PROFINET is the communication standard for automation from PROFIBUS & PROFINET International (PI). Its modular range of functions makes PROFINET a flexible system for all applications and markets. PROFINET is the networking solution of production and process automation, from applications with functional safety requirements and the entire spectrum of drive technology to isochronous motion control applications. The use of application profiles allows optimal usage of PROFINET in all areas of automation engineering.

PROFINET follows the provider/consumer model for data exchange. This means that both the IO controller and IO device spontaneously send cyclic data independently. The following device classes are defined for PROFINET (Figure 1):

Figure 1 – Communication paths for PROFINET

IO controller: This is typically the Programmable Logic Controller (PLC) in which the automation program runs. The IO controller provides output data to the configured IO devices in its role as provider and is the consumer of input data.

IO device: An IO device is a distributed IO field device connected to one or more IO controllers via PROFINET. The IO device is the provider of input data and the consumer of output data from the IO controller.

IO supervisor: This can be a programming device (PG), personal computer (PC) or human machine interface (HMI) device for commissioning or diagnostic purposes.

A system unit contains at least one IO controller and one or more IO devices. IO supervisors are usually integrated only temporarily for commissioning or troubleshooting purposes.

In its logical structure, a PROFINET field device is always modular in design. Modularity in the logical sense, however, does not require actual modularity in the electrical and mechanical design sense. An IO device is usually comprised of a communication module with Ethernet interface and (physical or virtual) modules assigned to it. The assigned modules handle the actual process data traffic. The access point for communication (Ethernet interface with data processing) is called the DAP (Device Access Point). The following structures are standardized for an IO device:

• The device model consists of slots, subslots, modules and submodules.
• The slot designates the insertion slot of a module in an IO field device. A field device usually has two or more slots.
• A module is comprised of one or more submodules or provides available subslots into which submodules can be inserted.
• The modules themselves have no task other than to provide structuring. The actual inputs and outputs (channels) are implemented in its submodules. The granularity of the channels (bitwise, bytewise or wordwise division of IO data) is determined by the manufacturer. Acyclic services always address submodules. Therefore, a module always contains at least one submodule.
• The data content of a submodule is always accompanied by status information. The index specifies the data within a submodule inserted into a slot/subslot which can be read or written acyclically using read/write services. For example, parameters can be written to a module, or manufacturer-specific module data can be read out based on an index. Specific indexes are defined in the standard here. Additional indexes can be freely defined by the manufacturer.

The submodule is the owner of the user data, diagnostics, channels, actual configuration, records and I&M data. Cyclic IO data of the submodule in the device is addressed by specifying the slot/subslot combination of the insertion slot. They can be freely defined by the manufacturer. For acyclic data communication via read/write services, an application can specify the data of the submodule to be addressed using slot, subslot and index (Figure 2).

Figure 2 – Addressing of IO data in PROFINET based on slots and subslots

To establish communication between the higher-level controller and an IO device, the communication paths must be established. These are set up by the IO controller during system startup based on the configuration data received from the engineering system. This specifies the data exchange explicitly.

All data exchange is embedded into an AR (Application Relationship) (Figure 3). Within the AR, CRs (Communication Relationships) specify the data explicitly. As a result, all data for device modelling, including the general communication parameters, are downloaded to the IO device. An IO device can have multiple ARs established from different IO controllers, for example, for shared devices.

Figure 3 – Addressing of IO data in PROFINET based on slots and subslots

The communication channels for cyclic data exchange (IO data CR), acyclic data exchange (record data CR) and alarms (alarm CR) are set up simultaneously. Multiple IO controllers can be used in a PROFINET system (Figure 4). If these IO controllers can access the same data in the IO devices, this must be specified during parameter configuration (shared devices and shared inputs).

Figure 4 – Application and communication relationships

An IO controller can establish one AR each with multiple IO devices. Within an AR, several IO CRs on different APIs can be used for data exchange. This can be useful, for example, if more than one user profile (PROFIdrive, ENCODER etc.) is involved in communication and different submodules are required.

OPC UA is an open and royalty free set of standards designed as a universal communication protocol. While there are numerous communication solutions available, OPC UA has key advantages:

• A state of art security model (see [OPC 10000-2]).
• A fault tolerant communication protocol.
• An information modelling framework that allows application developers to represent their data in a way that makes sense to them.

OPC UA has a broad scope which delivers for economies of scale for application developers. This means that a larger number of high-quality applications at a reasonable cost are available. When combined with semantic models such as PROFINET, OPC UA makes it easier for end users to access data via generic commercial applications.

The OPC UA model is scalable from small devices to ERP systems. OPC UA Servers process information locally and then provide that data in a consistent format to any application requesting data - ERP, MES, PMS, Maintenance Systems, HMI, Smartphone or a standard Browser, for examples. For a more complete overview see [OPC 10000-1].

As an open standard, OPC UA is based on standard internet technologies, like TCP/IP, HTTP, Web Sockets.

As an extensible standard, OPC UA provides a set of Services (see [OPC 10000-4]) and a basic information model framework. This framework provides an easy manner for creating and exposing vendor defined information in a standard way. More importantly all OPC UA Clients are expected to be able to discover and use vendor-defined information. This means OPC UA users can benefit from the economies of scale that come with generic visualization and historian applications. This specification is an example of an OPC UA Information Model designed to meet the needs of developers and users.

OPC UA Clients can be any consumer of data from another device on the network to browser based thin clients and ERP systems. The full scope of OPC UA applications is shown in Figure 5.

Figure 5 – The Scope of OPC UA within an Enterprise

OPC UA provides a robust and reliable communication infrastructure having mechanisms for handling lost messages, failover, heartbeat, etc. With its binary encoded data, it offers a high-performing data exchange solution. Security is built into OPC UA as security requirements become more and more important especially since environments are connected to the office network or the internet and attackers are starting to focus on automation systems.

OPC UA provides a framework that can be used to represent complex information as Objects in an AddressSpace which can be accessed with standard services. These Objects consist of Nodes connected by References. Different classes of Nodes convey different semantics. For example, a Variable Node represents a value that can be read or written. The Variable Node has an associated DataType that can define the actual value, such as a string, float, structure etc. It can also describe the Variable value as a variant. A Method Node represents a function that can be called. Every Node has several Attributes including a unique identifier called a NodeId and a non-localized name called a BrowseName. An Object representing a ‘Reservation’ is shown in Figure 6.

Figure 6 – A Basic Object in an OPC UA Address Space

Object and Variable Nodes represent instances and they always reference a TypeDefinition (ObjectType or VariableType) Node which describes their semantics and structure. Figure 7 illustrates the relationship between an instance and its TypeDefinition.

The type Nodes are templates that define all the children that can be present in an instance of the type. In the example in Figure 7 the PersonType ObjectType defines two children: First Name and Last Name. All instances of PersonType are expected to have the same children with the same BrowseNames. Within a type the BrowseNames uniquely identify the children. This means Client applications can be designed to search for children based on the BrowseNames from the type instead of NodeIds. This eliminates the need for manual reconfiguration of systems if a Client uses types that multiple Servers implement.

OPC UA also supports the concept of sub-typing. This allows a modeler to take an existing type and extend it. There are rules regarding sub-typing defined in [OPC 10000-3], but in general they allow the extension of a given type or the restriction of a DataType. For example, the modeler may decide that the existing ObjectType in some cases needs an additional Variable. The modeler can create a subtype of the ObjectType and add the Variable. A Client that is expecting the parent type can treat the new type as if it was of the parent type. Regarding DataTypes, subtypes can only restrict. If a Variable is defined to have a numeric value, a sub type could restrict it to a float.

Figure 7 – The Relationship between Type Definitions and Instances

References allow Nodes to be connected in ways that describe their relationships. All References have a ReferenceType that specifies the semantics of the relationship. References can be hierarchical or non-hierarchical. Hierarchical references are used to create the structure of Objects and Variables. Non-hierarchical references are used to create arbitrary associations. Applications can define their own ReferenceType by creating subtypes of an existing ReferenceType. Subtypes inherit the semantics of the parent but may add additional restrictions. Figure 8 depicts several References, connecting different Objects.

Figure 8 – Examples of References between Objects

The figures above use a notation that was developed for the OPC UA specification. The notation is summarized in Figure 9. UML representations can also be used; however, the OPC UA notation is less ambiguous because there is a direct mapping from the elements in the figures to Nodes in the AddressSpace of an OPC UA Server.

Figure 9 – The OPC UA Information Model Notation

A complete description of the different types of Nodes and References can be found in [OPC 10000-3] and the base structure is described in [OPC 10000-5].

OPC UA specification defines a very wide range of functionality in its basic information model. It is not expected that all Clients or Servers support all functionality in the OPC UA specifications. OPC UA includes the concept of Profiles, which segment the functionality into testable certifiable units. This allows the definition of functional subsets (that are expected to be implemented) within a companion specification. The Profiles do not restrict functionality, but generate requirements for a minimum set of functionalities (see [OPC 10000-7])

OPC UA allows information from many different sources to be combined into a single coherent AddressSpace. Namespaces are used to make this possible by eliminating naming and id conflicts between information from different sources. Namespaces in OPC UA have a globally unique string called a NamespaceUri and a locally unique integer called a NamespaceIndex. The NamespaceIndex is only unique within the context of a Session between an OPC UA Client and an OPC UA Server. The Services defined for OPC UA use the NamespaceIndex to specify the Namespace for qualified values.

There are two types of values in OPC UA that are qualified with Namespaces: NodeIds and QualifiedNames. NodeIds are globally unique identifiers for Nodes. This means the same Node with the same NodeId can appear in many Servers. This, in turn, means Clients can have built in knowledge of some Nodes. OPC UA Information Models generally define globally unique NodeIds for the TypeDefinitions defined by the Information Model.

QualifiedNames are non-localized names qualified with a Namespace. They are used for the BrowseNames of Nodes and allow the same names to be used by different information models without conflict. TypeDefinitions are not allowed to have children with duplicate BrowseNames; however, instances do not have that restriction.

An OPC UA companion specification for an industry specific vertical market describes an Information Model by defining ObjectTypes, VariableTypes, DataTypes and ReferenceTypes that represent the concepts used in the vertical market, and potentially also well-defined Objects as entry points into the AddressSpace.