# PLC **Domain:** Industrial Automation / Control Engineering **Doc Type:** Technical Concept Node **Maturity:** Developed ## Definition A **PLC**, or programmable logic controller, is a ruggedized industrial computer that executes deterministic control logic for machines and processes. ## Governing Variable The PLC governs local state transition: read inputs, evaluate logic, update outputs and repeat within timing constraints appropriate to the process. ## Material and Computational Manifestation PLCs connect sensors, switches, motors, valves and safety-related equipment to programmed sequences, interlocks and control loops. They can operate locally while exposing state and selected commands to supervisory systems. ## Relation to Governance The PLC illustrates competent execution below the [[wiki/Control Plane|control plane]]. A supervisory layer may change modes or setpoints, but the local controller retains process-specific logic and timing. Removing that competence in favor of remote micromanagement can reduce resilience. ## Constitutional Question and Failure Modes Change authority, version control, local override and safe failure must be explicit. Failure modes include unauthorized logic changes, hidden dependencies, input faults, unsafe assumptions and remote policies that violate local interlocks. ## Related Ontology [[wiki/Industrial Control System|Industrial Control System]] · [[wiki/Control Loop|Control Loop]] · [[wiki/SCADA|SCADA]] · [[wiki/Actuator|Actuator]] · [[wiki/Operational Autonomy|Operational Autonomy]]