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RIMMs Memory

With the introduction of Direct RDRAM (DRDRAM) in 1999 came the RIMM module (the name is not an acronym, but a trademark of Rambus Inc.). RIMM connectors have a form factor similar to DIMMs and fit within the same board area as the footprint for a DIMM connector. They have 184 pins compared to a DIMM’s 168, but use the same socket specification as a standard 100MHz DIMM. A PC’s BIOS will be able to determine what type of RAM is fitted, so 100MHz SDRAM modules should work in a RIMM-compatible system. However, systems can’t use RIMMs unless both BIOS and the chipset support it. In the case of the latter, after many delays this finally arrived in the shape of Intel’s 820 chipset in November 1999. SO-RIMM modules – which use the same form factor as small-outline SO-DIMMs – are available as one-to-eight RDRAM devices.

The major elements to a Rambus memory subsystem include a master device that contains the Rambus ASIC Cell (RAC) and Rambus Memory Controller (RMC), Direct Rambus Clock Generator (DRCG), RIMM connectors, RIMM memory modules, and RIMM continuity modules. The RIMM connector is an integral component in the Rambus Channel by providing the mechanical and electrical interface and signal integrity for the high speed Rambus signals as they move from the master ASIC to the appropriate RDRAM memory device component and vice versa. Since the Rambus Channel is a terminated transmission line, the channel must be electrically connected from ASIC to termination resistors. The net effect is that all RIMM connectors need to be populated with either a RIMM memory module or a RIMM continuity module to insure the electrical integrity of the Rambus Channel.