If you work in industrial automation, PLC integration, or SCADA system design, you will eventually encounter IEC 61158.
But IEC 61158 is not a single protocol.
It is a family of international standards that define how industrial fieldbus communication systems are structured — from the physical wiring all the way up to the application layer.
This article explains IEC 61158-1, the foundational part of the standard. We will break it down in clear, simple language while keeping the technical depth engineers need.
We will cover:
- What IEC 61158 really is
- Why it exists
- How it is structured
- How it maps to the OSI model
- How it relates to IEC 61784
- What it means for real-world system design
This is the architecture layer — the “big picture” of industrial fieldbus communication.
Table of Contents
1. Why IEC 61158 Exists
Before IEC 61158, industrial communication was fragmented.
Each vendor had:
- Their own fieldbus
- Their own frame formats
- Their own timing rules
- Their own device models
This created:
- Poor interoperability
- Vendor lock-in
- Complex integration projects
- Difficult certification paths
IEC 61158 was created to:
- Provide a formal structure
- Standardize how fieldbus technologies are documented
- Align industrial communication with the OSI reference model
- Allow multiple fieldbus technologies under one structured framework
It does not force one protocol.
Instead, it defines a standard way to define protocols.
That distinction is critical.
2. What IEC 61158-1 Actually Does
IEC 61158-1 is the overview and guidance document of the entire IEC 61158 series.
It explains:
- The concept of the IEC 61158 series
- The structure of the layered documents
- How the layers map to the OSI model
- How different “Types” fit into the system
- The relationship between IEC 61158 and IEC 61784
Think of IEC 61158-1 as the architectural blueprint.
It does not define voltages or frame formats.
It defines the structure of the standard itself.
3. The Core Concept: Layered Architecture
IEC 61158 follows the layered communication model approach.
It separates industrial communication into:
- Physical Layer
- Data-Link Layer
- Application Layer
These are clearly described in the structure of the series .
Unlike full IT networking stacks, IEC 61158 intentionally does not define:
- Network layer
- Transport layer
- Session layer
- Presentation layer
Why?
Because fieldbus systems are typically:
- Local networks
- Deterministic
- Flat (not routed like IP networks)
- Real-time control oriented
They do not need routing or TCP-style transport mechanisms.
4. Mapping IEC 61158 to the OSI Model
The OSI model defines 7 layers:
- Physical
- Data Link
- Network
- Transport
- Session
- Presentation
- Application
IEC 61158 maps only to:
- Layer 1 – Physical
- Layer 2 – Data Link
- Layer 7 – Application
This mapping is clearly described in IEC 61158-1 .
Why skip layers 3–6?
Because fieldbus communication is:
- Typically single-segment
- Deterministic
- Master/slave or producer/consumer
- Not IP-routed
Industrial control networks prioritize:
- Timing precision
- Deterministic response
- Low latency
- Electrical robustness
Not global routing.
5. Structure of the IEC 61158 Series
IEC 61158 is organized into parts.
Each part corresponds to a communication layer.
Here is the structure:
Part 1 – Overview and Guidance
Explains architecture and structure
Part 2 – Physical Layer
Defines:
- Media
- Signaling
- Encoding
- Interfaces
Part 3 – Data-Link Layer Service
Defines:
- Service primitives
- Communication services
Part 4 – Data-Link Layer Protocol
Defines:
- Frame formats
- Timing
- Protocol state machines
Part 5 – Application Layer Service
Defines:
- Application services
- Communication models
Part 6 – Application Layer Protocol
Defines:
- Application protocol machines
- Syntax rules
- Device structures
IEC 61158-1 explains how all of these fit together .
6. The Meaning of “Types” in IEC 61158
One of the most misunderstood parts of IEC 61158 is the concept of “Types.”
You will see references like:
- Type 1
- Type 2
- Type 3
- Type 4
- Type 8
- Type 12
- Type 16
- Type 18
- Type 20
- Type 24
Each “Type” represents a different fieldbus technology.
IEC 61158 does not define one universal fieldbus.
It defines a framework where multiple fieldbus technologies are structured in the same layered way.
Each Type has:
- Its own physical layer rules
- Its own data-link specifications
- Its own application layer definitions
But they all follow the same architectural separation.
This allows:
- Consistent documentation
- Clear certification structure
- Cross-technology understanding
7. Relationship Between IEC 61158 and IEC 61784
This is extremely important.
IEC 61158 defines:
- Communication building blocks
- Protocol definitions
- Layer specifications
IEC 61784 defines:
- Communication profiles
- Functional safety profiles
IEC 61158 = technical protocol definition
IEC 61784 = how those protocols are applied in profiles
IEC 61158-1 explains this relationship clearly .
In simple terms:
IEC 61158 describes the engine.
IEC 61784 describes how that engine is configured for specific use cases.
8. What Makes Fieldbus Different from Ethernet?
Modern engineers often ask:
Why not just use Ethernet?
Fieldbus systems are designed for:
- Deterministic timing
- Hard real-time control
- Low-level device control
- Electrical noise resistance
- Predictable cycle times
Traditional Ethernet (without real-time extensions) is:
- Best effort
- Non-deterministic
- Collision-based (in older forms)
IEC 61158 systems prioritize:
- Cyclic communication
- Scheduled timing
- Time-slot transmission
- Guaranteed update rates
The layered structure supports these requirements.
9. Benefits of the IEC 61158 Architecture
1. Clear Layer Separation
Physical changes do not break application logic.
2. Interoperability
Vendors can align with defined service and protocol boundaries.
3. Conformance Testing
Each layer can be tested separately.
4. Deterministic Design
Timing is engineered into the data-link layer.
5. Structured Documentation
Engineers can find exactly which layer defines what behavior.
10. Practical Impact for Engineers
If you are:
A PLC Programmer
You mostly interact with the application layer.
A Hardware Designer
You focus on Part 2 (physical layer).
A Protocol Stack Developer
You must implement:
- Data-link state machines
- Application protocol machines
- Service primitives
A System Architect
You use IEC 61158 to:
- Understand determinism
- Evaluate fieldbus technologies
- Plan integration paths
IEC 61158-1 provides the architectural map for all of these roles.
11. How IEC 61158 Enables Determinism
Determinism means:
The system behaves predictably.
In industrial control, that means:
- Every cycle happens on time
- No random delays
- No unexpected jitter
IEC 61158 supports this by:
- Defining data-link service structures
- Separating cyclic and acyclic communication
- Structuring frame handling
- Defining timing relationships
Even though IEC 61158-1 is an overview document, it defines the layered separation that enables deterministic implementation.
12. Common Misunderstandings
Misconception 1: IEC 61158 Is One Protocol
False. It is a structured standard containing multiple protocol Types.
Misconception 2: It Replaces Ethernet
No. Many industrial Ethernet systems coexist with IEC 61158 technologies.
Misconception 3: It Is Only About Wiring
No. It covers physical, data-link, and application layers.
Misconception 4: It Is Outdated
Not true. Many modern deterministic industrial systems are structured according to this framework.
13. Why This Matters for SCADA Systems
SCADA systems often sit above fieldbus layers.
But:
- PLCs talk fieldbus.
- Field devices use fieldbus.
- Determinism starts at the lower layers.
Understanding IEC 61158 helps you:
- Debug integration issues
- Understand update cycles
- Diagnose communication faults
- Evaluate vendor claims
- Design robust architectures
Without understanding the layered model, troubleshooting becomes guesswork.
14. The Big Picture
IEC 61158-1 gives us:
- A structured layered communication model
- A standardized way to define fieldbus technologies
- A bridge between industrial control and formal communication theory
- Alignment with the OSI model (partial mapping)
It is not about voltage levels.
It is about architecture.
15. Final Summary
IEC 61158-1 is the architectural foundation of industrial fieldbus communication.
It defines:
- The layered structure
- The OSI mapping
- The organization of the series
- The concept of Types
- The relationship to IEC 61784
- The guiding philosophy of deterministic industrial communication
If you understand IEC 61158-1, you understand how the rest of the IEC 61158 series fits together.
It is the blueprint behind industrial communication standards.