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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.

The PC100 modules require faster speed SDRAM components than the PC66. These parts may have the same footprint and look the same but the parts numbers will be different. The faster components can run at the slower 66MHz speed but the slower parts cannot be used on the PC100 modules. The faster parts have a lower dash number that usually signifies the clock cycle time.

There are two versions of the PC100 modules, one with a CAS latency of 2 and the other with a CAS latency of 3.
CAS latency is the number of clock cycles needed for data to be retrieved from the component. CAS latency is on both types of SDRAM modules. The components for the slower PC66 can run at a CAS latency of either 2 or 3 because of high yields at the SDRAM manufacturer.

The difficulty with the PC100 components is the lower yields on the SDRAM during manufacturing process. A fair number of parts will not run at the faster speed of CAS latency 2 but work fine at CAS latency 3.

The SPD content of the PC100 SDRAM modules is different than the content of the PC66 SDRAM modules. SPD or “Serial Presence Detect” contains the information about the module and the components on the module. The maximum rated speed or bus frequency information is contained in the SPD, along with the CAS latency parameters. A system will use this information to determine if it can support the module and how to configure the onboard memory controller.

Systems Issues
With the release of the Intel BX440 motherboards the memory interface can now run at 100MHz peak transfer rate. The CPU speed to the motherboard chipset is also running at 100MHz and Intel refers to this as a Front Side Bus (FSB).

Depending on the BIOS and chipset, a 100MHz motherboard may support the PC66 DIMM modules.

The BIOS will change the clock to the CPU and the rest of the system to 66MHz and run reliably but the performance has now been cut by 33%.
The impact of adding a slower memory into a fast machine not only slows down the system while accessing the slow memory but slows all the other components, including the faster memory and CPU. The 100MHz may work fine in the PC66 machines, providing of course that the system BIOS supports the configuration. There will be no performance gained by using a 100MHZ module in a 66MHz machine.

PC100 SDRAM modules are more costly to build and produce for a number of reasons. The 6 layer PCB used on the PC100 module is more expensive than the 4 layer PCB for the PC66. There is a price premium for the PC100 components. The PC100 CAS latency 2 SDRAM parts command a premium over the CAS latency 3 SDRAM parts. The CAS latency 2 parts provide a slight performance increase over the CAS latency 3 parts.

Using the wrong SDRAM module in a system that it has not been designed for will result in a number of problems.

1.The system may not boot.
2.The system performance will be significantly decreased.
3.No performance gained by installing faster more expensive memory.

For maximum performance and cost effectiveness always use the correct memory supported by the system.

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