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Chapter 3 Processor/Memory Subsystem
Compaq Deskpro EXS and Workstation 300 Personal Computers
Featuring the Intel Pentium 4 Processor
First Edition - December 2000
3-6
3.3.1 RAMBUS ATTRIBUTES
To ensure signal quality during high-speed memory transfers, the Rambus interface design departs
from previous memory interface designs in several key aspects (Figure 3-4). Rambus uses a daisy-
chained signal distribution system that requires that all memory sockets be populated with either a
RIMM or a continuity module (CRIMM) in order to maintain constant load impedance. Rambus
Signaling Levels (RSL) uses a 1.4-volt reference with a 0.8-volt swing between logic 0 at 1.8 V
and logic 1 at 1.0 V.
On these systems RIMMs (or CRIMMs) must be installed in pairs (one module for each channel).
A maximum of two gigabytes of memory may be installed using 512-MB RIMMs. These systems
ship with PC800 (400-MHz) RIMMs but will also accept PC700 or PC600 RIMMs. A mix of
ECC and non-ECC RIMMs may be installed, although all RIMMs must be ECC to realize ECC
benefits.
Rambus Signal Attributes (Each Channel)
Signal Name
No. of
Lines
Input/
Output [1]
Signal
Level Impedance Function
ROW 2..0 3 O RSL 28 ohms Row address
COL 4..0 5 O RSL 28 ohms Column address
DQA 8..0 9 I/O RSL 28 ohms Data byte A (w/parity or ECC bit)
DQB 8..0 9 I/O RSL 28 ohms Data byte B (w/parity or ECC bit)
CFM, CFMN 2 O RSL [2] 28 ohms 400-MHz Clock-from-master for writes
CTM, CTMN 2 I RSL [2] 28 ohms 400-MHz Clock-to-master for reads
Vref 1 -- 1.4 V -- Reference voltage for RSL signals
SIO 1 I/O CMOS -- Serial I/F for initialization & pwr cntrl.
SCK 1 O CMOS 56 ohms SIO clock; 1 MHz for configuration,100
MHz for power management.
CMD 1 I/O CMOS 56 ohms Serial I/F config. & power control
Vdd -- 2.5 V -- Power for Rambus circuitry
NOTES:
[1] Relative to the memory controller.
[2] Differential pair with Ep-p swing of 400 to 600 mV.
Figure 3–4. Rambus Channel Signal Distribution and Key Attributes
Bus Termination
RIMM Sockets
RIMM
MCH
RIMM or CRIMM (Shown)
Channel 1
Channel 2
RIMM