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
- Calm HMI
- Situation awareness
- Tag (instrument tag)
- Mounting symbol
- Multi-state lamp
- ESD
- SIS / Safety Instrumented System
- Fieldbus
- OPC UA
- Modbus
- Operator display
- Display levels
- MCC
- VFD / VSD
- Interlock
- Permissive
- Trip
- 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.
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.
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).
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.