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Allen-Bradley PLC racks (often also referred to as backplanes, backplates, base units, or chassis) act as a common bus that may be shared by multiple different subsystems, allowing those systems to distribute both data and power. In this way, the rack acts as the "heart" of a PLC system. Racks commonly contain a slot for the power supply, one or two for the CPU, and a number of additional slots for I/O modules. While it is recommended to select a rack with a number of slots optimal for the desired application, serial expansion modules placed in the final slot of a rack can expand the PLC system beyond the rack's slot limit.