Networking in C&A

TopicsControl & Automation System ConfigurationsPLCsSCADADCSRTUsTraditional Methods of Signal FlowField-BusesModbus, CAN, Profibus, FFIndustrial Ethernet

Control & Automation System Configurations A look at the classical loop

Control & Automation System Configurations Alternative controllersPneumatic controllerRelay logic hardwiredElectromechanical cam timersDiscrete Electronic ControllersDigital / Microprocessor / Microcontroller / PLC based

Control & Automation System Configurations PLCsPLC was invented in response to the needs of the American automotive industry1968 GM Hydramatic PLC 084 by Bedford Associates of BostonModicon an industrial computer control system thatcontinuously monitors the state of input devices andmakes decisions based upon a custom programto control the state of devices connected as outputs

Control & Automation System Configurations PLCsAllen-Bradley SLC 500 Processors Siemens Simatic S7-300

Control & Automation System Configurations PLCsBuilt in communications ports forat least RS232, and optionally for RS485 and nowadays ethernetModbus is the lowest common denominator communications protocolOthers various field-buses such as ProfibusPLCs package I/O capabilities: Modular (limited no. of I/Os and expansion slots)Rack (high speed serial I/O link is used so that racks can be remotely mounted from the processor, reducing the wiring costs for large plants)P2P (with serial commn link + SCADA servers)

Control & Automation System Configurations SCADANot a full control system, but rather focuses on the supervisory levelThe bulk of the site control is actually performed automatically by a PLC or by an RTUHost control functions almost always restricted to basic site over-ride or supervisory level capabilityPurely software package positioned on top of hardware to which it is interfaced, in general via PLCs

Control & Automation System Configurations SCADADistributed database which contains data elements called pointsa single input or output value monitored or controlled by the system can be either "hard" or "softnormally stored as value-timestamp combinationsNot only HMI for distributed PLCs but alsoto provide instant trending, diagnostic data, scheduled maintenance procedures, logistic information, detailed schematics for a particular sensor or machine, and expert-system troubleshooting guides

Control & Automation System Configurations SCADAServer layerPLCs connected to data servers directly or through field-busesClient layerHMI consoles connected to data servers via ethernet normally

Control & Automation System Configurations SCADAServers have RTDBsresponsible for data acquisition and handling (e.g. polling controllers, alarm checking, calculations, logging and archiving)

Control & Automation System Configurations SCADACommunicationsServer-client and server-server communicationTCP/IP protocol normally over ethernet Access to devicesPolling the controllers at a user defined polling rateNormally thru a field-bus (eg Modbus; Profibus etc)Also use combinations of radio and direct serial or modem connectionsIP over SONET is also frequently used at large sites such as railways and power stationsSome customers wantSCADA data to travel over their pre-established corporate networks or to share the network with other applications

Control & Automation System Configurations SCADAMid 1990stypical DAQ I/O manufacturers own proprietary communications protocols over a suitable-distance carrier like RS-485Late 1990sshift towards open communications with I/O manufacturers offering support of open message structures like Modicon MODBUS over RS-4852000most I/O makers offered completely open interfacing such as Modicon MODBUS over TCP/IP

Control & Automation System Configurations SCADANow it is possibleto purchase a SCADA system from a single supplierto assemble a SCADA system from components like Wonderware HMI, Allen-Bradley & GE PLCs, Ethernet communication devices, etcPLC manufacturers offer integrated HMI/SCADA systems (using open and non-proprietary communications protocols)Third-party HMI/SCADA packagesoffer built-in compatibility with most major PLCs

Control & Automation System Configurations DCSA control system which collects the data from the field and decides what to do with thembe stored for future referenceused for simple process controluse in conjunction with data from another part of the plant for advanced control strategiesComponentsOperator ConsolesControl ModulesI/O ModulesHistory Module

Control & Automation System Configurations DCSpractical and technological boundaries between a DCS, PLC and PC control are blurring

ABB - Advant Master Control Systems Honeywell - Experion PKS Emerson - DeltaV Hybrid Systems

Control & Automation System Configurations RTUDevice which interfaces objects in the physical world to a DCS or SCADA systemby transmitting telemetry data to the system and/oraltering the state of connected objects based on control messages received from the systemtypical RTU has a communications interface (usually serial, Ethernet, proprietary, or any combination), a simple processor, some environmental sensors, some override switches, and a device-bus or field-bus which it uses to communicate with devices and / or interface boardsCAN, Modbus etcAllen-Bradley's Data Highway and HSQ Technology's MISERnet

Control & Automation System Configurations RTUInterface boardsanalog and digital flavors"DI" (digital input), "AO" (analog output) etc.subdivided based on the range of inputs, the amount of protection against voltage surges, the amount of intelligence on the interface boardRTUs and PLCs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa

Control & Automation System Configurations HeirarchyUsually needs an organized hierarchy of controller systems to functiona central/distributed computer / SCADA system at the top, where an operator can monitor or program the whole system (linked by automation / plant network)a middle layer of PLCs / RTUs linked via a control bus / network systema bottom control chain is a device-bus or field-bus which links the PLCs / RTUs to the components which actually do the work such as sensors, actuators, motors, console lights, switches and contactors

Control & Automation System Configurations Heirarchy

Control & Automation System Configurations Historical perspective of signal flow1940s - process instrumentation relied upon pressure signals of 3-15 psi for the monitoring / control of devices1960s - 4-20 mA analogue signal standard was introduced for instrumentation1970s - use of computers to monitor and control a system of instruments from a central point1980s - smart sensors began to be developed and implemented in a digital control, microprocessor environmentneed to integrate the various types of digital instrumentation into field networks to optimise system performance1990s efforts to standardiseInstrument Society of America (ISA), the International Electrotechnical Commission (IEC), Profibus (German national standard) and FIP (French national standard), form the IEC/ISA SP50 Fieldbus committee

Traditional Methods of Signal Flow The Ubiquitous Current Loop4-20mA loopCurrent vis-a-vis VoltageNoise immunityLong distancesSimple implementation

Traditional Methods of Signal Flow The Ubiquitous Current Loop

Traditional Methods of Signal Flow The Workhorse Asynchronous Serial ConnectionLow cost, low development solution for low bandwidth, low volume data transferFlavoursRS-232RS-422RS-485

Traditional Methods of Signal Flow Asynchronous Serial Connection RS-232Defined by EIA in 1962Available on almost all devicesUses bipolar voltages (+12V to -12V)Suitable for low noise environments and distances below 30.5 meters (100 feet), Transceivers capable of 460k baud and higher are available (practical 115.2k baud)

Traditional Methods of Signal Flow Asynchronous Serial Connection RS-232

Traditional Methods of Signal Flow Asynchronous Serial Connection RS-422/485RS-422Suited to longer distance communications, up to 1200 meters (4000 feet) without repeaters Using a balanced differential pair (two wires) results in higher noise immunity (valid signal down to 200mV) Commonly used for point-to-point communications, although up to 10 receivers may be connected to a single transmitterRS-485 Similar to RS-422offers a multidrop capability, up to 32 nodes can be connectedmultidrop feature also allows "two-wire" (in addition to signal ground) half-duplex data connection to be made.

Field-busIndustrial network system for real-time distributed control used in process control and industrial automation New industrial digital communications network intended to replace the existing 420mA analog signal standardA Field-bus Network is DigitalBi-directionalMultidropSerial-bus communications network used to link isolated field devicesEach field device has low cost computing power installed in it, making each device a "smart" deviceable to execute simple functions on its own such as diagnostic, control, and maintenance functions as well as providing bi-directional communication capabilitiescan report if there is a failure of the device or manual calibration is required

Field-busThe field-bus technology promises to Improve qualityDigital more accurate than analogReduce costsReduction in maintenance. (enables online diagnostics to be carried out on individual field devices, including functions such as open wire detection and predictive maintenance and simplifies tasks such as device calibration)One field-bus device could be used to replace a number of devices using the 420mA analogue standard Only a single twisted pair wiring scheme is required (the 420mA analogue signal standard requires each device to have is own set of wires and its own connection point) - 5:1 reduction in field wiring expense Boost efficiencySmart devices execute own functions

Field-busAround since 1988In 2000, all of the interested parties converged to create a the IEC fieldbus standard, 61158 with eight (8) different protocol sets called "Types" as follows:Type 1 - FOUNDATION Fieldbus H1 Type 2 ControlNetType 3 ProfibusType 4 - P-NetType 5 - FOUNDATION Fieldbus HSE (High Speed Ethernet)Type 6 InterbusType 7 - SwiftNet (a protocol developed for Boeing, since withdrawn)Type 8 - WorldFIP

Field-busOrganisations InvolvedIEC/ISA SP50 Fieldbus committeeInstrument Society of America (ISA)International Electrotechnical Commission (IEC)Profibus (German national standard)FIP (French national standard)Two major groupsWorld Factory Instrumentation Protocol (WorldFIP)Honeywell (Arizona); Bailey Controls (Ohio); Cegelec (Paris); Allen Bradley Corporation (Ohio); Telemecanique (Paris); Ronan Engineering Co. (California); Square D Electricite de France (France); Elf (France) Interoperable Systems Project (ISP)Siemens (Germany); The Rosemount Group (Minnesota); Fisher Controls, Inc. (Texas); Foxoboro Co. (Massachusetts); ABB Co. (Sweden); Yokogawa Electric Corporation (Tokyo)

Field-busLayers definedPhysical Layer: This defines the media that communication occurs over and could be viewed as the 4-20mA standard replacement. Data Link Layer: This monitors the communications taking place among the various devices and detects errors. Application Layer: This formats the data into messages which all devices connected to the network can understand and provides the services for process control, supplying them to the user layer. User Layer: This connects the individual plant areas and provides an environment for applications. It is implemented using high level control functions (product interoperability)

Field-busExamplesExamples of field-bus networks are

MODBUS CAN DEVICENETPROFIBUSFOUNDATION FIELDBUSIndustrial Ethernet

ModbusCommunications protocolpositioned at the level 7Master/slave or client/server architecture Designed by Modicon for use with its PLCs (Schneider)de facto standard communications protocol in industryUsesmost commonly available means of connecting industrial electronic devicesexample a system that measures temperature and humidity and communicates the results to a computer often used to connect a supervisory computer with a RTU in SCADA systemsVersions exist for serial port and EthernetModbus RTU - binary representation with CRCModbus ASCII ASCII representation with LRCModbus/TCP - similar to Modbus RTU but transmitted within TCP/IP data packetsModbus Plus (Modbus+ or MB+) - proprietary to Modicon similar to EIA RS485

ModbusEach device using Modbus has a unique addressAny device can send out a Modbus command (usually only one master device does so)Modbus command contains the Modbus address of the device it is intended forOnly the intended device will act on the command, even though other devices might receive itAll Modbus commands contain checking information, ensuring that a command arrives undamagedBasic Modbus commands can instruct a RTU to change a value in one of its registers, as well as commanding the device to send back one or more values contained in its registers

CANOriginally developed in the 1980s by Robert Bosch GmbH, for connecting electronic control units (ECUs) of automobilesIn 1992First Mercedes Benz car with CAN rolled outCAN in Automation CiA group formedIn 1994DeviceNet protocol introduced by Allen-BradleyIn 1995CANopen protocol introduced by CiAIn 2000Development of TTCAN

CANMulticast shared serial bus standardDesigned to be robust in electromagnetically noisy environments and can utilize a differential balanced line like RS-485Bit rates 1 Mbit/s at networks length below 40 mDecreasing the bit rate allows longer network distances (e.g. 125 kbit/s at 500 m)ProvidesMulti master hierarchy (still operates even when one node defective)Broadcast communications (message oriented message identifier rather than stations and station addresses)Sophisticated error detection & retransmission (transparent to user)Usually links two or more mC based devices

CANData link layer protocol is standardized in ISO 11898-1 (2003)LLC & MAC + some PHY descriptionsOther layers left to network designersuch as flow control, device addressing, and transportation of data blocks larger than one messagemany implementations of higher layer protocols have been createdDeviceNetCANopenCANaerospaceCAN Kingdom.

CANCAN transmits data througha binary model of "dominant" bits (logical 0) and "recessive" bits (logical 1)If one node transmits a dominant bit and another node transmits a recessive bit then the dominant bit "winsCarrier Sense Multiple Access/Bitwise Arbitration (CSMA/BA) schemeif two or more devices start transmitting at the same time, there is a priority based arbitration scheme to decide which one will be granted permission to continue transmitting

DeviceNetProcess Field Busmost popular type of field-busmore than 10 million nodes (2004) in use worldwide (In Europe PROFIBUS dominates - more than 60% of the factory automation market)Developed in 1989result of a German research project involving 15 firms and research institutionsDifferent versions:PROFIBUS-FMS (Fieldbus Message Specification) is used based on the Client-Server model for the communication between automation devices. PROFIBUS-DP (Decentrallised Periphery) is used for fast remote inputs and outputs, to connect sensors and actuators to a controlling device. PROFIBUS-PA (Process Automation) is used for the connection of field devices and transmitters to a process control device. It allows intrinsic safe transmission and power on the line

PROFIBUSProcess Field Busmost popular type of field-busmore than 10 million nodes (2004) in use worldwide (In Europe PROFIBUS dominates - more than 60% of the factory automation market)Developed in 1989result of a German research project involving 15 firms and research institutionsDifferent versions:PROFIBUS-FMS (Fieldbus Message Specification) is used based on the Client-Server model for the communication between automation devices. PROFIBUS-DP (Decentrallised Periphery) is used for fast remote inputs and outputs, to connect sensors and actuators to a controlling device. PROFIBUS-PA (Process Automation) is used for the connection of field devices and transmitters to a process control device. It allows intrinsic safe transmission and power on the line

Foundation FieldbusUsing a distributed architecture where the control is in the devices themselvesFoundation fieldbus integrates low-speed sensors and actuators with high-speed controllers and servers in a single systemLower speed H1provides communication and power over standard twisted-pair wiring is a bus-powered, serial, multi-drop topology at 31.25 Kbps that replaces 4-20 mA circuitsbut can communicate with 4-20 mA devices via convertersIntroduced in 1995provides services at layers 1, 2 and 7 of the OSI modelHigher speed HSEusing standard Ethernet cabling provides 100 Mbps between PLCs, host servers and workstationsalso connects to H1 networks via the linking device that multiplexes H1 segments togetherIntroduced in 2000provides services at layers 1, 2, 3, 4(Ethernet and TCP/IP) and layer 7

Industrial EthernetNormally PLC communicates with a slave machine using open or proprietary protocols, such asModbus, Profibus, CANopen, DeviceNet or Foundation FieldbusHowever, there is now increasing interest in the use of Ethernet as the link-layer protocol, with one of the above protocols as the application-layer

Industrial Ethernet Factors Favouring EthernetReady availabilityLow costHardwareTrained manpowerAbility to use standard access points, routers, switches, hubs, cables and optical fiber, which are immensely cheaper than the equivalent serial-port devicesSpeedIncreased speed, up from 9.6 kbit/s with RS232 to 1 Gbit/s with IEEE 802 over Cat5e/Cat6 cables or optical fiber Fast Ethernet has higher data rates than DeviceNet & ProfibusPeer-to-peer architectures may replace master-slave ones

Industrial Ethernet Factors Against EthernetInteroperabilityEthernet only provides layer 1 & 2It is the upper layers which define the functionality of the network & devices it can connect toa standard application layer, common object model and universal device profiles dont existNeed for Determinism and Real Time controlsLacking in ethernet inherentlyNo. of network nodes continually changeEnvironment SuitabilityDifficultiesMigrating existing systems to a new protocol (however many adapters are available) Managing a whole TCP/IP stack is more complex than just receiving serial data

Industrial Ethernet Fatter pipes of Ethernet

Industrial Ethernet Evolution of EthernetHalf Duplexone pair of cables used for communication.Can either be for transmission or reception, but not both at the same time.Full Duplextwo pairs of cables used for communication.One pair for transmission, one for reception.Dual Duplexone pair of cables used for communication.The pair can be used for transmission and reception at the same time.

Industrial Ethernet Switching in Ethernet1990 Switches introduced which allow multiple simultaneous transmission pathsOne collision domain in original EthernetMany collision domains in switched ethernet

Industrial Ethernet Increasing Speeds

Industrial Ethernet QoS in EthernetQoS introduced by CISCO for VoIPTag field introduced in frame3 bits are reserved for User Priority and 12 bits for the VLAN Original Ethernet Frame - maximum length 1518 bytes New Ethernet Frame - maximum length 1522 bytes

Industrial Ethernet QoS in Ethernet

Industrial Ethernet VLANs in Ethernet

Industrial Ethernet Redundancy in EthernetSpanning Tree (802.1D)allows the connection of Ethernet switches into a tree structureduplicate paths are then deactivated, and only reactivated when the active one failsproblem is the time it takes to reconfigure - approximately 30 secondsFast Spanning Tree, is expected to get this time down to 5 secondsLink Redundancy / Aggregation involves double wiring each connectionHirschmanns Methodring structure and one of the switches to monitor the integrity of the ring by sending short frames the entire way aroundOne of the links connected to this Redundancy Monitor switch/hub will be deactivated, but will still pass these special integrity check framesand only be reactivated when the integrity of the link fails. By this method, we can get reconfiguration times as low as 50ms

Industrial Ethernet Hardening in EthernetTIA/EIA's 569 Committee's TR 42.9 Working Group is seeking to modify 568 to meet industrial requirements To provide 100-Mbps Ethernetdevelopers had to lower the amplitude and encode the data using a tertiary signaling method called MLT3 bit encodingalready low-amplitude voltage is split into three distinct levels, which allows it to run faster, while reducing radiated emissionsHigh mode rejection (CMRR) on cabling and magnetics on the network interface card (NIC) can further enhance performance and reduce noiseEnvironmentally hardened equipment

Gigabit EthernetGuaranteeing determinism with intelligent switchesIn the avionics field, standards such as AFDX (avionics full-duplex switched Ethernet) and ARINC 664 specify how to create a deterministic and profiled avionics data network with 100 Mbps switchesIf Gigabit Ethernet provided this can surely be achievedReducing bottlenecks and eliminating unnecessary network delays can be achieved transparently byusing QoS techniques, such as VLAN segmentation, access restriction, traffic prioritization and guaranteed bandwidth allocationEnsuring fault-tolerance with a resilient network (can be addressed with MST, RST and link aggregation)

Gigabit EthernetGuaranteeing determinism with intelligent switchesTraffic prioritization, based on the traffic classGuaranteed bandwidth allocation per VLAN basis

Gigabit EthernetGuaranteeing determinism with intelligent switchesResilience & RedundancyUsing MST & RST algorithmsUsing link aggregation

Real Time EthernetAccepting variance in Ethernet transmission propagation timesone solution to guarantee deterministic system behaviour might be to make available a precise clock in all terminal devices synchronised with all other systems IEEE 1588new IEEE Standard Precision Time Protocol (PTP)it is possible to synchronise to less than a microsecond local clocks in sensors, actuators and other terminal devices using the same network that also transports process dataUses IP multicast on ethernet

Choosing the Right, Best NetworkWhat is the distance requirement?What physical cabling arrangement makes sense for this application?What is the speed (i.e., response time) requirement for the most time-critical devices? Do all devices require that speed or should some have a higher priority than others?Does the network allow you to prioritize messages?Do the devices you want to use support the same network standard? Are there open versus closed architecture considerations?How much electrical noise is present in the application and how susceptible is the cabling to such interference?What is the maximum required packet size for the data that will be sent? If the data can be fragmented over several packets, how fast does a completed message have to arrive?What type(s) of device relationships are desired: master/slave, peer-peer, or broadcast?Does the network need to distribute electrical power? If so, how much current?What kind of fault tolerance needs to be built into the network architecture?