Server memory buyers are our most tech-savvy customers. Even so, there is a lot of confusion surrounding the different types of ECC memory available. So let's try to clear things up!
There are 3 different types of ECC memory currently in circulation: ECC Registered (a.k.a. buffered), ECC Unbuffered, and ECC Fully Buffered. Many machines that use one of these ECC types are not compatible with the others.
If you are uncertain which ECC memory type you have, take a look at your currently installed RAM. You should be able to correctly identify the type by referring to the information below.
ECC Registered RAM
ECC Registered RAM is often designated by the letter "R" or "P" on label specifications, for example PC2-3200R or PC2-5300P. (R for registered or P for parity a.k.a. registered).
ECC Registered RAM (a.k.a. ECC Buffered) has 9 or 18 chips per side plus additional smaller chips called "registers" which act as a buffer between the DRAM and the system memory controller. Read/write functions are buffered for one cycle to provide greater data integrity and stability for mission critical applications such as servers. Standard ECC Registered RAM will not fit the slots of machines that require Fully Buffered FB-DIMMs.
For machines that use DDR2 or earlier, it is common for server models that use ECC Registered to be incompatible with other ECC memory types.
Many servers that use DDR3 can accept either ECC registered or ECC unbuffered but you cannot use both types in the same machine at the same time. Most DDR3 generation servers require ECC registered memory to reach the server's full memory capacity. Check your server manual for model specific information.
DDR3 ECC Registered modules in capacities of 4GB or larger usually have heat spreaders which visually resemble the heat spreaders on Fully Buffered FB-DIMM RAM. Pay attention to the label designation, if it is "R" or "P" then the memory is standard ECC Registered.
ECC Unbuffered RAM
ECC Unbuffered RAM is often designated by the letter "E" on label specifications, for example PC2-5300E.
ECC unbuffered RAM looks similar to Non-ECC unbuffered desktop RAM, but instead of 8 chips per side it has 9 chips. The 9th chip handles the error correction function. This memory does not have the additional "register" chips found on ECC Registered RAM.
Machines that use ECC Unbuffered DDR2 or earlier are usually incompatible with ECC registered memory.
Many servers that use DDR3 can accept either ECC Unbuffered or ECC registered but you cannot use both types in the same machine at the same time.
ECC Fully Buffered RAM (FB DIMMs)
Fully Buffered FB-DIMM memory is often designated by the letter "F" on label specifications, for example PC2-5300F.
FB-DIMMs (aka fully buffered) are a type of ECC RAM which use an Advanced Memory Buffer (AMB) between the memory controller and the memory module. The notch on FB DIMM memory is offset to prevent these modules from being installed in systems which use standard ECC Registered RAM. Future development of this memory type is uncertain.
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