Compare PLC and DCS systems for process control.

PLC (Programmable Logic Controller): originated for discrete/sequential control, fast scan times (ms), typically standalone or networked, strong for logic and motion control, programming via ladder logic, function blocks. DCS (Distributed Control System): designed for continuous process control, integrated HMI and historian, extensive regulatory control libraries, distributed architecture, engineering tools for large systems. Convergence: modern systems blur distinctions. PLCs adding analog/PID; DCS adding sequence control. Selection based on: application (discrete vs continuous), system size, integration needs, and existing platform.