R5500 Backplane 2.5 or 3.5?

I keep reading about different cables for 2.5 or 3.5 backplanes.  Is the R5500 backplane a 2.5 or 3.5?  Wasn't sure since no one ever mentions like inch or something if that is determined by the size of the drives that "can" go into the slot.  So is because there can only be 2.5" hard drives in the Precision R5500 that the backplane is a 2.5"?

Trying to figure out the right cables to get for an H700 to replace the 6/i and apparently this is important.  Not sure what the difference between a 2.5 cable and a 3.5 cable is but trying to figure what I need.

Thanks.

JR