SDR SDRAM (Single Data Rate Synchronous Dynamic Random Access Memory) is predecessor to EDO RAM. It became widely accepted in industry in around 1993. It gets synchronized with the system bus; i.e., it accepts one command and transfers one word of data per clock cycle. Long-DIMM (or Desktop RAM) comprises of 168 connector pins and has two notches in its PCB, whereas SO-DIMM (Laptop RAM) comprises of 144 connector pins. It typically has data transfer rates of PC-66 (66MT/s), PC-100 (100MT/s), PC-133 (133MT/s).