As I recall there were instructions in the last one I set up regarding how to install...something about the order of doing things. Looking at install manual now, says disconnect existing DEIs, power system up, connect one DEI at a time, no modules slotted, then go back in and insert modules. It also says plug in just one at a time (removing previous) to upgrade software before making final connection, but I'm sure I didn't do that.
Also be sure the 2 or 4 DEI mode choice is correct on processor (SYSTEM/CONTROLLER/DIGITAL EXPANSION INTERFACE, then DEI MODE choose 2 or 4).