|The ram slot required and the modules are the exactly the same length, but|
- the PC100 ram has 168 contacts, the PC2700 has 184 (some of them in the same places), half on each side.
- the ram slot for DDR ram has....
- 184 contacts
- one bump in the bottom that lines up with the one slot in the bottom of a DDR ram module, only when the module is installed in one direction, so that it can sit flat in the slot - the PC100 module has two slots in the bottom, in two different places than that for DDR ram - it can't go all the way down in a DDR ram slot, and visa versa.
You can install the PC100 module in the DDR ram slot and it will still connect, or it or the DDR ram module can be installed in the wrong direction, but you can't install it properly - the module can't sit flat in the slot and at least one latch at the ends of the slot can't be latched (unless you force it to, bending the mboard). (Installing a module in the wrong directions fries the module and the slot circuits the backwards module is in, at the very least, when the computer is booted, instantly.)
- the ram voltage provided in the slot is a lot lower than that for the PC100 ram. The PC100 ram module can't run at the minimum mhz the mboard and cpu require (min. 200mhz probably). Even if installing PC100 didn't harm the module or the slot (I've never tried that) the PC100 module would not be recognized.
However, there were some mboard models made that had ram slots for and could use either 168 or 184 "pin" ram.