CSC 8260-Chapter 17: Real-Time Ethernet for Automation Applications

Yuehua Wang

Outline

Introduction

Structure of the IEC Standardization

Real-Time Requirements

Practical Realizations

Conclusion

Introduction Internet of things

Enabled by Ethernet and TCP/IP protocol

Low cost

Widely deployed

Everything, anywhere

Make life much easier

Automation world

Dedicated field buses (the name of a family of industrial computer network protocols used for real-time distributed control, standardized as IEC 61158)

Introduction (cont’d.)

Fieldbuses

Become barriers to access devices from Ethernet

Question：

Is it possible to use Ethernet also in the field of automation world?

More convenient for remote users to control field devices via web browser.

Answers?

What are scenarios in Automation?

Time deterministic communication

Time-synchronized actions between field devices

Efficient and frequent exchange of very small data records

However, Ethernet and its applications

Best-effort model

No QoS guarantees

Jitters, packet loss, and long packet delay

Migration of the Ethernet to Real-Time Ethernet (RTE)

Become a necessity in the field of automation world

Existing studies

Introduce quality of service

Modify packet processing in switch

Synchronize field devices.

The IEC/SC65C committee

in addition to the maintenance of the international fieldbus and its profiles

Finished a standardization project and defined aspects of RTE

Outline

Introduction

Structure of the IEC Standardization

Real-Time Requirements

Practical Realizations

Conclusion

Structure of the IEC Standardization

IEC 61158

Standard including all industrial protocols

structured according to the Open System Interface (OSI) reference model in seven parts

all networks identified by types

24 different types in 6 different parts

IEC 61784

Standard collecting sets of profiles

some versions based on Ethernet technology are also defined.

ISO/IEC 8802-3(Ethernet)-based

To identify all these profiles,

CPFs denotes communication profile families

CPs represents communication profiles

which is a set of policies that determine how to communicate with other devices.

Full table can be found on Pages 17-4 and 17-5

Table 17.2 covers

functional safe communications, secure communications, installation profiles for communication networks

not discussed in this talk (Ch17)

Outline

Introduction

Structure of the IEC Standardization

Real-Time Requirements

Practical Realizations

Conclusion

Real-Time Requirements

The users of an RTE network

different requirements

different applications

Performance indicators

Requirements defined in IEC 61784-2 (Table 17.2)

Defines the requirements for a class of applications

Have limits or ranges

Are interdependent

PIs are defined in the CPs for RTE (IEC 61784-2):

Delivery time

Number of RTE end-stations

Basic network topology

Number of switches between RTE end-stations

RTE throughput

Non-RTE bandwidth

Time synchronization accuracy

Non-time-based synchronization accuracy

Redundancy recovery time

Delivery time

is the time needed to convey a service data unit (SDU, message payload) from one node (source) to another node (destination). The delivery time is measured at the application layer interface.

The maximum delivery time shall be stated for the two cases of no transmission errors and one lost frame with recovery.

Using delivery time as a performance indicator, there are three classifications

Three classifications

Low speed class for human control

delivery times around 100 ms

Typical for the case of humans involved in the system observation

Most processes in process automation and building control fall into this class

Use Basic Ethernet and TCP/IP protocol

Class for process control

Delivery times below 10 ms

for most tooling machine control system like PLCs or PC-based control, special effort has to be taken in the RTE equipment

Powerful and expensive computer resources are needed to handle the TCP/IP protocol in RT

Class for motion control

Delivery times below 1 ms

A jitter of not more than 1 us

Ethernet network with a minimal bit rate of 100 Mbit/s

Modify Protocol medium access and hardware structure

More definitions of PIs can found on Page 17-7 in the textbook

Outline

Introduction

Structure of the IEC Standardization

Real-Time Requirements

Practical Realizations

Conclusion

Practical Realizations

Note that Standard Ethernet is not able to reach the requirements of the RTE.

hypertext transfer protocol (HTTP) or file transfer protocol (FTP) for the NRT applications

Solution:

Modify the Ethernet technologies

Communication interfaces structured indifferent layers.

Three categories:

On top of TCP/IP

keep the TCP/UDP/IP protocols unchanged and concentrate all RT modification in the top

layer;

On top of Ethernet

the TCP/UDP/IP protocols are bypassed

the Ethernet functionality is accessed directly

Modified Ethernet

Ethernet mechanism and infrastructure itself are modified to ensure RT performance

“On Top of TC P/IP” Protocols

Realization of …

use the TCP/UDP/IP protocol stack

nondeterministic delays

Need extra resource in processing power and memory

Examples:

Modbus/TCP (Profile 15/1 and 15/2)

EtherNet/IP (Profile 2/2 and 2/2.1)

P-NET (Profile 4/3)

Vnet/IP (Profile 10/1)

Modbus/TCP (Profile 15/1 and 15/2)

probably one of the most widely used Ethernet solutions in industrial applications today

Modbus defined by Schneider Electric in 1979

a request/reply protocol using port 502

send a request frame

get back a reply frame

Modbus/TCP

the actual definition must be extended with service definitions for the integration in international standards

New RT extensions (profile 15/2)

real-time publisher subscriber (RTPS) protocol

two main communication models

publish–subscribe：transfers data from publishers to subscribers

composite state transfer (CST) protocol transfers state informationfrom a writer to a reader.

RTPS

One or several subscribers

publication service objects describe services’ attributes

Topic, type, quality

Subscribe defines a minimum separation time

Best effort

No concrete indication for the values for PIs

Strongly depends on the performance and implementation of the UDP/IP communication stack

EtherNet/IP (Profile 2/2 and 2/2.1)

defined by Rockwell and supported by the Open DeviceNet Vendor Association (ODVA)†and ControlNetInternational

standardized in IEC 61784-1 as profile 2/2 (using type 2 specifications in IEC 61158)—already provides ISO/IEC 8802-3-based RT communication.

makes use of the common industrial protocol (CIP)

An industrial protocol for industrial automation applications

Time synchronizing with IEEE 1588 protocol (accuracy of 0.5 μs)

Defines objects to transport control-oriented data and other related information (e.g., parameters and diagnostics)

P-NET (Profile 4/3)

proposed by the Danish national committee

RT communication wrapped into UDP/IP packages

P-NET packages routed through both IP networks and any type of P-NET network

IP specification just defines how to tunnel over UDP/IP network

No special mechanism for real-time delivery guarantee

Vnet/IP (Profile 10/1)

developed by Yokogawa and is included in the IEC document as profile 10/1

Uses the UDP/IP protocol suite to transport the RTP application protocol.

A Vnet/IP network consists of one or more domains connected to each other by routers.

Uses IP unicast and multicast addresses

Not an RTE protocol; No special measures to get a deterministic or even RT behavior; not control other traffic using the bandwidth

one or more domains connected to each other by routers

10 ms: the min cycle time of scheduling of RT traffic

“On Top of Ethernet” Protoocols

Don’t have Ethernet communication hardware modification

Ethertype (=0x0800 protocol type) instead of TCP/IP protocol stack

RTE protocols has their own protocol type

Examples:

Ethernet Powerlink (Profile 13/1)

TCnet (Profile 11/1)

EPA (Profile 14/1 and 14/2)

PROFINET CBA (Profile 3/3)

Ethernet Powerlink (Profile 13/1)

EPL was defined by Bernecker + Rainer (B&R)

is now supported by the Ethernet Powerlink Standardisation Group (EPSG).*

A protected Ethernet

No slaves can talk without permission

Master–slave scheduling

SCNM: slot communication network management

MN: managing node, i.e., the master

CNs: controlled nodes sending on request by the MN

SoC: start-of-cycle frame issued by the MN to start the cycle

Preq: Poll-Request frame (unicast) for configuration

PRes: Poll-Response frame (mulicast)

SoA: Start-of-Asynchronous frame

UDP/IP

(+) No interference and guaranteed communication time

Multiple Ethernet segments

MNs exchange messages

Synchronize using distributed clocks

routing functionality specified by the MNs, e.g., IP

Application

CANopen: define process data objects (PDOs) to control the physical process and service data objects (SDOs)

PDOs use isochronous EPL communication

SDOs use UDP/IP protocol

TCnet (Time-Critical Control Network) (Profile 11/1)

is a proposal from Toshiba

Uses modified standard MAC* access CSMA/CD†

A cycle is called a high-speed-transmission period

Node 1broadcast Node 2

high-, medium-, and low-speed cyclic data transmission

First send high-speed cyclic data

TCnet handles redundant transmission media, for low recovery time

two inputs of received frames

two outputs to two redundant transmission media

Application: common memory system (virtual memory)

EPA (Profile 14/1 and 14/2) a Chinese proposal

Two phases:

periodic message transferring phase

nonperiodic message transferring phase (Tn)

two kinds of application processes

EPA function block application processes

NRT application processes (TCP/IP)

PROFINET Component-Based-Automation (CBA) (Profile 3/3)

is defined by several manufacturers (including Siemens) and supported by PROFIBUS International†

The mechanical, electrical, and functional elements of an automation device are grouped together into components.

Components have inputs and outputs

The values of the input and output VARs of the components are transmitted over the standard TCP/IP connection using the RPC* and DCOM† protocol from the office world. If cycle times of less than 100 ms are required, the RT protocol is used.

“Modified Ethernet” protocols

SERCOS (Profile 16/3)

EtherCAT (Profile 12/1 and 12/2)

PROFINET IO (Profile 3/4, 3/5, 3/6)

SERCOS (Profile 16/3)

a daisy chain (line structure) or a ring (ring structure)

Only the free port of the last slave in a line structure may be connected to a switch if required by the configuration, for example, for communication with devices via TCP/IP or UDP/UDP.

consists of two different logical communication channels:

RT channel

IP channel

The communication cycle consists of

up to four master data telegrams (MDTs)

up to four device telegrams (ATs¶) in the RT channel and the IP channel

MDTs are transmitted by the master and received by each slave

Synchronization information and a data record for each slave

The ATs are transmitted by the master as an empty frame with predefined fields but without information. Each slave inserts its data into data fields allocated to it in the ATs.

EtherCAT (Profile 12/1 and 12/2)

A line topology

In order to achieve maximum performance,

the Ethernet frames should be processed “on the fly.”

To realize such a node, a special ASIC* is needed for medium access, which integrates a two-port switch into the actual device.

PROFINET IO (Profile 3/4, 3/5, 3/6)

defined by several manufacturers (including Siemens) and supported by PROFIBUS International†

isochronous RT (IRT)

all IRT frames are transmitted based on a a predefined and configured timetable

RT

address-based communication and devices behave as standard Ethernet switches

devices are synchronized by means of a modified IEEE 1588 mechanism with “onthe fly” stamping

PROFINET IO

needs a 100 Mbit/s switched full duplex Ethernet network

For IRT, a special PROFINET-Ethernet switch is needed

Outline

Introduction

Structure of the IEC Standardization

Real-Time Requirements

Practical Realizations

Conclusion

Conclusion A long discussion in standardization process:

why is it not possible to reduce the number of technical solutions to three or four RTE profiles?

More than a technical selection or decision

Depend on the market

More different industrial networks in the field No upper limit visible