What’s the difference between buffered and unbuffered DIMMs?

What’s the difference between buffered and unbuffered DIMMs?

High density DIMMs have lots of chips on them and therefore possess a higher capacitive load on the address and control signals in comparison to lower density DIMMs. Some designers use re-drive buffers on the DIMM to boost the signals to reduce system loading when compared to the same high density module without buffers. But, the buffers introduce a small delay into the electrical signal, so adding buffers to a standard density module would have the effect of slowing down the signal, compared to the same low density module without buffers.