(#5073) 1063Mbps (Ultra SCSI) System Unit Expansion Tower Adds I/O cards and disk units (#5058 required when adding disk units). #5073 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 (#2624 or #6513 required as a prerequisite), one battery backup, one 400-watt base power supply, two 500-watt power supplies, two I/O regulators and featurable disk units using #5058. 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 on the Model 720; 18 on the Models 730 and 740. Also Supported on all 6xx series machines except 600, all Sxx machines except S10, and model 530