Compare direct digital control versus limit switching with a thermostat in simple terms.

• A. DDC is a purely on/off mechanical switch at fixed thresholds; limit switching is software-based.
• B. DDC uses only local thermostats while limit switching uses a PLC.
• C. DDC uses software-based control with sensors/actuators and can implement advanced strategies; a simple thermostat just switches a device on or off at setpoints.
• D. DDC ignores setpoints; limit switching enforces exact temperature.

Answer: (C)

Direct digital control uses software-based logic that reads sensors, processes data, and sends precise signals to actuators. This lets it run control loops (like PID), manage multiple variables, sequence operations, optimize energy use, and even adapt based on schedules or occupancy. A thermostat that uses a limit switch is a simple, binary device: it turns equipment on or off when the temperature crosses a setpoint, often with some small hysteresis to prevent rapid cycling. It doesn’t process data, adjust strategies, or coordinate with other equipment.

So the best description is that direct digital control relies on software-based control with sensors and actuators and can implement advanced strategies, while a simple thermostat simply switches a device on or off at setpoints. The other descriptions either mischaracterize DDC as purely mechanical or imply that limit switching has more complexity than it does.