FBDIMM to "regular" MBO's

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Greetings to all,

A guy who's selling 2GB FBDIMM's for 30$ a piece swears to me that those
DIMMs work in *any* ECC capable board. Although I worked with both types
of memory, not in my wildest dreams I have thought about that idea, since
operation of those two types of RAM is completely different. However, he
also claims he was able to make those DIMMs work in some Asus MBO. What
shed the doubt is that they DDR2 vulgaris and FBDIMM are physically

So, under the risk I become the biggest lamer in the alt.c.h history, is
it possible that FBDIMM works on Bad Axe 2, for example?


Documentation is like sex: when it's good, it's very, very good; when it's
bad, it's better than nothing.

Re: FBDIMM to "regular" MBO's

Bubba wrote:
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Desktop motherboards have a parallel interface. 64 bit data, all transferred
in one tick. Commands on a separate bus. Half duplex operation (on a
given cycle, can do one of: read, write, or do nothing).

There are some FBDIMM pictures here:

FBDIMMs are packet oriented, high speed serial, more signals in one direction
than the other. Full duplex operation is possible to the motherboard.
(Read and write in-flight at the same time.) Between the controller
and the memory chips is half-duplex.

       FBDIMM       ------->  FBDIMM     -------> to next FBDIMM on channel
       Motherboard  <-------  controller <-------
                    separate  chip
                    busses      |
                             Memory chips on FBDIMM
                             have a conventional interface
                             to the controller. This
                             allows commodity chips to be

A conventional desktop looks like this

      DDR2         <---------> Memory chips on DDR2 DIMM

The two are quite different. There is no "controller chip" on a
desktop DDR2 DIMM. The motherboard talks straight to the RAM
chips. On the FBDIMM, the FBDIMM controller is an intermediary
and buffer. The FBDIMM controller decides whether a transaction is
for it, or should be passed to the next FBDIMM on the channel.
FBDIMMs daisy-chain, as I show in the figure above.
The desktop motherboard needs no such mechanism.

The FBDIMM is fully protected in terms of ECC like functions (called
CRC or cyclic redundancy check). Desktop memory comes in protected
and unprotected flavors.

There is a pinout here for FBDIMM. Yes, it has 240 pins. No, the
key is in the wrong place, to allow you to insert it in the
wrong kind of slot. The pinout is entirely different.
Things like power pins might well be in the wrong places.


If you try it, the "magic smoke" is going to come pouring out :-)
And you'll need a hammer to make it fit (plastic key stops it).

Your friend should sell them on Ebay, as they're going to
fetch more than $30 each.


Re: FBDIMM to "regular" MBO's

Paul's log on stardate 03 pro 2007

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/snip, thx!

Thanks Paul. It was only that this guy was so bloody convincing... :) It
made no logic to me in the first place, but I really thought I was dumb
and was missing something.

Documentation is like sex: when it's good, it's very, very good; when it's
bad, it's better than nothing.

Re: FBDIMM to "regular" MBO's

Sorry, but not a chance in the world.  Wrong specs.

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