A Fully Buffered DIMM (FB-DIMM) is a type of memory module used in computer systems. It is designed to improve memory performance and capacity by allowing multiple memory modules to be each connected to the memory controller using a serial interface, rather than a parallel one. Unlike the parallel bus architecture of traditional DRAMs, an FB-DIMM has a serial interface between the memory controller and the advanced memory buffer (AMB). Conventionally, data lines from the memory controller have to be connected to data lines in every DRAM module, i.e. via multidrop buses. As the memory width increases together with the access speed, the signal degrades at the interface between the bus and the device. This limits the speed and memory density, so FB-DIMMs take a different approach to solve the problem.
240-pin DDR2 FB-DIMMs are neither mechanically nor electrically compatible with conventional 240-pin DDR2 DIMMs. As a result, those two DIMM types are notched differently to prevent using the wrong one.
As with nearly all RAM specifications, the FB-DIMM specification was published by JEDEC.