PC100 vs PC66

PC100 vs PC66

Use only PC66 Memory in TX and LX systems requiring 4 clock RAM Intel
Ver 1.0 Compliant. Do not use for VX systems. Specially made SDRAM
modules are required for VX systems.
Using the wrong memory speed in a system has a number of problems
associated with it, the first being it may not work. If the slower
memory does work in your faster machine then you have slowed down the
entire system and defeated the purpose of the new machine. For maximum
performance and cost effectiveness only use the correct memory per the
system specifications.

In an effort to answer some of the many questions regarding the "New"
PC100 bus memory I have assembled this white paper. I will focus only on
the differences and try to avoid a technical paper on how it works. The
paper is broken down into two parts with the first being the physical
differences between the two types of memory modules. The second part
deals with the systems issues with PC100 and PC66.

Physical Similarities and Differences
Intel created both the specifications for the PC66 and the PC100 SDRAM
DIMMs which tightly define their electrical characteristics. Both the
PC100 and PC66 SDRAM modules are 168 pin, 3.3 volt DIMM with SPD "Serial
Presence Detect". The signal pinouts or where the signal pins on the
connectors' locations is the same for both types of modules.

Both start out with the same basic concepts and similar physical
appearances but both are really quite different from performance
aspects. On the module itself, the PC100 specification requires that the
length of the traces be kept shorter than the PC66 modules. The shorter
the traces the faster the electrical signals can travel on and off the
module. The routing of the traces on the PC100, because they need to be
kept shorter, have to use a 6 layer PCB as
compared to a 4 layer PCB used on the PC66.