Control unit
Control units are connected to the channels with "bus and tag" cable pairs. The bus cables carried the address and data information and the tag cables identified what data was on the bus. The general configuration of a channel is to connect the devices in a chain, like this: Mainframe—Control Unit X—Control Unit Y—Control Unit Z. Each control unit is assigned a "capture range" of addresses that it services. For example, control unit X might capture addresses 40-4F, control unit Y: C0-DF, and control unit Z: 80-9F. Capture ranges had to be a multiple of 8, 16, 32, 64, or 128 devices and be aligned on appropriate boundaries. Each control unit in turn has one or more devices attached to it. For example, you could have control unit Y with 6 disks, that would be addressed as C0-C5.