# What is a Programmable Logic Controller (PLC)?

> A programmable logic controller (PLC) is an industrial solid-state controller with user-programmable memory for executing functions such as logic, sequencing, timing, counting, arithmetic, communication, input and output control, and proportional-integral-derivative control.

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## Content

A PLC reads signals from sensors or other inputs, executes its configured program, and drives outputs such as motors, valves, relays, or alarms to control a physical process.

PLCs can act as local controllers within SCADA or distributed control systems and can be the primary controllers in smaller automation systems. Their programs, set points, operating modes, communication paths, and engineering workstations are high-consequence assets: an unauthorized or mistaken change may alter physical behavior even when the device continues to appear healthy.

### Key points

- **Control role:** PLC scan cycles and control logic translate measured process conditions into timely output actions; performance and deterministic behavior may be operational requirements.
- **Engineering path:** Restrict and monitor programming software, project files, downloads, firmware changes, keys and mode switches, and access from engineering workstations.
- **Network and identity controls:** Remove default credentials where supported, allow only required peers and protocols, segment control networks, and prevent direct internet exposure.
- **Integrity and recovery:** Maintain verified logic and configuration backups, record authorized baselines and changes, monitor feasible indicators, and test restoration on compatible equipment.
- **Important limitation:** Many installed PLCs have constrained security features or legacy protocols, and active scanning, patching, rebooting, or blocking communications can interrupt control or safety functions. Changes require engineering review, testing, and an operational recovery plan.

### Related terms

[Industrial control system (ICS)](<https://yellowcube.eu/glossary/industrial-control-system/>) · [Supervisory control and data acquisition (SCADA)](<https://yellowcube.eu/glossary/supervisory-control-and-data-acquisition/>) · [Human-machine interface (HMI)](<https://yellowcube.eu/glossary/human-machine-interface/>) · [Safety instrumented system (SIS)](<https://yellowcube.eu/glossary/safety-instrumented-system/>) · [Operational technology (OT) security](<https://yellowcube.eu/glossary/operational-technology-security/>)

### Sources

[NIST glossary: PLC](https://csrc.nist.gov/glossary/term/plc) · [NIST SP 800-82 Rev. 3: Guide to Operational Technology Security](https://csrc.nist.gov/pubs/sp/800/82/r3/final)

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