JEDEC has formed a task force to investigate the development of a standard for fully buffered memory modules in an effort to ease the way for the adoption of DDR2 SDRAM in servers.
The group, part of the JC42 memory committee of the JEDEC Solid State Technology Association, is expected to present an initial concept for a standardized fully buffered DDR2 DIMM in September. Desi Rhoden, JEDEC chairman, said present buffered DIMMs will be unable to handle the memory requirements of the type of high-speed servers that would run on DDR2 at speeds of 667 to 800Mbits/s.
"PCs, which don't have the very large memory size of some servers, can use DDR2 without the buffer, just as they do now," Rhoden said. "It's a server issue."
A conventional buffered DIMM includes added logic that acts as a buffer to reduce the loading on the system address and control bus lines. A system designed for buffered DIMMs can use more modules, since the buffer chips absorb some of the load.
Most DIMMs used in servers buffer the address, control, and clock signals, but not the data signal. Mian Quddus, director of technology enabling for Samsung Semiconductor Inc., San Jose, said the new DDR2 DIMM would include data buffering capability and could also be daisy-chained so that data signals sent to the first module could be passed onto subsequent modules in the system.
Bill Gervasi, technology analyst with Irvine, Calif., module maker Netlist Inc. and vice chairman of the JEDEC task force, said the problem with current buffered DIMM designs is that they can compromise signal integrity as more and higher-speed modules are added to the memory bus line.
The point-to-point path of the fully buffered DIMM eliminates many of the loading problems of the conventional bus line at data rates exceeding 667Mbits/s, he said.
Samsung's Quddus said servers with fully buffered DIMMs would accommodate four DDR2 DIMMs per memory channel. A server using today's memory modules is able to use two DIMMs per channel, in the case of 667Mbit/s DDR2, and one DIMM in 800Mbit/s DDR2.
The first fully buffered DDR2 DIMMs based on a new standard should come in 2005, according to Arthur Sainio, strategic marketing manager at Smart Modular Technologies Inc., Fremont, Calif.
"A major challenge will be making the module 'hot-swappable' so DIMMs could be pulled out or put in while the server is still running," Sainio said.
Others said fully buffered DIMMs will be available a year or more after receiving JEDEC approval and will carry an initial price premium.