IEC 62424, CAEX.
IEC 62424 is the international standard for representing process control engineering (PCE) requests on a P&ID and for exchanging that data between P&ID tools and control engineering (PCE-CAE) tools. It defines its own tables of PCE categories and processing functions, the letters a PCE request is written with, and a CAEX-based exchange file that carries every request to the engineering tool that turns it into a control loop.
What IEC 62424, CAEX means.
IEC 62424 addresses a narrow, recurring handover problem on process plants: a process engineer decides a control task is needed, holding a level or tripping on high pressure, and that decision has to reach control engineering as a defined requirement rather than a note read off a drawing. The standard gives the process side a way to state the request directly on the P&ID, as a PCE request carrying a PCE category (the variable, from the standard's own table) and one or more processing functions (indicate, control, switch, alarm and so on, also from the standard's own table), and it gives both disciplines a shared electronic handover, a CAEX file that a P&ID tool exports and a PCE-CAE tool imports, carrying every PCE request without retyping. It is the international successor to the German DIN 19227 representation of EMSR-Stellen. IEC 62424 does not decide how the control system implements a request, and it does not replace ISA 5.1 on a project that tags to ISA 5.1; which convention a drawing set uses is stated on its legend sheet.
What a PCE request is and where it appears on the P&ID
A PCE request is the P&ID-side statement of one process control engineering task: a process engineer has decided a variable needs to be measured, controlled or monitored, and marks that decision directly on the drawing rather than leaving it to be inferred from the instrument symbols around it. The request carries a PCE category, the variable being handled, and one or more processing functions, the kind of task asked for, both drawn from tables the standard itself defines, plus a PCE loop identifier. It is drawn as a request symbol with a location indication (field, local panel, control room) and it does not prescribe how the control system implements the task. On a P&ID drawn this way, a reviewer sees which instruments exist and which of them are grouped into one requested control task, which a plain instrument symbol set does not carry on its own.
How CAEX carries the request to the control engineering tool
IEC 62424 pairs the drawing-side PCE request with an electronic handover so the requirement does not have to be re-keyed by the control engineering team. The exchange format is CAEX, Computer Aided Engineering Exchange, an XML-based structure for describing engineering objects and their relationships. A P&ID tool exports the PCE requests it holds as a CAEX file, and a PCE-CAE tool, the control engineering CAE application on the receiving side, imports that file and uses it as the starting point for engineering the control function: assigning the loop, the logic, and the hardware that will actually carry out the request. The standard's scope stops at that interface. It defines what a CAEX export of a PCE request must carry so the two tools agree on meaning, not how the receiving tool builds the control logic once the request has arrived.
PCE categories and ISA 5.1 letters are two systems, not one
A PCE request and an ISA 5.1 tag answer different questions. An ISA 5.1 tag identifies an individual instrument: what it measures and what function that device performs. A PCE request states a control task the process side is asking for, and the letters it carries come from IEC 62424's own tables, the PCE category for the variable and the processing functions for the task, which descend from the DIN 19227 letter system rather than from ISA 5.1. The two letter systems are similar in spirit and overlap on many variables, but they are not identical, and reading an IEC 62424 request as though it were an ISA 5.1 tag is the common misreading on a mixed-convention set. The legend sheet states which convention a drawing set is written in; a reviewer moving between an ISA 5.1 project and an IEC 62424 project should expect two separate systems doing two separate jobs.
IEC 62424, DEXPI, and AutomationML
IEC 62424 and DEXPI both move P&ID data between tools, but they are not the same standard solving the same problem. IEC 62424 is narrowly scoped to the seam between the P&ID and control engineering: it carries PCE requests, and only PCE requests, from a P&ID tool to a PCE-CAE tool over CAEX. DEXPI is a later and considerably wider specification: it exports the whole P&ID, equipment, piping, instrumentation, and the connectivity between them, as a structured, vendor-neutral dataset that builds on ISO 15926 concepts for its classes and attributes. A project can use both without conflict, IEC 62424 for the control-engineering handover of specific requests, DEXPI for the broader asset-data handover downstream. CAEX itself is not unique to IEC 62424: AutomationML, the engineering data-exchange format used across mechanical, electrical, and control disciplines, uses CAEX as its own top-level container format, so a CAEX file authored to IEC 62424 shares its base structure with the wider AutomationML ecosystem even where the two are used for different purposes.
What a PCE request means for the instrument index and I/O list
A PCE request is upstream of the instrument index and the I/O list, not a replacement for either. It states the control task; the instrument index and the I/O list record the individual tagged devices and signals that end up carrying that task out once control engineering has done its work. Where a project hands over PCE requests in CAEX, that file is a legitimate seed for the instrument register: each request maps to one or more resulting instrument tags and signal types, so a receiving system can start the index from the handover rather than re-reading the P&ID from nothing. What the CAEX file does not carry is the final tag identity or signal class in ISA 5.1 or any other tagging convention; that assignment still happens on the engineering side, and the index has to reconcile against the as-engineered drawing set once the control functions are built out.
PCE request, PCE control function, and instrument tag
Three different objects sit between a control decision and a wired device, and each one belongs to a different discipline.
| Term | What it is | Where it lives | Who owns it |
|---|---|---|---|
| PCE request | The process engineer's statement that a control task is needed, carrying a PCE category, one or more processing functions and a PCE loop identifier | On the P&ID, as a PCE request symbol | Process engineering |
| PCE control function | The control-engineering realisation of that request: the engineered loop, logic, and hardware that carry out the task | In the PCE-CAE tool, and in the resulting control-system documentation | Control and instrumentation engineering |
| Instrument tag | The individual physical device, a transmitter, valve, or switch, that measures or acts within the resulting loop | On the P&ID as an ISA 5.1 or project tag, and in the instrument index | Instrumentation engineering |
IEC 62424, DEXPI, and ISO 15926 compared
All three touch process-plant data exchange, but at different scopes and in different formats.
| Standard | Scope | Format | What it exchanges |
|---|---|---|---|
| IEC 62424 | The P&ID to control-engineering interface for PCE requests specifically | CAEX, XML-based | PCE requests and their category, so a PCE-CAE tool can engineer the control function |
| DEXPI | The whole P&ID as a dataset: equipment, piping, instrumentation, and connectivity | XML, built on ISO 15926 concepts | A structured, vendor-neutral P&ID export for downstream tools and asset systems |
| ISO 15926 | Process-plant lifecycle data generally, not one document type | A reference data model and library, not a single file format | The common classes and meaning that other standards, including DEXPI, build on |
Common questions
What is IEC 62424 for.
What is a PCE request.
What is CAEX.
How does IEC 62424 relate to DEXPI.
How does IEC 62424 relate to ISA 5.1 tags.
Does IEC 62424 replace the instrument index.
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