In ladder logic conventions, input devices function to do what?

• A. Provide signals to the control logic circuit
• B. Power the system
• C. Directly drive output devices
• D. Store state information

Answer: (A)

Input devices are sensing elements that convert real‑world events into signals the PLC can read. Their job is to provide the control logic with the current state of external conditions. When a button is pressed or a sensor detects something, the corresponding input point becomes active, and the ladder rung uses that signal to decide whether to energize its output coil. In other words, input signals feed the control logic so it can determine what the outputs should do.

They don’t power the system—the power supply does that. They don’t directly drive outputs—outputs are energized only after the control logic evaluates the input signals. They also don’t store state information; any memory of state resides in the controller’s memory or in the output devices once energized. For example, pressing a start button sends a signal to the PLC, the logic evaluates the rung, and if conditions are met, the motor output is energized.