8 sticks. Run CPU-Z and get the SPD info from the current RAM - you can use tools-save as text file rather than screenshot and then we can copy/paste the manufacturer code into Google to find out the spec for you.
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Danny ChildIT ManagerCommented:
I tend to go to www.crucial.com .. Memory.. and then use their System Scanner to tell me what's compatible.  

If upgrading multiple similar machines, I'll take the RAM out of one, and add it to another, and then fit 2 new sticks to the "empty" PC.  Just to keep matched pairs...
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sasllcAuthor Commented:
Can I tell if RAM is single or dual rank by looking at it?  If so, how?
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nobusCommented:
if you post the pc model, or mobo model, we can tell better what can be used
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Gary CaseRetiredCommented:
"... Can I tell if RAM is single or dual rank by looking at it?  If so, how? " ==>  Many modules will have 1R or 2R as part of the module's identifying number.    If not, you can Google the exact part number and should be able to find out from the description.    But the memory maker's don't always make it easy to pin down the exact specs of their modules  (not sure why that is).
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