Which of the following best describes discrete inputs in a PLC?

Discrete inputs in a Programmable Logic Controller (PLC) are best described as representing only on or off states. This means they are binary inputs that can be clearly defined as either a high signal (on) or a low signal (off). Discrete inputs are typically used to read the status of simple devices like switches, pushbuttons, or sensors that produce a clear, quantifiable signal indicating a single condition.

The nature of discrete inputs allows PLCs to make straightforward decisions based on these binary states. For example, if a switch is closed, the PLC recognizes this as a '1' (or on state), whereas an open switch would be recognized as a '0' (or off state). This simplicity is crucial for applications requiring precise control and monitoring of basic on/off situations in industrial automation.

Other options introduce characteristics that do not apply to discrete inputs. Continuous and variable inputs suggest the presence of a range of values, which pertains to analog signals, but does not fit the binary nature of discrete inputs. Analog signals represent a spectrum of values rather than just two states, and complex signals imply a combination or processing of multiple signals, which extends beyond the functionality of simple discrete inputs. Thus, the definition of discrete inputs as only on or off states