Overview
ISO 3511-4:1985 - "Industrial process measurement control functions and instrumentation - Symbolic representation - Part 4" defines basic graphical symbols to represent process computer, interface, and shared display/control functions on technical drawings. It is a supplemental part to ISO 3511/1 and ISO 3511/2 and is intended for use on process flow diagrams (PFDs), piping and instrument diagrams (P&IDs), schematic diagrams and similar documentation. The standard provides a universal, concise way to identify software-based control functions, operator interfaces and time-shared display/control systems without depicting specific instrument hardware.
Key topics and requirements
- Scope and use: Symbols are for identification only - they do not illustrate detailed instrument construction or parts. They must be used in conjunction with ISO 3511/1 and ISO 3511/2.
- Function identification: Letter codes for function identification are taken from table 1 of ISO 3511/1 and ISO 3511/2; numbers can be added for easier identification.
- Basic symbols:
- Computer-based functions: represented by a thin-line hexagon (approx. 10 mm between parallel sides) - used for programmable digital systems handling real-time process data.
- Shared display/control functions: represented by a circle inside a square - intended for configurable, time-shared operator/display systems.
- Notation rules: Symbols that touch imply communication between functions. Single and double horizontal lines indicate operator interface accessibility (central control room vs local panel), consistent with ISO 3511/1 and /2 conventions.
- Definitions: Includes important terms such as programmable, configurable, process computer, shared display/shared control system, distributed control system (DCS) and software link.
- Examples: Application methods and representative examples (pressure/temperature/flow signals, cascade control, backup arrangements, local/central indications) illustrate common uses.
Applications and users
- Typical users: process engineers, instrumentation & control (I&C) designers, P&ID drafters, automation engineers, plant operators, and technical document authors.
- Practical uses:
- Standardizing P&IDs and flow diagrams for chemical, oil & gas, power, and process industries.
- Documenting DCS, process computers and operator interface architecture.
- Clarifying relationships between software-based controls and hard-wired backup or local instruments.
- Supporting procurement, installation, maintenance and safety documentation by providing consistent symbol conventions.
Related standards
- ISO 3511/1 and ISO 3511/2 (basic and extended symbolic requirements) - mandatory complements for letter codes and conventions.
- Coordination with IEC graphical-symbol publications (notably IEC committees on instrumentation and diagrams) is noted in the standard.
Keywords: ISO 3511-4:1985, process computer symbols, shared display/control, symbolic representation, P&ID symbols, DCS, programmable/configurable systems, process instrumentation.
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