Communication Basics

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Communication Basics

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  • Communication BasicsTopicsISO OSI Layers Sajan JosephRS 232, 485 Sajan JosephHART Sajan JosephModbus RTU Lalit VarshneyEthernet Lalit Varshney

    The problem with communication ... is the illusion that it has been accomplished.G.B. Shaw

  • ISO OSI 7 Layer ModelPeople Don't Need This Stuff Presented AnywayENCAPSULATIONOpen Systems Interconnection Model Describes how information from a software application in one computer moves through a network to a softwareApplication in another computer.Application LayerE.g.:- HTTP, SMTP, FTP Presentation LayerStandard formats (GIF, TIFF, MPEG), Data Compression, Data conversion to ASCII, EBCDICSession LayerEstablish, Manage, Terminate sessions

    Transport LayerSegmenting of data, data sequencing, flow control. E.g.:- TCPNetwork LayerDefines network Addresses (E.g.:-IP address)Opens shortest path, keep track of number of hopsData Link LayerMedia Access Control (MAC Address) , Error notificationPhysical LayerDefines voltage levels, baud rates etc. E.g.:- Network interface cards.

  • Serial Communication - TermsBalanced vs. Unbalanced StandardsUnbalanced - An unbalanced signal is represented by a single signal wire where a voltage level on that one wire is used to transmit/receive binary 1 and 0. Balanced - A balanced signal is represented by a pair of wires where a voltage difference is used to transmit/receive binary information Simplex, Half Duplex, Full DuplexSimplex - A simplex system is one that is designed for sending messages in one direction only.Half-Duplex - Half duplex communications occurs when data flows in both directions; although in only one direction at a time.Full-Duplex - In a full duplex system, the data can flow in both directions simultaneously.

  • RS-232 Serial CommunicationInterfaces between Date Terminal Equipment (DTE) and a Data Communication Equipment (DCE).E.g.:- Communication between a Computer (DTE) and a Modem (DCE)RS stands for Recommended StandardCharacteristics Point to Point Unbalanced Low noise immunity Full Duplex Available in 9 Pin or 25 Pin configurations

    Disadvantages Max. speed of transmission 19.2 Kbps Susceptible to noise Distance Limitation 50 ft. Multi drop is not possible

  • RS-485 Serial CommunicationCharacteristics Multi-drop Balanced High noise immunity Half-Duplex or Full Duplex 2 wire or 4 wire twisted pair cable Max. number of devices on the network 32 Distances Up to 1200m Speeds Up to 100KbpsEven though RS-485 is available in both 2-wire and 4-wire configurations, 2-wire is more common.RS 485 2-wire configuration is Half Duplex, whereas 4 wire configuration is Full Duplex.

  • HART CommunicationHART Highway Addressable Remote TransducerHART is a master-slave communication protocol, which means that during normal operation, each slave (transmitters, actuators) communication is initiated by a master (DCS, PLC) communication device. Based on the Bell 202 telephone communication standard Operates using the frequency shift keying (FSK) principle The digital signal is made up of two frequencies 1,200 Hz and 2,200 Hz representing bits 1 and 0, respectively. Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications. Because the average value of the FSK signal is always zero, the 420 mA analog signal is not affected. The digital communication signal has a response time of approximately 23 data updates per second without interrupting the analog signal. A min. loop impedance of 230 Ohms is required for communication.Each HART device has a 38-bit unique address encoded at the time of manufacture that consists of the manufacturer ID code, device type code, and device-unique identifier. A HART master must know the address of a field device in order to communicate with it.

  • HART CommunicationBenefits - The traditional 420 mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using the HART protocol. The HART communication digital signal gives access to secondary variables and other data that can be used for maintenance, and diagnostic purposes. Backward compatibility of HART ensures that investments in existing cabling and current control strategies will remain secure.

  • MODBUS PROTOCOLThe Modbus protocol Modbus is an application layer messaging protocol developed by Modicon in 1979, positioned at level 7 of the OSI model that provides client/server communication between devices connected on different types of buses or networks. Modbus is a request/reply protocol and offers services specified by function codes. Modbus function codes are elements of Modbus request/reply PDUs (Protocol Data Unit).

  • RS-232 VS RS-485The original Modbus interface ran on RS-232, but most later Modbus implementations used RS-485 because it allowed longer distances, higher speeds and the possibility of a true multi-drop network. In a short time hundreds of vendors implemented the Modbus messaging system in their devices and Modbus became the de facto standard for industrial communication networks.

  • Serial Transmission Modes ASCII / RTUSerial Modbus connections can use two basic transmission modes, ASCII or RTU, remote terminal unit. The transmission mode in serial communications defines the way the Modbus messages are coded. With Modbus/ASCII, the messages are in a readable ASCII format. The Modbus/RTU format uses binary coding which makes the message unreadable when monitoring, but reduces the size of each message which allows for more data exchange in the same time span. All nodes on one Modbus network segment must use the same serial transmission mode. A device configured to use Modbus/ASCII cannot understand messages in Modbus/RTU and vice versa

    Modbus RTU is an open, serial (RS-232 or RS-485) protocol based on master/slave or client/server architecture. It is popular, well established, relatively easy to implement and reliable. Since it is so easy to implement, Modbus RTU has gained wide market acceptance wherever Industrial Automation Systems (IAS) or Building Management Systems (BMS) need to communicate with other devices. In fact, Modbus RTU is probably the most implemented automation protocol of all, and a quick search of Google will confirm this.

  • Modbus AddressesLegal Modbus RTU node addresses are 1-254. 0 is reserved for broadcast messages, and useable for writes only. This is very seldom used since there is no confirmation that the message was successfully received at the server node. If the physical layer is RS-232 then only one node can be implemented anyway. The RS-485 specification limits the number of nodes to 32, although some RS-485 drivers will allow this limit to be extended somewhat.

    Device addressDescription1...10000Coils10001...20000Inputs 3000140000Input Registers40001...50000Holding Registers

  • Modbus Configuration Examples with Delta V DCS system

  • Modbus Configuration Examples with Delta V DCS system

  • Modbus Configuration Examples with Delta V DCS system

  • Modbus Configuration Examples with Delta V DCS system

  • Modbus Configuration Examples with Delta V DCS system

  • Modbus Configuration Examples with Delta V DCS system

  • Modbus Configuration Examples with Delta V DCS system

  • EthernetMost widely deployed LAN technology60% + of all LAN installationsMost common office automation LANWill encounter in a variety of applications including plant LANs, sub-networks, and as a vehicle for control device connectionInexpensiveWidely understoodExcellent migration path to future technologiesFast ethernetGigabit ethernetMost typically LANs complying to IEEE 802.3 CSMA/CD specification; there are slight differences between ethernet at IEEE 802.3

  • EthernetEthernet is a contention LAN -- CSMA/CDaccess -- although typically fast -- is not guaranteed this performance profile is well suited to file transfer applicationsCSMA/ CD imposes minimal controls, allows fast access, and provides collision detection and resolution mechanismsFour (4) implementations:10base-T - twisted pair media via hub connection10base-2 - thin coaxial cable media10base-5 - thick coaxial cable media10base-FL - fiber optic mediaOvation will integrate/ interface to ethernet as follows:Printer support - either via a bridge or via a sub-netPLC integration - either via a bridge or via a controller sub-netPlant LAN integration

  • Ethernet10base-TUTP mediaStar topologyMost common implementationRJ-45 cable connection Connection via hubOne (1) device per hub portHub isolates individual end stations from the main networkMaximum of 100 meters per segment - installation can span 200 meter diameter without repeatersVariety of hub features and capabilities

  • Ethernet10base-2Coaxial cableBus architecturePassive tap connection200 meters max segment length32 devices per segmentStation connection may require a transceiver or media converterNIC may support 10base-T and 10base-2Network span can be increased through the use of repeaters

  • Ethernet10base-5Thick coax mediaBus topologyTypically backbone implementation500 meters max segment length100 devices per segmentStation requires transceiver for connection -- typically vampire tapsAUI on NIC is transceiver connection

  • Fast Ethernet100 Mbps extension of ethernetLogical extension of 10 Mbps ethernetProvides an excellent migration path to higher performanceRetains CSMA/CDNo impact on higher level protocols or applicationsBackbone application is typical 10/100 auto-sensing NICs available 10 times the bandwidth at twice the price of 10 MbpsFiber and UTP media supportDue to higher speed has a smaller application radius than 10 Mbps ethernetImplementations:100baseT4 - four (4) pairs of Category 3,4,or 5 UTP100base-TX - two (2) pairs of Category 5 UTP or STP100base-FX - two (2) strands of 62.5/125 multi-mode fiber

  • Ethernet Application ExampleLocalized Ethernet information highwayWestationsHub - Provides physical connection for printer and Westations via UTP. Printer - Communicates with Westations via UTPHUB connection.UTP cables, hub, Westation on-board ethernet controllers, and printer on boardethernet controller constitute informationhighway implementation...10baseT application using hub and UTP - for segment length less than 100 meters

  • Ethernet Application Example10base-T connected via fiber optic cableWestationsWestnet II data highwayPrinterFiber optic segment linking two hubs and connected equipmentUse of fiber optics assumes separation of >500 meters and/ orhigh electrical noise environment. Media conversion from localUTP to fiber accomplished in the hub Printers are ethernet-based via hub connection and integral ethernet controller.Network configuration allows any Westation to direct print output to eitherprinter.WestationsWestnet II data highwayHubPrinterMedia Conversionin HubWestationsWestnet II data highwayFiber optic segment linking two hubs and connected equipmentUse of fiber optics assumes separation of >500 meters and/ orhigh electrical noise environment. Conversion from localUTP to fiber accomplished using a media converterWestationsWestnet II data highwayPrinterMedia converterMedia converterUTPUTPMedia Conversionwith external convertersHubHubHub