The ControlLogix Chassis is the physical device in which the I/O Modules, Communication Modules, and Power supply are placed into.  The chassis is available in five different sizes:  4-slot, 7-slot, 10-slot, 13-slot, and 17-slot.  You may have other types of chassis in your system besides ControlLogix, such as Flex, SLC, and PLC chassis.  The ControlLogix communication modules give you the opportunity to interface with these other systems.

There is no configuration on the ControlLogix module, and the chassis uses a backplane architecture known as ControlBus.  You can think of the backplane as being it’s own network, and each slot number is the unique identifier for each module on the backplane.