Which type of input is designed to handle gradually varying signals rather than just on/off?

• A. Analog input
• B. Digital input
• C. Unipolar input
• D. Bipolar input

Answer: (A)

Signals that vary gradually require capturing a continuous range of values, not just ON or OFF. An analog input is designed to read that continuum, converting a voltage or current that can take many values into a proportional number the PLC uses. This lets the controller respond to changes in real quantities like temperature, pressure, or light level. In contrast, digital inputs only detect whether a signal is present or not, so they stay at a fixed high/low state and can't represent gradual changes. The terms unipolar and bipolar describe the allowed polarity or range of an analog signal (for example 0 to +10 V versus −10 V to +10 V), but both still rely on an input type capable of resolving many levels—an analog input. Therefore, the input type that handles gradually varying signals is the analog input.