HMI / SCADA / P&ID Glossary
The fast-reference for engineers buying, designing or migrating an HMI / SCADA. Each term in 2-4 sentences with cross-references and examples from real plant equipment.
Quick navigation
- HMI
- SCADA
- DCS
- PLC
- P&ID
- ISA-101
- ISA-5.1
- ISA-5.5
- ISA-18.2
- Alarm priorities P1-P4
- Alarm flood
- BPCS
- HART
- Calm HMI
- Situation awareness
- Tag (instrument tag)
- Mounting symbol
- Multi-state lamp
- ESD
- SIS / Safety Instrumented System
- Fieldbus
- OPC UA
- Modbus
- RTU
- Profibus
- PROFINET
- EtherNet/IP
- FOUNDATION Fieldbus
- Operator display
- Display levels
- MCC
- VFD / VSD
- Interlock
- Permissive
- Trip
- PID control loop
- Setpoint
- Deadband
- Hysteresis
- Cascade control
- Engineering units
- Ladder logic
- Faceplate
- Watchdog
- Redundancy
- Trend (historian)
- OEE
- HMI style guide
HMI /eɪtʃ em aɪ/
Human-Machine Interface. The graphical layer an operator interacts with to monitor and command a process. Modern HMIs are typically web-based panels (Weintek cMT, Siemens WinCC Unified, Ignition Perspective) or dedicated hardware (Rockwell PanelView, Pro-face GP). The HMI shows tank levels, pump states and alarms; the operator sees what's happening and gives commands.
SCADA /ˈskeɪdə/
Supervisory Control and Data Acquisition. A plant-wide system that gathers data from multiple PLCs / RTUs across distance (sometimes geographically dispersed: water utilities, oil pipelines) and presents it on operator workstations. SCADA is broader than HMI — it includes the historians, alarm management, batch control, and the supervisory logic. Vendors: AVEVA Wonderware, Inductive Automation Ignition, Rockwell FactoryTalk View SE, Schneider Citect.
DCS /diː siː ɛs/
Distributed Control System. A tightly-integrated control system used in continuous processes (refineries, chemical plants, pulp & paper, large utilities). Unlike PLC+SCADA, the DCS is engineered as one product — controllers, I/O, HMI and historian come from one vendor (Honeywell Experion, Yokogawa Centum, ABB Ability System 800xA, Emerson DeltaV). Used when reliability and tight integration matter more than flexibility. The HMI in a DCS is called the operator display.
PLC /piː ɛl siː/
Programmable Logic Controller. A ruggedized industrial computer running deterministic logic on a scan cycle (typically 10-50 ms). PLCs handle discrete control (motor on/off, valve open/closed) and batch sequences. Examples: Siemens S7-1500, Rockwell ControlLogix, Schneider M580, Mitsubishi MELSEC, Beckhoff TwinCAT. PLCs are programmed in the IEC 61131-3 languages (Ladder Diagram, Function Block, Structured Text, SFC, Instruction List).
P&ID /piː ən aɪ diː/
Piping & Instrumentation Diagram. The master engineering drawing of a process plant: every pipe, valve, instrument and equipment item is shown with its tag, connection and operational state. P&IDs use standardized symbols (ISA-5.1, ASME Y32.11, BS 1646) so the same drawing is readable across vendors and countries. Modern HMI screens often mirror P&ID layouts so operators can cross-reference quickly.
ISA-101 /aɪ ɛs eɪ wən oʊ wən/
ISA-101.01-2015 — Human Machine Interfaces for Process Automation Systems. The standard for HMI design. Defines the lifecycle (style guide → design → implementation → operation → maintenance), display hierarchy (overview → unit → detail → diagnostic), color philosophy (color = state, not decoration), alarm priorities, and typography. Compliance reduces alarm fatigue and operator error. Full ISA-101 guide.
ISA-5.1 /aɪ ɛs eɪ faɪv pɔɪnt wən/
ISA-5.1-2009 — Instrumentation Symbols and Identification. The standard for tag letters and instrument bubbles on P&IDs. Defines: function letters (P=pressure, T=temperature, F=flow, L=level, A=analytical, Z=position…), modifiers (I=indicator, C=controller, T=transmitter, S=switch, V=valve), and mounting bubbles (field circle, DCS circle with line, PLC hexagon, shared circle with double line). Full ISA-5.1 reference.
ISA-5.5
ISA-5.5 — Graphic Symbols for Process Displays. The standard for equipment shapes (vessels, columns, pumps, heat exchangers) on P&IDs and HMI screens. Complements ISA-5.1 (which covers instrumentation tags) by defining shapes for the process equipment itself. Used widely in conjunction with ASME Y32.11 (US) and BS 1646 (UK) / DIN 28004 (Germany).
ISA-18.2
ISA-18.2-2016 — Management of Alarm Systems for the Process Industries. Defines the alarm lifecycle: identification, rationalization, design, implementation, operation, monitoring, management of change, audit. Sets prescriptive limits (max 1 alarm per 10 minutes per operator) and distinguishes nuisance alarms from real ones. Often referenced alongside ISA-101 for HMI design.
Alarm priorities P1-P4
The 4-tier classification used by ISA-101 / ISA-18.2:
- P1 Critical (red) — immediate action required (< 1 min); safety, environmental or major equipment damage at stake
- P2 High (orange) — action within minutes; production loss or secondary equipment risk
- P3 Medium (yellow) — action within tens of minutes; off-spec product, efficiency loss
- P4 Low (blue) — end-of-shift; diagnostic or predictive maintenance
Color-coding redundantly with shape (triangle / diamond / hexagon / circle) so colorblind operators can still distinguish.
Alarm flood
A burst of more than 10 alarms in 10 minutes per operator (ISA-18.2 threshold). During an alarm flood, the operator cannot read, prioritize or act — situational awareness collapses. Common causes: cascading trips, missing rationalization, alarms used for status. The fix is rationalization (delete, suppress, raise threshold) and shelving rules, not a bigger screen.
BPCS · Basic Process Control System
Basic Process Control System. The DCS or PLC+SCADA layer that runs continuous and discrete control of the plant under normal conditions — the day-to-day "brain". Distinguished from the SIS by IEC 61511, which mandates physical, logical and administrative independence between the two so a BPCS failure cannot defeat the safety layer.
HART
Highway Addressable Remote Transducer. Layered on top of the legacy 4-20 mA loop: the analog signal still carries the primary variable, while a low-amplitude FSK signal carries the digital diagnostics, configuration and secondary variables. The pragmatic upgrade path that lets plants reuse existing wiring — most modern transmitters speak HART, and HART-IP / WirelessHART extend it.
Calm HMI
The HMI design philosophy of ISA-101 / High Performance HMI Handbook: most of the screen is gray and white; saturated color (red, orange, yellow, blue, green) is reserved for abnormal conditions and equipment state. The opposite of "Christmas-tree" HMIs of the 1990s where everything was colored decoratively. Calm HMIs reduce alarm fatigue by a measurable margin. Read the calm color palette guide.
Situation awareness
Mica Endsley's 1995 model: an operator is effective only if they can perceive what's happening, comprehend what it means, and project what will happen next. ISA-101 explicitly maps every design rule to one of these three levels. A good HMI helps the operator project (not just react).
Tag (instrument tag)
The unique ID of an instrument or final control element: FCV-104 = flow control valve, loop 104. Format XX[X]-NNN where the letters describe function (see ISA-5.1) and numbers identify the specific loop. Tags are sacred — they appear on P&ID, HMI, control logic, alarm summaries, maintenance work orders. Changing a tag is a Management of Change exercise.
Mounting symbol (instrument bubble)
The shape of the bubble around the tag tells you where the instrument lives:
- Plain circle = field-mounted (visible only at the equipment)
- Circle with horizontal line = DCS-mounted (visible in the control room)
- Hexagon inside circle = PLC-mounted
- Circle with double horizontal line = shared display / shared control
Multi-state lamp
The HMI graphic object that maps a single PLC tag (typically UINT16) to a list of pictures or styles. Used to render the 9-state matrix per motorized equipment (stopped / starting / running / no-feedback / manual / disabled / +P1 / +P2 / fault). In Weintek EasyBuilder Pro: object name Multi-state Lamp; in WinCC: Multi Lamp; in FactoryTalk: Multi-state Indicator; in Ignition: bind icon to expression. 9 motor states explained.
ESD
Emergency Shutdown — the independent system that brings a plant or unit to a safe state when normal control fails. ESD trips bypass the regular HMI / DCS and fire safety-rated solenoids directly. Drawn on P&IDs with bold red ESD valves. Required by IEC 61511 / ISA-84 in hazardous processes.
SIS · Safety Instrumented System
Safety Instrumented System. The collection of sensors, logic solver and final elements that implement a Safety Instrumented Function (SIF) to bring the process to a safe state. Operates independently from the basic process control system (BPCS) — different hardware, different power, different cabling. Governed by IEC 61511 / ANSI ISA-84.
Fieldbus
Digital networks that replace 4-20 mA analog wiring at the field level: Profibus DP/PA, Profinet, EtherCAT, EtherNet/IP, Modbus TCP, FOUNDATION Fieldbus, HART. Fieldbuses move multiple variables per device (signal + status + diagnostic), reducing wiring cost and enabling predictive maintenance.
OPC UA
OPC Unified Architecture — the modern industrial communication standard from the OPC Foundation. Supersedes OPC Classic (DCOM-based). Platform-independent, secure (TLS by default), supports companion specifications (PackML, MTConnect). The lingua franca for connecting PLCs, HMIs, MES, ERP and IoT cloud services.
Modbus
The oldest still-relevant industrial protocol (1979). Two variants: Modbus RTU (over RS-485 serial) and Modbus TCP (over Ethernet). Simple, ubiquitous, supported by nearly every device. Function codes 03 (read holding registers) and 06 (write single register) are 90% of the use cases.
RTU
Remote Terminal Unit. A ruggedized field controller that collects sensor data and actuates equipment at a remote, often unmanned, site and reports back to a central SCADA master over a wide-area link (cellular, radio, satellite, leased line). RTUs dominate geographically dispersed infrastructure — water/wastewater networks, pipelines, electrical substations — where a full PLC cabinet is overkill. The line between RTU and PLC has blurred; many modern devices do both.
Profibus
Process Field Bus. A serial fieldbus standardized in IEC 61158, historically dominant in Siemens-based plants. Two profiles: Profibus DP (Decentralized Peripherals) for fast cyclic I/O over RS-485, and Profibus PA (Process Automation) for instruments in hazardous areas with bus-powered, intrinsically-safe wiring. Largely superseded by PROFINET for new installations but still widespread in the installed base.
PROFINET
The industrial Ethernet standard from PROFIBUS & PROFINET International (PI), the Ethernet successor to Profibus. Runs standard TCP/IP for configuration alongside real-time (RT) and isochronous real-time (IRT) channels for deterministic, low-jitter I/O and motion control. The default fieldbus on Siemens S7-1500 systems and a leading choice for plant-floor Ethernet.
EtherNet/IP
Industrial Ethernet that carries the Common Industrial Protocol (CIP) over standard Ethernet and TCP/UDP — the "IP" stands for Industrial Protocol, not Internet Protocol. Managed by ODVA and the dominant fieldbus on Rockwell/Allen-Bradley ControlLogix and CompactLogix platforms. Uses implicit (cyclic I/O) and explicit (on-demand messaging) connections.
FOUNDATION Fieldbus
An all-digital, two-way process fieldbus (now governed by FieldComm Group, which also stewards HART). Its H1 layer (31.25 kbit/s, bus-powered, intrinsically safe) connects instruments in continuous process plants and can distribute control logic into the field devices themselves (Control in the Field). Competes with Profibus PA in refinery and chemical applications.
Operator display
The DCS term for what is called HMI in PLC+SCADA architectures. Same concept, different vendor vocabulary.
Display levels (Level 1-4)
ISA-101 four-level hierarchy of HMI screens:
- Overview — entire site or plant area on one screen, KPI-only
- Unit / Area — one process unit; routine operator working screen
- Detail — single equipment with all loops, interlocks, controls
- Diagnostic / Maintenance — instrument-level data, valve travel
MCC
Motor Control Center — the cabinet that houses contactors, overload relays, soft-starters, VFDs and control circuits for plant motors. Connected to the PLC for start/stop commands and feedback. The MCC is where the 9-state motor matrix originates.
VFD / VSD
Variable Frequency Drive (US) or Variable Speed Drive (EU) — converts fixed-frequency line power to variable-frequency, allowing motor speed control. Saves energy (pumps, fans on demand) and reduces mechanical wear. Common brands: ABB ACS, Siemens Sinamics, Schneider Altivar, Yaskawa A1000, Rockwell PowerFlex.
Interlock
A safety logic that prevents a command (typically motor start, valve open) unless preconditions are met: e.g., a pump cannot start unless the suction valve is open. Implemented in PLC ladder; visible on HMI as locked controls or red banners. Distinguished from trips which are reactive.
Permissive
Same idea as interlock but typically describing the positive condition: "all permissives met, start enabled". Operators see a "Permissives" panel on detail screens.
Trip
The reactive shutdown of equipment when an alarm or safety condition fires. A motor trip removes contactor power immediately. Different from interlock (which blocks the command beforehand).
PID control loop
Proportional-Integral-Derivative — the workhorse feedback algorithm of process control. It continuously computes the error between the measured process variable (PV) and the setpoint (SP), then drives an output (CO) using three tuned terms: proportional (reacts to present error), integral (eliminates steady-state offset), and derivative (anticipates change). Most loops run PI only, with derivative reserved for slow temperature processes. Operators tune and monitor each loop from its faceplate.
Setpoint
The target value an operator or supervisory program wants a controlled variable to hold — e.g. a tank level setpoint of 60%, a reactor temperature setpoint of 180 °C. The control loop manipulates its output to drive the measured process variable toward the setpoint. On the HMI the setpoint is the entry field on a loop faceplate; in cascade control one loop's output becomes the setpoint of another.
Deadband
A band around the setpoint (or an alarm threshold) within which no control action or state change occurs. Deadband stops actuators and alarms from chattering on small fluctuations or sensor noise — e.g. a pump set to start at 80% and stop at 20% has a 60% deadband. In alarm management it is the amount the variable must retreat below a trip point before the alarm clears, preventing repeated re-annunciation.
Hysteresis
The property where a switching threshold depends on the direction of travel: the turn-on point differs from the turn-off point. Closely related to deadband and often used interchangeably in practice, hysteresis is what gives a thermostat or level switch its two distinct trip points so it does not rapidly cycle near a single value.
Cascade control
A control strategy where the output of an outer (master/primary) loop sets the setpoint of an inner (slave/secondary) loop, rather than driving a valve directly. The fast inner loop rejects disturbances before they reach the slow outer variable — for example a reactor temperature loop cascading onto a faster jacket-flow loop. On the HMI the operator can put the inner loop in "cascade" mode (driven by the master) or break the cascade to manual.
Faceplate
The standardized pop-up panel an operator opens to interact with a single object — a control loop, motor or valve. It exposes the process variable, setpoint and output, the auto/manual/cascade mode buttons, alarm status and tuning. Because every loop of a given type uses an identical faceplate, operators build muscle memory; defining faceplates is a core style-guide deliverable. Vendor analog of the multi-state lamp for interactive control.
Engineering units
The real-world units a raw signal is scaled into for display — bar, °C, m³/h, % — as opposed to the raw counts a PLC reads from the I/O card (e.g. 0–27648 for a Siemens analog channel, or 4–20 mA). Scaling raw counts to engineering units (and defining the EU range, or span) is a basic configuration step for every analog tag; the HMI and trends always present engineering units to the operator.
Ladder logic
The most widely used PLC programming language, one of the five defined by IEC 61131-3. Programs are drawn as rungs between two vertical "power rails", with contacts (inputs) and coils (outputs) that visually echo the relay schematics it replaced — which makes it readable to electricians and maintenance staff. Ideal for boolean interlock and permissive logic; less suited to math, handled by Structured Text or Function Block.
Watchdog
A timer that must be reset ("kicked") at regular intervals by healthy software or a communicating partner; if the reset stops arriving, the watchdog expires and forces a safe action — restarting a controller, or driving outputs to a fail-safe state. In SCADA/HMI a heartbeat watchdog detects a dead PLC link and flags every tag from that device as stale rather than showing frozen, misleadingly "good" values.
Redundancy
Duplicating critical components — controllers, power supplies, network paths, servers — so a single failure does not stop the process. Common schemes are hot standby (a backup controller mirrors state and takes over within one scan, used in DCS and safety systems) and redundant network rings. Redundancy raises availability; it is distinct from the functional independence IEC 61511 requires between the BPCS and the SIS.
Trend (historian)
The graphical view of one or more tags over time. Powered by the historian (a time-series database storing every tag value with timestamp). Critical for tuning loops, root-cause analysis and OEE reporting. Examples: AVEVA PI System (formerly OSIsoft), Honeywell PHD, Wonderware Historian, InfluxDB.
OEE
Availability × Performance × Quality, expressed 0-100%. The most widely-quoted manufacturing KPI. World-class is ~85%; typical plants 60-70%. Often shown on HMI Overview screens as a KPI tile.
HMI style guide
The project-specific document that locks down palette, fonts, symbol library, alarm priorities, naming conventions before screens are drawn. ISA-101 Phase 1 deliverable. A good style guide is what makes 50 screens look like one product instead of 50 different visual experiments. See live HMI style guide editor.