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IEC PAS 62953 — Industrial Fieldbus Specifications (ADS-NET Type 25)
Standardised communication protocol elements for real-time industrial automation networks
Industrial automation systems demand deterministic, real-time communication between controllers, drives, I/O devices, and sensors. IEC PAS 62953 addresses this need by defining the complete communication stack for ADS-NET (Automation Device Specification Network) Type 25 fieldbus systems. Published as a Publicly Available Specification (PAS), this standard specifies the data-link layer services and protocols (Parts 3-25, 4-25) and application layer services and protocols (Parts 5-25, 6-25), together with the CPF 20 communication profile for real-time Ethernet networks based on ISO/IEC 8802-3.
1. Standard Architecture and Scope
IEC PAS 62953 follows the layered architecture defined by the ISO/OSI reference model, focusing specifically on the lower and upper layers critical for real-time industrial communication. The standard is organised into four main technical parts plus a communication profile annex:
The Type 25 fieldbus is designed to support cycle times as low as 31.25 μs and jitter below 1 μs, making it suitable for high-speed motion control, robotics, and synchronised multi-axis drive systems.
| Part | Title | Scope |
|---|---|---|
| Part 3-25 | Data-link layer (DLL) service definition | Services provided to the application layer at the DL-service interface |
| Part 4-25 | Data-link layer protocol specification | Encoding, sequencing, error control, and medium access for Type 25 |
| Part 5-25 | Application layer service definition | AL-service primitives for variable access, program invocation, event management |
| Part 6-25 | Application layer protocol specification | PDU structures, encoding rules, and state machines for Type 25 applications |
| CPF 20 | Communication profile for real-time Ethernet | Mapping of Type 25 to ISO/IEC 8802-3 (Ethernet) networks |
2. Data-Link Layer Specifications
2.1 Service Definitions (Part 3-25)
The data-link layer service definition establishes the primitives exchanged between the application layer entity and the DL-service provider. Key services include DL-connection establishment and release, data transfer with confirmed and unconfirmed modes, and expedited data delivery for time-critical messages. The standard defines the DL-address structure, quality-of-service parameters, and connection-end-point identifiers tailored to the deterministic requirements of industrial automation.
2.2 Protocol Specification (Part 4-25)
The data-link layer protocol specifies the exact encoding of Protocol Data Units (PDUs), the state machines governing connection-oriented and connectionless data transfer, and the medium-access control mechanism. Type 25 employs a hybrid TDMA-based access scheme that guarantees deterministic slot allocation for isochronous data while preserving bandwidth for asynchronous (acyclic) communication. The protocol supports both single-segment and multi-segment network topologies.
Engineers designing Type 25 networks must pay careful attention to the maximum segment length and propagation delay budget. The standard specifies a maximum of 200 m per segment for copper media at 100 Mbit/s, with optical-fibre extensions permitted up to 2 km using 100BASE-FX physical layers.
3. Application Layer Specifications
3.1 Service Definition (Part 5-25)
The application layer service definition provides a comprehensive set of service primitives for industrial automation applications. These are grouped into several key service clusters:
- Variable Access Services: Read, write, and information services for process variables, device parameters, and configuration data objects.
- Program Invocation Management: Start, stop, resume, and reset services for control program execution across distributed devices.
- Event Management: Notification, acknowledgement, and alarm handling services for diagnostic and safety-related events.
- Connection Management: Association establishment and release between application entities with negotiable service characteristics.
3.2 Protocol Specification (Part 6-25)
The application layer protocol defines the exact PDU formats, encoding rules using a compact Abstract Syntax Notation (ASN.1-derived), and the protocol state machines. A notable feature is the support for segmented data transfer, allowing large data objects exceeding the maximum APDU size to be fragmented, transmitted, and reassembled transparently at the receiving end.
A key engineering insight from implementing Type 25 is that the handshake overhead for connection-oriented services accounts for approximately 15–20 % of the achievable throughput at 1 ms cycle times. For sub-millisecond cycles, connectionless data transfer (unconfirmed services) should be preferred wherever reliability guarantees are not critical.
4. CPF 20 Profile and Real-Time Ethernet Mapping
The CPF 20 communication profile annex specifies how Type 25 fieldbus elements are mapped onto standard Ethernet networks conforming to ISO/IEC 8802-3. This mapping enables coexistence with standard TCP/IP traffic on the same physical network while preserving real-time performance. The profile defines three traffic classes:
| Traffic Class | Description | Typical Cycle Time |
|---|---|---|
| Isochronous (IRT) | Time-deterministic, jitter-free data exchange | 31.25 μs – 1 ms |
| Real-Time (RT) | Cyclic data with guaranteed delivery | 1 ms – 10 ms |
| Best-Effort (NRT) | Standard TCP/IP, configuration, diagnostics | 10 ms – 100 ms |
Mixing isochronous and best-effort traffic on the same segment requires a managed Ethernet switch that supports IEEE 802.1Q VLAN tagging and priority queuing. An unmanaged switch will introduce frame collisions and jitter that can exceed the 1 μs tolerance of the IRT class, leading to communication timeouts and drive faults.
5. Engineering Design Insights
Practical deployment of IEC PAS 62953-based systems reveals several critical engineering considerations:
- Cycle Time vs. Payload Trade-off: At 31.25 μs cycle time, the maximum application payload per frame is limited to approximately 40 bytes. Designers must carefully partition data across multiple frames when larger payloads are required.
- Cable Redundancy: The CPF 20 profile supports media redundancy using a ring topology with a reconfiguration time below 10 ms, critical for safety-related applications.
- Diagnostic Integration: Type 25 devices must implement the mandatory diagnostics object (Object ID 0xF800) to report communication errors, device status, and protocol version information for network management tools.
- Backward Compatibility: The Type 25 specification maintains backward compatibility with legacy fieldbus systems through a gateway profile defined in an informative annex, enabling gradual migration strategies.
When planning a Type 25 network, allocate at least 20 % headroom in bandwidth utilisation. This margin accommodates acyclic diagnostic traffic during commissioning and firmware updates without disrupting the deterministic cyclic data exchange.
6. Frequently Asked Questions
Q: What is the difference between IEC PAS 62953 and IEC 61784?
A: IEC 61784 defines communication profiles (CPFs) that specify protocol sets for industrial networks. IEC PAS 62953 provides the detailed data-link and application layer service and protocol specifications for the Type 25 profile, which is one of the profiles referenced by IEC 61784-2.
A: IEC 61784 defines communication profiles (CPFs) that specify protocol sets for industrial networks. IEC PAS 62953 provides the detailed data-link and application layer service and protocol specifications for the Type 25 profile, which is one of the profiles referenced by IEC 61784-2.
Q: Can Type 25 devices coexist with standard Ethernet devices on the same switch?
A: Yes, but only with managed switches supporting IEEE 802.1Q VLANs and priority queuing. The CPF 20 profile assigns the highest priority (7) to isochronous frames and recommends a separate VLAN for real-time traffic to isolate it from best-effort broadcasts.
A: Yes, but only with managed switches supporting IEEE 802.1Q VLANs and priority queuing. The CPF 20 profile assigns the highest priority (7) to isochronous frames and recommends a separate VLAN for real-time traffic to isolate it from best-effort broadcasts.
Q: What physical layer options does Type 25 support?
A: Type 25 supports 100BASE-TX (copper, 100 m segments), 100BASE-FX (multimode fibre, 2 km), and 1000BASE-T (gigabit, 100 m) as the physical layer, in accordance with ISO/IEC 8802-3. The choice depends on the required cycle time and network span.
A: Type 25 supports 100BASE-TX (copper, 100 m segments), 100BASE-FX (multimode fibre, 2 km), and 1000BASE-T (gigabit, 100 m) as the physical layer, in accordance with ISO/IEC 8802-3. The choice depends on the required cycle time and network span.
Q: How is device configuration managed in Type 25 networks?
A: Devices are configured using electronic data sheets (GSD/EDS files) that describe device parameters, communication capabilities, and available objects. The standard defines an object directory structure indexed by 16-bit Object IDs, with the first 0x0000–0x0FFF range reserved for mandatory communication objects.
A: Devices are configured using electronic data sheets (GSD/EDS files) that describe device parameters, communication capabilities, and available objects. The standard defines an object directory structure indexed by 16-bit Object IDs, with the first 0x0000–0x0FFF range reserved for mandatory communication objects.
