4 Fundamentals
4.1 Introduction to Sercos

Sercos (Se rial R ealtime Co mmunication S ystem) is a digital automation bus that interconnects motion controls, drives, I/Os, sensors and actuators for numerically controlled or motion controlled machines and systems. It is designed for high-speed serial communication of standardized real-time data over a noise-immune, fiber optic ring (Sercos I & II) or Industrial Ethernet cable (Sercos III). Sercos is an international standard defined in the IEC61784/ 61158 standard series.

Sercos III is the open, IEC-standardized third-generation of Sercos that uses Industrial Ethernet for real-time control, combining the best of both Ethernet and previous Sercos designs to provide the highly deterministic bi-directional real time motion and I/O control required by modern production equipment. It overcomes the wasted bandwidth in other TCP/IP-based Ethernet bus solutions, because it is based directly on Ethernet frames, defining a new, registered EtherType for Sercos. In addition to real-time communications between all drives and the motion control, Sercos III provides rich I/O communication capabilities, while also enabling other protocols, such as EtherNet/IP, TCP/IP, UDP and others, to be transmitted over the same Ethernet network efficiently in parallel with Sercos real-time communication or even without Sercos real-time communication.

Sercos Parameters can be accessed in different ways, e.g. by using:

  • a Sercos master device and the Sercos real-time protocol providing cyclic and acyclic services to exchange data with Sercos (slave) devices,
  • an S/IP client and the S/IP protocol providing access to all data of Sercos devices which incorporate an S/IP server using the TCP/IP and UDP/IP protocol, not requiring a Sercos master and the Sercos real-time protocol, but also working in parallel to the Sercos real-time protocol,
  • an OPC UA server integrated in a Sercos (slave) device to provide direct OPC UA access to a Sercos device, not requiring a Sercos master and the Sercos real-time protocol, but also working in parallel to the Sercos real-time protocol.
4.2 OPC UA

The main use case for OPC standards is the online data exchange between devices and HMI or SCADA systems using Data Access functionality. In this use case the device data is provided by an OPC server and is consumed by an OPC client integrated into the HMI or SCADA system. OPC DA provides functionality to browse through a hierarchical namespaces containing data items and to read, write and to monitor these items for data changes. The classic OPC standards are based on Microsoft COM/DCOM technology for the communication between software components from different vendors. Therefore classic OPC server and clients are restricted to Windows PC based automation systems.

OPC UA incorporates all features of classic OPC standards like OPC DA, A&E and HDA but defines platform independent communication mechanisms and generic, extensible and object-oriented modelling capabilities for the information a system wants to expose.

The OPC UA network communication part defines different mechanisms optimized for different use cases. The current version of OPC UA is defining an optimized binary protocol for high performance intranet communication as well as Web Services. It allows adding new protocols in the future. Features like security, access control and reliability are directly built into the transport mechanisms. Based on the platform independence of the protocols, OPC UA servers and clients can be directly integrated into devices and controllers.

The OPC UA Information Model provides a standard way for Servers to expose Objects to Clients. Objects in OPC UA terms are composed of other Objects, Variables and Methods. OPC UA also allows relationships to other Objects to be expressed.

The set of Objects and related information that an OPC UA Server makes available to Clients is referred to as its AddressSpace. The elements of the OPC UA Object Model are represented in the AddressSpace as a set of Nodes described by Attributes and interconnected by References. OPC UA defines eight classes of Nodes to represent AddressSpace components. The classes are Object, Variable, Method, ObjectType, VariableType, DataType, ReferenceType and View. Each NodeClass has a defined set of Attributes.

This specification makes use of almost all OPC UA NodeClasses.

Objects are used to represent real-world entities such as Devices and (communication) Networks as well as software entities such as Blocks. An Object is associated to a corresponding ObjectType that provides definitions for that Object.

Variables are used to represent values. Two categories of Variables are defined, Properties and DataVariables.

Properties are Server-defined characteristics of Objects, DataVariables and other Nodes. Properties are not allowed to have Properties defined for them. Examples for Properties of Objects are the device serial number and the block tag.

DataVariables represent the contents of an Object. DataVariables may have component DataVariables. This is typically used by Servers to expose individual elements of arrays and structures. This specification uses DataVariables to represent the Parameters of both Blocks and Devices.

4.3 Conventions used in this document
4.3.1 Conventions for Node descriptions

Node definitions are specified using tables (See Table 1)

Table 1 – Type Definition Table

Attribute

Value

Attribute name

Attribute value. If it is an optional Attribute that is not set “--“ will be used.

References

NodeClass

BrowseName

DataType

TypeDefinition

ModellingRule

ReferenceType name

NodeClass of the TargetNode.

BrowseName of the target Node. If the Reference is to be instantiated by the server, then the value of the target Node’s BrowseName is “--“.

Attributes of the referenced Node, only applicable for Variables and Objects.

Referenced ModellingRule of the referenced Object.

Notes –

Notes referencing footnotes of the table content.

Attributes are defined by providing the Attribute name and a value, or a description of the value.

References are defined by providing the ReferenceType name, the BrowseName of the TargetNode and its NodeClass.

  • If the TargetNode is a component of the Node being defined in the table the Attributes of the composed Node are defined in the same row of the table.
  • The DataType is only specified for Variables; “[<number>]” indicates a single-dimensional array, for multi-dimensional arrays the expression is repeated for each dimension (e.g. [2][3] for a two-dimensional array). For all arrays the ArrayDimensions is set as identified by <number> values. If no <number> is set, the corresponding dimension is set to 0, indicating an unknown size. If no number is provided at all the ArrayDimensions may be omitted. If no brackets are provided, it identifies a scalar DataType and the ValueRank is set to the corresponding value (see OPC 10000-3). In addition, ArrayDimensions is set to null or is omitted. If it can be Any or ScalarOrOneDimension, the value is put into “{<value>}”, so either “{Any}” or “{ScalarOrOneDimension}” and the ValueRank is set to the corresponding value (see OPC 10000-3) and the ArrayDimensions is set to null or is omitted. In Table 2 examples are given.

Table 2 – Examples of DataTypes

Notation

DataType

ValueRank

ArrayDimensions

Description

Int32

Int32

-1

omitted or NULL

A scalar Int32

Int32[]

Int32

1

omitted or {0}

Single-dimensional array of Int32 with an unknown size

Int32[][]

Int32

2

omitted or {0,0}

Two-dimensional array of Int32 with unknown sizes for both dimensions

Int32[3][]

Int32

2

{3,0}

Two-dimensional array of Int32 with a size of 3 for the first dimension and an unknown size for the second dimension

Int32[5][3]

Int32

2

{5,3}

Two-dimensional array of Int32 with a size of 5 for the first dimension and a size of 3 for the second dimension

Int32{Any}

Int32

-2

omitted or NULL

An Int32 where it is unknown if it is scalar or array with any number of dimensions

Int32{ScalarOrOneDimension}

Int32

-3

omitted or NULL

An Int32 where it is either a single-dimensional array or a scalar

If the NodeId of a DataType is provided, the symbolic name of the Node representing the DataType shall be used.

If no components are provided, the DataType, TypeDefinition and ModellingRule columns may be omitted and only a Comment column is introduced to point to the Node definition.

Components of Nodes can be complex, i.e. containing components by themselves. The TypeDefinition, NodeClass, DataType and ModellingRule can be derived from the type definitions, and the symbolic name can be created as defined in 4.3.2.1. Therefore those containing components are not explicitly specified; they are implicitly specified by the type definitions.

4.3.2 NodeIds and BrowseNames
4.3.2.1 NodeIds

This “OPC UA for Sercos Devices Companion Specification” is mandatory using String as IdentifierType for all Objects / instance NodeIds and any NodeIds, e.g. Numeric, for the ObjectType NodeIds.

The symbolic name of each Node defined in this document is its BrowseName, or, when it is part of another Node, the BrowseName of the other Node, a “.”, and the BrowseName of itself. In this case “part of” means that the whole has a HasProperty or HasComponent Reference to its part. Since all Nodes not being part of another Node have a unique name in this document, the symbolic name is unique.

In this “OPC UA for Sercos Devices companion specification”, the Identifier for each Node starts with one out of the three possible connection / addressing types

  • Topological (defined in Sercos Communication Profile Specification).

The Sercos topology index (topological address) describes the position of a slave (device) in a Sercos network. It uses a daisy chain numeration starting with value 1 at the first slave behind the master.

  • Sercos (defined in Sercos Communication Profile Specification).

The Sercos address is a freely selectable address of a slave (device) in the range between 1 and 511. It has to be unique in the Sercos network.

  • SercosIP (defined in Sercos Internet Protocol Services Specification).

This is the IP address of a Sercos device within an IP network, e.g. Sercos network or “non-Sercos” network.

Each connection type has its own NodeId composition that can be explained as follows. Each of the following examples addresses the variable MinValue from parameter S-0-0001 (SercosParameterType).

Topological:

Topological,[Sercos Master No.], [Sercos Topological Address]

e.g. Topological,0,1.ParameterSet.”S-0-0001”.MinValue

Sercos:

Sercos,[Sercos Master No.], [Sercos Slave Address]

e.g. Sercos,0,1.ParameterSet.”S-0-0001”.MinValue

SercosIP:

SercosIP,[IP Address], [SlaveIndex], [SlaveExtension]

e.g. SercosIP,”192.168.0.11”,0,0.ParameterSet.”S-0-0001”.MinValue

(The SlaveIndex will be derefered to a SlaveAddress using the IDN S-0-1046. For further information please refer to the Sercos Communication Profile Specification.)

The namespace URI for Sercos Types is “http://sercos.org/UA/”.

The namespace URI for Sercos Instances is arbitrary, e.g. “http://[YourCompanyName.com]/UA/Sercos/”.

4.3.2.2 BrowseNames

The text part of the BrowseNames for all Nodes defined in this specification is specified in the tables defining the Nodes. The NamespaceIndex for all BrowseNames defined in this specification is server specific and depends on the position of the namespace URI in the server NamespaceArray.

4.3.3 Data types and lengths transformation between Sercos and OPC UA

In order to access data of Sercos devices via OPC UA, data types and lengths are transformed between Sercos and OPC UA. Table 3 shows the data types and data lengths transformation rules between Sercos and OPC UA.

Table 3 – Transformation rules between Sercos and OPC UA

Sercos

OPC UA

Data Type

Data Length

Data Type

Extended Character Set

1 Octet List

String

Floating-Point number

4 Octet

Float

8 Octet

Double

Integer

2 Octet

Int16

4 Octet

Int32

8 Octet

Int64

Unsigned Integer

or

Binary Number

1 Octet

Byte

2 Octet

UInt16

4 Octet

UInt32

8 Octet

UInt64

Sercos Time

8 Octet

UtcTime

The Sercos list data types are transformed analogue to the single data types, e.g. S-0-0017 (Data type: Integer, Data length: 4 Octed list) is transformed to an array of Int32.