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Different types of memory

Posted on 1998-12-13
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Last Modified: 2010-04-27
My manual for my motherboard says it does not recommend the use of both DRAM and SIMM memory type AT THE SAME TIME. Can someone explain to me what may or will happen if I try to use both at the same time anyway? I always thought this was OK to do.
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Question by:chinman
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by:joopv
ID: 1130940
What kind of mainboard are you talking about ?  what cpu do you have : 386 / 486 / pentium ?

Do you mean with dram the small 16 pin black chips that are on the mainboard (mostly in rown of 9 pieces) ?
And what kind of modules do you use ?  The 31pin of the 72pin version ? Or even the latest DIMM versions ?

There are many different types of mainboards.  The more recent versions can support both DIMM and 72p SIMM modules.  These can not always be mixed because of power supply voltage conflicts (3.3V / 5V)  In such a case it is possible that you destroy memory modules.


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wayneb earned 200 total points
ID: 1130941
Some DRAM Specification You Should Know  

Can 5V SIMMs and 3V DIMMs coexist peacefully on a mainboard?
To protect your rights! Please use the right way to protect your system and do not give this word "Possible" a chance to damage your possession.  
Some people claim it's feasible to mix 3V DIMM modules with 5V SIMM modules on a mainboard. However, we do not recommend to use both kinds of modules on a mainboard for the reasons below:  
 
 As the semiconductor process advances, the thickness of gate oxide has become thinner and thinner, 5V I/O may cause the 3V DIMM to degrade. This degradation will change the CMOS properties, especially the CMOS threshold voltage, and ultimately damaging the gate oxide. This means the DIMM modules may fail (burn out) after a period of time due to the gradual degradation process. Exact duration can not be determined, because it also depend on individual semiconductor vendors on the semiconductor processes used for their products.
By inserting a serial resistor between 5V DRAM and 3V DRAM can not solve the problem, since it's the high voltage not the current that will damage the gate oxide.  
 
 5V SIMM I/O data pin will drive 5V signal to 3V data bus. This 5V signal goes through the protection diode to the 3V DIMM power plane and causes the possible IC latch-up effect to occur. To test for the IC latch-up effect is easy, you only need to drive a 5V line into the data line of a 3V DIMM module. If it fails the test, it shows the latch-up effect has occured. However, the latch-up effect may not show up all the time because the signal may not be high enough to go over the threshold every time.
Technology used in the process of producing the DRAM plays an important role also. NMOS process or CMOS process with dual N channel for I/O pin can avoid this effect.
 
Most of the DRAM vendors do not guarantee and will not recommend to mix 5V SIMM and 3V DIMM, because it will decrease the 3V DIMM module life cycle. To protect yourself and your possession please make your right choice and using the right way to protect your system.
If you really want to use 5V SIMMs and 3V DIMMs together, please contact your DRAM vendor to confirm with the 3V DIMM module chips you are using, do not have the above problem issues. However after the confirmation, you can mix 5V SIMMs and 3V DIMMs on ASUS TX97 series mainboards as other mainboard manufactures do.

The following table lists 3.3V DRAM specifications from various DRAM vendors. You can contact your DRAM vendors for the infomation if they are not in the list.
 


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by:wayneb
ID: 1130942
Here is the Url I posted for you to look at, including the sdram table listing the different memory types.
http://www.asus.com.tw/Products/Techref/Memory/Dram/dimmsimm.html
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