MP 1,1-5,1 Mac Pro 4,1 RAM compatibility

[Taipan]

Hi!

I have a Mac Pro 4,1 flashed with 5,1 firmware, 2x 6core CPUs and 6x 4GB registered DIMMs. Now I tried to add some more RAM and bought 3x IBM 16GB registered DIMMs. I learned quickly that mixing the 16FB modules with different ones doesn't work, but I couldn't even get the three 16GB ones to run on their own. No matter which slots I used (including the recommended ones) and whether I used two or three modules, the machine always recognized only one of them. Sometimes in slot 1, sometimes in slot 2, and it wasn't always the same physical DIMM.
Is there any reason why these modules shouldn't work? One thing that struck me was that they are 3,5V modules. Is that a problem? Would it be safer to get 8GB modules instead of 16GB?

And a general thing I haven't quite understood: I know the modules should ideally be used in sets of three to get triple channel access. But I keep reading about matched "pairs", and there are two CPUs after all. So, do the modules in slot 1 (of CPU1) and slot 5 (=slot1 of CPU2) not need to match? For example, if I had 3x 4GB and 3x 8GB, would it be correct to plug the former into the slots of CPU1 and the latter into the slots of CPU2?

Thank you!

--
Timo

 

[tsialex]

When you buy DIMMs for a Mac, you buy the same identical one for all slots that you gonna use for compatibility.

The Mac Pro requirements are:

1. DDR3 or DDR3L,
2. 1333MHz,
3. dual rank = 2Rx4,
4. ECC,
5. RDIMM or UDIMM (UDIMM are extremely costly with sizes over 8GB)
6. at least 3 DIMMs per CPU socket, since Nehalem/Westmere Xeons are tri-channel.

 

[h9826790]

I think the memory reference code on the cMP only support 1,2,4Gbit chips DIMM. Therefore, 16GB UDIMM that build by 8Gbit chips also won't work on the cMP.