Central direct digital control systems are typically used for:

• A. Supervisory control instead of closed loop control.
• B. Pure local control at individual devices.
• C. Open-loop monitoring only.
• D. Only data logging tasks.

Answer: (A)

Central direct digital control systems are designed to oversee and coordinate operations rather than perform the main feedback control themselves. A central computer handles supervisory tasks: monitoring process variables, setting setpoints, sequencing operations, and generating alarms and trends. The actual closed-loop control—the continuous adjustment of actuators based on feedback—is usually carried out by local controllers at each device or loop, with the central system guiding and supervising those loops. So this approach is about supervisory control, not replacing the local closed-loop control. The other options don’t fit because central DDC isn’t just pure local control, isn’t limited to open-loop monitoring, and isn’t defined only by data logging tasks—its primary role is to supervise and coordinate multiple loops.