TSN, OPC UA combine to address growing demand for real-time communications
The automation world’s drive to do more intricate tasks at increasingly higher speeds is driving interest in Time Sensitive Networking, which lets systems handle real-time communications. PROFIBUS/PROFINET International (PI) plans to start developing ways to use TSN to facilitate the integration of operational technology networks into the IT networks of companies.
TSN, developed by the IEEE802.1 Working Group that manages Ethernet-based communication, makes it possible to fulfill hard real-time requirements needed by closed loop applications within motion control applications or safety applications. TSN defines a Layer 2 for communication, making PROFINET a desirable application protocol for seamlessly integrating TSN.
TSN is garnering a lot of attention, especially in the context of Industry 4.0, which increases the amount of data sent over networks. Through further development of Ethernet in IEEE 802.1, future devices with standard Ethernet controllers can be developed that still meet all the robustness and determinism requirements of industrial automation.
There’s already plenty of activity that will help end users and equipment makers combine TSN and protocols like PROFINET. PI has set mid-2019 as its goal for publication of the specification for use of TSN with PROFINET. Many observers feel chipmakers will embed TSN in controllers, making it relatively simple to utilize the standard.
One focus of the working group for PROFINET was defining which of the numerous IEEE 802.1 standards will be required for integrating TSN into PROFINET. Synchronization using 802.1 ASrev, TAS (time aware shaper, 802.1Qbv) and preemption (802.1Qbu) are among the most important functions. Through the early selection of supported standards, device manufacturers can prepare for TSN now and plan their next generation of devices with TSN.
However, a significant finding of work so far is that the configuration of TSN network parameters will be among the decisive criteria for success and acceptance by users. TSN will only be easily integrated into systems if it uses a Plug & Work approach that makes extensive adjustment unnecessary. PI therefore primarily pursues the IEEE decentralized configuration model, which allows the creation of flexible, efficient system networks and will be tested, for example, on the test bed of Labs Network Industrie 4.0.
Thanks to the proven architecture of PROFINET using standard Ethernet, user perceptions on issues such as IO data, parametrization, diagnosis, etc. remain unchanged. In view of the large installed base of PROFINET Devices, this is an important compatibility issue for our users.
There’s also a lot of ongoing work that links TSN and OPC UA, a framework defined by the OPC Foundation based on service-oriented architecture (SOA) principles. The OPC-UA protocol will also take advantage of TSN to extend its capabilities to real-time communications.
OPC-UA provides an object model with a base data type system. Following an object-oriented approach objects can be composed by other objects, variables and methods and objects can have references to other objects. The base type system can be extended by using inheritance mechanisms.
A fixed set of base services are specified to guarantee interoperability. Those base services are grouped in ten logical service sets. For example, the Attribute Service Set is used to read and write attributes of specific object or the Method Service Set is used to invoke methods, which are a component of an object, within the servers address space.
The architecture of OPC-UA allows organizations to extend the information model by adding additional functionality or semantic to the object model. Those add-ons are called companion specifications. The device integration model is probably the most popular companion specification by defining an object model for devices independent from their underlying communication protocol. Other companion specifications are already specified, for example the OPC UA PackML Companion Specification or the OPC UA PLCopen Information model.
OPC UA and PROFIsafe
Recently the OPC Foundation and the PI announced that the organizations are forming a working group with the avowed goal to create a PROFIsafe over OPC UA specification which shall become an integral part of the OPC UA specification.
PROFIsafe is the leading standard for fail-safe communication and named as the first and only communication-based safety standard in China. It allows the transfer of standard and fail-safe data via the same network and because it relies on the black channel principle it can also be used in a wireless communication environment.
Combined with PROFIsafe technology, OPC-UA will then be able to run fail-safe communication between machines allowing a complete new approach for implementing a consistent fails-safe communication concept, which up to now, was only possible to a certain extent by using additional hardware.
Both protocols OPC UA PubSub and PROFINET will play a significant role in upcoming Siemens products.
The fact that PROFINET (with or without TSN) already fulfills requirements like determinism, flexibility and openness, and implements a well-defined fieldbus application layer covering services such as cyclic/acyclic data exchange, alarms/diagnosis and parametrization, makes it unnecessary to replace PROFINET at field level in the long run. Due to it being a proven and reliable technology, PROFINET will continue to play a significant field-level role in the future.
OPC-UA is a favored protocol for vertical acyclic communication which, when integrated with TSN and the PROFIsafe extension, may well become the technology of choice for machine to machine (M2M) communication.
Siemens will gradually adapt its portfolio in that direction, with PROFINET and TSN as the standard at field level and OPC-UA based on TSN as the standard for M2M communication and vertical communication above controller level.
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