OT Glossary

A design principle where if a system fails, it does so in a way that defaults to a safe condition. For example, a fail-safe valve might close (shutting off flow) if power is lost, to prevent an uncontrolled process. In OT, fail-safe mechanisms ensure that if something goes wrong, whether a component fails or communication is lost, the process will shut down or revert to a state that avoids harm to people, environment, or equipment. (Contrast with fail-secure, where on failure a system locks in a secure state, like a door locking during a power outage – but in industrial control, fail-safe is usually about safety.)

A general term for equipment on the plant floor or in the field that interacts directly with the physical process. Field devices include sensors (which measure things like temperature, pressure, flow) and actuators (like valves, motors, relays). They often connect to controllers (PLC/RTU) via I/O modules or fieldbus networks. 

A category of industrial network protocols designed for connecting field devices (sensors/actuators) to controllers, usually in a daisy-chain or bus topology (as opposed to each device having a direct wire to the controller). Examples include Profibus, Foundation Fieldbus, DeviceNet, and Modbus. 

A network security device (software or hardware) that monitors and filters network traffic based on predefined security rules. In an ICS environment, firewalls are used to segment networks (for instance, between the corporate IT network and the OT network, or between control levels) and to restrict traffic to only what’s needed. They can range from simple devices allowing only specific IP/port combinations, to more advanced ones that understand industrial protocols (industrial next-gen firewalls that can, say, allow Read commands but block Write commands to a PLC).