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Posted on 1999-06-30
Medium Priority
Last Modified: 2013-11-10
1)  Can a 100MHz motherboard use 66Mhz SDRAM?

2)  How can I tell if I have 66Mhz or 100Mhz SDRAM if I do
not know what kind of motherboard I have?  Any designation
on the chip itself?

3)  Are all Pentium Motherboards(P-66 to P-300)
( Not Pentium II) 66Mhz?
Question by:lllao
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Expert Comment

ID: 1158264
Dear Iiiao:

First of all, when you indicate that you have a 100 MHz
motherboard, do you mean that the bus speed is 100 MHz, or
do you mean that the CPU is clocked at 100 MHz internally?

Anyway, you must use memory that's rated at least as fast as the motherboard. In other words, if your motherboard's bus is 100 MHz, then your memory must be rated at 100 MHz or higher.

As for your 2 other questions, I'm not sure, so instead of
giving you a "hit-or-miss right or wrong" answer, I won't
attempt to answer these questions.

Author Comment

ID: 1158265
Questions 2 & 3 are more important.  Please answer.

Expert Comment

ID: 1158266
1) Can you use PC66 SD on a 100MHz board *if* you only intend to run at 66 bus.  A good PC66 SD can usually handle up to 83MHz but not 100.

2) There's really no definate way to tell if it's 66 or 100.  On Intel PC100 spec, it calls for SPD and EPPRom as standard on PC100, but lots of manufacturer make without these to cut down cost.  So, you can try to look for SPD and EPPRom on the SD module.  But the easliest way is still to run it at 100 to see if it's stable.  Or give me the brand and the numbers on the module to see if I can find anything.

3) Most Pentium motherboards are capable of 50/66 since this is the Intel spec for pentium.  But most should also have 75/83 settings for overclockers.  And none have 100 settings except for the non Intel chipset super-7 boards.
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Expert Comment

ID: 1158267
Q1. Yes, if you can dumb down the motherboard speed. On my FIC VA-503+, I can set speeds of 66, 75, 83, 95, 100, 112, and 124 MHz.

Q2. If a DIMM is PC-100, there usually will be a sticker on it proclaiming that fact. Short of plugging the RAM into a dedicated hardware tester, there's no other *quick* way to figure if it's SDRAM or not.

To calculate speed, on the memory chips there will be a part number, then a second number after the hyphen. The second number is the speed in nanoseconds. Take its reciprocal to get the speed in megahertz. (-15 = 66 MHz, -12 = 83 MHz, -10 = 100 MHz, -8 = 125 MHZ). It is good practice *not* to run the memory at its design limit. I've successfully run 100MHz non-PC100 SDRAM at 66MHZ, and in my current setup, I have a PC100 SDRAM (8ns) which I run at 100MHz.

Q3. The answer to your question depends on the chipset installed on your motherboard. All the Intel chipsets in the 430 series had a max speed of 66MHz. Usually the motherboard gave you the options of 50, 60, and 66 MHz. The 430TX and VX were the only two which were capable of running SDRAM.

Other 3rd party chipsets, such as VIA, Acer Labs, and SiS, had support for all the slower speeds, and added 75, 83, and 100MHz.

Hope this helps.


Expert Comment

ID: 1158268
To lllao and larbel,

I didn't see larbel's answer until after I submitted my comment, and then the page refreshed and voila!

Stuff happens


Expert Comment

ID: 1158269
Slight correction to the stuff mentioned above...

A1: no additional comment

A2: Use Ramcomp's "formula". It may also be marked "BX memory" to denote that it was designed for the 440 BX chipset, the first 100 MHz sidebus. Also note that is now selling PC133 SDRAM (7.5 ns!)

A3: Pentium chipsets (not Super7) runs at  60 or 66 MHz, depending on clockspeed. Super 7 chipsets can go up to 100, maybe higher.

Expert Comment

ID: 1158270
There is one other thing to check for.....
Some early boards had the DIMM socket, but are configured to use 168-pin EDO memory. (5 volts)
Try to determine that yours is NOT one of those before installing SDRAM, which is rated for 3v operation.
Some have jumper pins labeled with the voltage settings.
Good luck,

Author Comment

ID: 1158271

Here is the info off the 32MB 168 Pin module:

TI             -10
2E  81A3VP P

Note:  This is a computer probably built about March 1998.
          It has an Intel 200Mhz Cpu .  It is a Pentium MMX board.(non Intel, I think           FIC or Via)

I just want to add on compatible memory.  An extra 64 MB.

Expert Comment

ID: 1158272
that is a 66Mhz chip, not PC100.  So just buy a plain old SDRAM chip for your added memory, although I think at this point PC100 memory is actually cheaper, and will usually work just fine when clocked at 66mhz.  but if you want to be absolutely safe, buy a 66Mhz SDRAM DIMM with EPROM.


Accepted Solution

sp00n earned 1080 total points
ID: 1158273
1) A 100mhz motherboard CAN use 66mhz RAM
as long as your clock multiplier is set to 66 or lower
and that you have the correct voltage setting for the
RAM .. If not, you will fry your RAM.

2) Usually on each memory chip it will have a long
type of CODE. and at the end of the code -66 or -100.

Most good RAM making companies will stamp their RAM in that way.

3) The answer is NO, Some boards are 75mhz or anywhere up to 133mhz in Socket 7. Yet they are AMD or Low end Pentium boards. E.g the ASUS P5-AB.

Hope this helps you out.


Expert Comment

ID: 1158274
sp00n... Half your answers are wrong! 1) The correct term is "Front Side Bus" or "FSB". Clock multiplier is for the CPU itself!  2) the memory is NOT stamped -66 or  -100! It's stamped -10 (10 ns), -8 (8 ns), and so on!  3) ASUS P5 series is a Super-7 board, not a "low-end Pentium board".

Expert Comment

ID: 1158275
Umm hate to be a burden on kschlang but you are INCORRECT.
Yah I agreee with you on number one .. musta been off in some other world .. sorry. Clock Multiplier = 4x 5x 6x etc.
The FSB is Megahertz ratio.

2) I DISAGREE TOTALLY ... 99% of memory i have stamped IF IT IS SDRAM<which is what we are talking about> has a 66 or 100 stamping ... ns stamping is for SIMM ram.

3) The ASUS P5A-B (which if you looked above is what was mentioned) IS a board that runs Lower end Pentiums or AMDS, YES it can run a FSB & Clock multiplier for late model cpus... but it means u are clocking the Clock up to like 5 or 6 times multiply <DANGEROUS>. Usually its about a 300mhz or 233 or round that which is used comfortably on the boards.

Expert Comment

ID: 1158276
sp00n....  I've never seen any SD with stamping of 66 or 100 on the module, only the speed -10ns, -8ns so on like kschang described.  And Asus P5A-B *is* a Super7 board, though it's backward compatable but still doesn't make it a lower end pentium board!

Expert Comment

ID: 1158277
Gees its not a DIRECT stamping its a long line of code with
either 66 or 100 at the end of the code.

Check out the below link about SDRAM info.

Also I can suggest i mean if you know your board takes PC100 or 66mhz RAM, try installing it .. if its 66mhz RAM it will just fail to boot if your board needs PC100

About the motherboard - Direct from the ASUS Site. I do believe the BELOW listed cpus are now all Lower-End .. Thank You Very Much.

"Multi-Processor / Multi-Speed Support

AMD-K6TM-2 (PR266 ~ Pr350), AMD-K6TM (PR166 & faster), AMD-K5TM (PR75 ~ PR133), IBM®/Cyrix® 6x86MXTM/ M IITM (PR166 & faster), IBM®/Cyrix® 6x86-PR166+ (Rev 2.7& faster), Intel Pentium® 90 ~ 233MHz (P55C - MMXTM)  "

Expert Comment

ID: 1158278
Just adding to my comment above.

HY5116100A <- from a Hyundai DIMM of Ram .. Note to 100 Specifies the SPEED <PC66 or PC100>. I rest my case, even though some RAM does not have those markings, decent well made ram does.
see if that helps

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