What type of input operation requires conversion to digital form before input?

The type of input operation that requires conversion to digital form before input is analog input. Analog signals are continuous signals that can represent a range of values, such as temperature, pressure, or sound. However, digital systems, like most modern control systems, operate using discrete values (often represented as binary data).

To use an analog input in these digital systems, it must be converted into a digital format, typically through an Analog-to-Digital Converter (ADC). This conversion allows the system to process and interpret the analog signals in a way that is compatible with the digital controls and processing techniques used in modern automation and control systems.

The other types of input operations listed do not require such conversion. For example, disc input usually refers to data stored on physical media and is already in a digital form. Contact courses and limit switches are examples of digital input devices, as they operate using on/off states without the need for conversion.