What mode should be selected to modify code but not control the I/O modules?

Selecting Program Mode is the correct choice for modifying code without impacting the operation of I/O modules. In this mode, you can make changes to the logic and programming of the ControlLogix system while the system's processes continue to run without interruption.

When you are in Program Mode, the processor will not execute the current program; instead, it allows for editing and updating the logic. This is particularly useful when you need to make modifications to your control logic or programs safely, without the risk of affecting live operations or data collection tied to the I/O modules.

In contrast, Run Mode is where the controller actively executes the logic and manages I/O; thus, any programming changes would affect the real-time operation of the system. Remote Mode typically involves remote access for monitoring and configuration, while Test Mode is often used for simulating processes and behaviors without the risk of altering the actual I/O state. Therefore, Program Mode is specifically designed for safe, uninterrupted code modifications.