This document discusses various addressing modes in PLCs, including direct, indirect, indexed, and indexed indirect addressing. It provides examples of each addressing mode using SLC500 and ControlLogix PLCs. Indirect addressing allows data to be accessed using a reference address rather than a direct address. Indexed addressing uses a base address plus an offset value from an index register to calculate the final address. ControlLogix uses arrays instead of indexed addressing, where arrays can have one, two, or three dimensions to store multiple values of the same data type.