Differences Between DDR and DDR SDRAM

Can someone tell me the differences between DDR and DDR SDRAM?  I'm trying to purchase a server with 2 GB of memory, but if offers different ways of getting it.  I would like to know the differences between the 2.

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Luc FrankenEMEA Server EngineerCommented:
This link should give you a little more info:
You can see there that DDR ram runs at twice the clockspeed compared to SD
DDR stands for Double Data Rate, if you can affort it choose the DDR.


Glen A.IT Project ManagerCommented:
Well, first you should understand that DDR IS SDRAM.  yes, it is.  Regardless of what anyone will tell you, or wants to argue here, DDR is SDRAM.

However . . .  

DDR does stand for double data rate, as LucF mentioned.  that's how it differs from standard SDRAM.   It also comes on a 184pin module whereas standard SDRAM comes on a 168pin module.

SDRAM is available in PC66 (66Mhz), PC100 (100MHz) and PC133 (133MHz) varieties.

DDR is available in different configurations:  PC1600 (200MHz), PC2100 (266MHz), PC2700 (333MHz), PC3200 (400MHz), PC3500 (433MHz), PC3700 (466MHz) , PC4000 (500MHz) etc.    Currently only the first four that I listed are certified Jedec RAM specifications.
Luc FrankenEMEA Server EngineerCommented:
>Regardless of what anyone will tell you, or wants to argue here,
I won't I totally agree with you... ;-)
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Glen A.IT Project ManagerCommented:
Double data rate (DDR) SDRAM does exactly what it sounds like, it doubles the rate of speed at which standard SDRAM can process data. This means DDR memory is roughly twice as fast as standard SDRAM.  The big difference between DDR SDRAM and standard SDRAM is that DDR reads data on both the rising and falling edges of the clock signal.  Standard SDRAM, or single data rate (SDR) SDRAM, only carries information on the rising edge of a signal.  Basically this allows the DDR module to transfer data twice as fast as standard SDRAM.  For example, instead of a data rate of 133MHz, DDR memory transfers data at 266MHz.

Regarding the PCxxxx spec:  (from cluboverclocker.com) The guys in suits who sit in their plush offices and do nothing but think up of ways to make us purchase their new technology have come up with a new way of naming DDR memory to make it "sound faster".  Don't be confused, DDR memory has not changed.  The suits have renamed the specs of DDR memory after the peak bandwidth, not the bus frequency.  Hang on, this is where it gets confusing!  PC1600 DDR is really PC200 DDR or PC100 DDR and PC2100 is actually PC266 DDR or PC133 DDR!  Why the change?  It's all because the 100MHz bus transfers two bits of data per wire per clock (double data rate), which yields 1600MB (1.6GB) per second over a 64‑bit bus.  The same goes for PC2100 (PC133), 133MHz multiplied by two bits per clock multiplied by eight bytes equals 2100MB (2.1GB) per second.  Don't be fooled, DDR memory has not changed.  This all boils down to a sad attempt to make DDR memory sound more impressive when it actually runs on the same bus speeds as before.

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Glen A.IT Project ManagerCommented:
So to correctly answer your question: >>Can someone tell me the differences between DDR and DDR SDRAM?

There is no difference between "DDR" and "DDR SDRAM".  they are one in the same.  All DDR RAM is Synchronous Dynamic Random Access Memory (or SDRAM).
Glen A.IT Project ManagerCommented:
LucF: >>DDR stands for Double Data Rate, if you can affort it choose the DDR.

Um . . .  he's being offered DDR.   Sounds like he's only being offered DDR, imho.
Glen A.IT Project ManagerCommented:
LucF: >>I won't I totally agree with you... ;-)

Um . . .  sorry to say this, but if you don't agree that "DDR" is still SDRAM, then you need to do some research.  No offense.
Luc FrankenEMEA Server EngineerCommented:
>LucF: >>I won't I totally agree with you... ;-)
That was meant as:
I won't argue here,
I totally agree with you...

>LucF: >>DDR stands for Double Data Rate, if you can affort it choose the DDR.
I assumed he's offered SD and DDR ( as in PC133 and PC2100 )

Let's first find out what Joe_27 really means.

Glen A.IT Project ManagerCommented:
Oops, didn't see the second 'I', and thought it said "I won't totally agree with you".

Sorry, LucF

Looks like the specs he's given are (from one supplier) "DDR" and from another supplier "DDR SDRAM", is what I got from what was written.  Joe_27, is that correct??

>>Can someone tell me the differences between DDR and DDR SDRAM?
DDR is a version of SDRAM which transfers data on both clock edges.
SDR SDRAM is the original single transfer version.
Joe_27Author Commented:
Actually, the specs are from the same supplier.  They list 1GB of memory, but offer choices in the types like this:

 1 GB DDR Memory
 1 GB DDR SDRAM Memory

Luc FrankenEMEA Server EngineerCommented:
Then I suggest you print this page and show it to that supplier, he doesn't know what he's talking about. Let me guess, the DDR SDRAM is more expensive ;-)

Does he give more information ( brand, frequencies, latency, etc.)?

Glen A.IT Project ManagerCommented:
Public:  >>DDR is a version of SDRAM which transfers data on both clock edges.
            >>SDR SDRAM is the original single transfer version.

Hmm...  wasn't that exactly what I said yesterday??

Glen A.IT Project ManagerCommented:
Joe_27, my guess is that the supplier is copying/pasting specs from their distributor, and that it's the distributors who are listing it differently.

But I assure you, as I said earlier, there is no difference between 1 GB DDR Memory and 1 GB DDR SDRAM Memory.  They are one and the same.   As I mentioned, ALL DDR memory is SDRAM.
SDRAM does not mean PC-100 or PC-133 ram.  DDR SDRAM is DDR ram.  SDR SDRAM is single data rate ram.  Thanks, Beef.  I'm sorry if I've repeated anyone's claims.  I just skimmed through this question; it seems to me no one has mentioned SDR SDRAM.  That's probably because it's not ever typed out like that.  That would be SDR DIMM, as opposed to DDR DIMM.  That's the same type of memory...  It's just a specification of another part of the ram.  I guess you could say you'd like a DDR SDRAM DIMM or two.

Thank you,
Radomir Jordanovic
Glen A.IT Project ManagerCommented:
>> it seems to me no one has mentioned SDR SDRAM.

Radomir: go back and you'll see where I did.  It's the comment that's headed with:

  Comment from AlbertaBeef
  Date: 11/17/2003 01:50PM MST

1st paragraph, 2nd and 3rd sentences :-)
I have another question.... can I use both ddr and sdr in the same motherboard?... I mean I have a motherboard that has 4 slots 2 ddr and 2 sdr, is it ok if I use 4 of them?

Glen A.IT Project ManagerCommented:
No, you cannot.  They cannot and should not be mixed.
Because of different voltages and speeds, the motherboard won't turn on with all of the ram slots filled.
Glen A.IT Project ManagerCommented:
Sometimes it will turn on fine and simply ignore one of them, other times it won't.  Depends on how the system is designed.  But you cannot use them together, even if you wanted to, and the main reason is because of the speed differences, yes.
Glen A.IT Project ManagerCommented:
No comment has been added lately, so it's time to clean up this TA.
I will leave the following recommendation for this question in the Cleanup topic area:

Accept: AlbertaBeef{9708584}

Please leave any comments here within the next seven days.

EE Cleanup Volunteer
Page Editor, Desktops/Microchips TA's
Glen A.IT Project ManagerCommented:
Joe_27:  fyi, all the information provided here was everything you needed -  Why did you award a 'B' grade?
Joe_27Author Commented:
Sorry for that AlbertaBeef....my mistake.
Glen A.IT Project ManagerCommented:
no problem, thanks.
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