(#5072) 1063Mbps System Unit Expansion Tower Adds SPD cards and disk units (#5052 required when adding disk units). #5072 includes a 1063Mbps optical bus card, various cables (including an optical cable) and the I/O Tower. The I/O Tower includes 13 SPD card slots, space for four removable media devices (#6513 required as a prerequisite), one battery backup, one 400-watt base power supply, one 400-watt power supply for availability, two I/O regulators and featurable disk units using #5052. #5072 is supported on model upgrades only, and cannot be ordered new. Prerequisites: One port on the OLP (#2688) in the System Unit Expansion #5064/#9364 on the Model 720. #2688 and #2695, or #2688 and an open IOA slot in the base optical adapter on Models 730 and 740. Maximum: A total of four expansion towers of all types are supported the Model 720; 18 on the Models 730 and 740. All supported on 620, 640, 650, S20, S30, S40, & 530 Systems.