CX-Programmer Ver.2.0 or higher displays "I:" and "Q:" in front of addresses in a ladder diagram to help you differentiate between inputs and outputs when reading the ladder diagram.

Note:Programmable Controllers that do not have I/O tables (i.e., CQM1H and CPM2A) do not have this feature.
When an address used in a ladder program operand is an I/O bit, and the address is allocated to a Basic I/O Unit or to a High-density I/O Unit classified as a Special I/O Unit in the I/O tables, then "I:" will be automatically added to in front of the address for real inputs, and "Q:" will be added to the front of the address for real outputs.

In CX-Programmer Ver.3.0 or higher, the "I:" and "Q:" in front of the addresses that are used to differentiate between inputs and outputs can be changed to "X:" and "Y:". The setting can be changed on the Ladder Diagram Tab Page (Tools - Options - Ladder Diagram).