SOFTWARESOFTWARE & NETWORKS & NETWORKS DigitalDigital BusesBuses ForFor Digital Plants Digital plant architectures have transformed the face of modern process plants. Jonas Berge, Senior Manager, PlantWeb Consulting, Emerson Process Management Asia Pacifi c Pte Ltd, explains the underlying bus technologies. Emerson igital communications technology over the bus, enabling plants to adopt the input and output values to the reduces wiring and improves a predictive maintenance program. digital automation system, the bus D end-to-end signal accuracy and Further, digital values may be trans- must enable confi guration of the integrity in modern digital plants. ferred in engineering units, allowing many settings that determine how Digital technology enables new transmitters to be used over their full transmitters and positioners operate innovative and more powerful devices, range and eliminating range mismatch. and must give access to the wealth wider measurement range, elimination Access to more information is also key of diagnostics information in these of range mismatch, and access to more to intelligent device management. devices as and when required. Lastly, information. Overall, use of digital many process industries require re- technology can reduce automation Application Areas dundant interface cards for increased project costs by as much as 30 percent Buses are used in factory automation, reliability. All of these requirements as well as providing a two percent process automation and building are met by FOUNDATION fi eldbus H1. operational improvement. This article automation. Tasks may vary from explores considerations to be made in motion control, to machine control, Factory Automation Digital Buses selection of bus technology for optimal to distillation column control. Since A bus for proximity switches and digital plant architecture. the tasks are varied, different buses solenoids in a factory environment were designed to meet a broad range has different requirements. The appli- ForDigital Plants Why Digital? of requirements. cation is not in a hazardous area There are many advantages of using so intrinsic safety is not required. digital communications and multi-drop Process Control Machinery may be fast moving fi eldbus wiring instead of conventional A bus for regulatory process control but unlike PID control, logic is not wiring, including reductions in cable requires input/output sampling to be sensitive to aliasing. Communication and connections as many devices precisely periodic and synchronised to the PLC can even be event-driven. connect to a single bus. Other benefi ts with communications and PID control Since these simple switches and include better accuracy for control to avoid aliasing, prolonged response solenoids have little or no settings and loops as no precision is lost in D/A and time, and instability due to jitter. The diagnostics, continuous I/O communi- A/D conversion, and higher integrity bus must also provide intrinsically cation with the PLC is suffi cient. In as distortions can be detected using safe power since the devices are this application ASI and DeviceNet 8 or 16 bit error checking. Two-wire often operating in hazardous areas buses connect directly to proximity devices get more current, allowing and require live maintenance. In switches and solenoid valves. delivery of new and faster diagnostics addition to continuously updating A common hybrid bus solution www.iaasiaonline.com April 2007 | industrial automation asia 41 SOFTWARE & NETWORKS is illustrated by applications having Control-Network Level Safety conventional sensors and actuators At the control-level, digital automation Most bus organisations have developed wired one-by-one to a remote-I/O system system controllers have other net- communication diagnostics extensions which in turn uses PROFIBUS-DP or working requirements. The control to their protocols to enable use in DeviceNet to connect to the PLC. network carries data gathered from Safety Instrumented Systems up to Note also that both PROFIBUS-DP and all the devices in the plant or area and SIL3. The use of fieldbus in Safety DeviceNet may be used for variable must therefore have a high bandwidth Instrumented Functions (SIF) is speed drives. capacity to cope with the data volume. expected to increase over the years A control network supports peer-to- to come. Until that time 4-20 mA with System Hierarchy peer communication between digital HART remains the most popular Even within a control system there is a automation system controllers to option for transmitters in safety, and need for different connection charac- enable cascade and sophisticated HART is also used to communicate teristics at each tier of the control control schemes. Digital automation diagnostics from the shutdown valves. hierarchy. FOUNDATION fieldbus is system controllers are separately used at one level, Ethernet at another, powered so there is no need to Acyclic Bus Communication and the two complement each other provide power over the network. All fieldbuses that provide cyclic com- perfectly. The plant information and Today most control systems use munication of process I/O inputs and business networks again have other Ethernet media and IP network layer outputs are continuously updated by requirements. protocol for the control-network. repeated reads and writes. This may Controllers, data servers, and work- be sufficient for simple switches and H1: Fieldbus Level stations are typically in close proximity actuators that have no configuration A process plant has hundreds or of each other so 100 m distance pro- settings and no diagnostics. However, even many thousands of sensors, vided by Ethernet on copper wire is intelligent devices such as transmitters, transmitters, valve positioners, and sufficient, but fibre optics can be used positioners, and drives need to be actuators. Wiring is significantly re- for longer distances. Several different configured, and the information they duced by connecting these many application layer protocols exist for supply includes valuable information devices with fieldbus. The fieldbus Ethernet in factory automation and such as diagnostics. The bus tech- also delivers power to operate the process control including PROFINET- nology selected to connect these devices. CBA, PROFINET-IO, EtherNet/IP, Modbus/ devices must therefore, in addition to In hazardous areas, it is required TCP, FOUNDATION fieldbus HSE and cyclic I/O communication, also support to use a fieldbus that supports intrinsic several more. Since reliability is acyclic communication of other data in safety or non-incendive wiring to paramount in process control, digital the device. Digital plant architecture permit live connection/disconnection automation systems support full uses digital device managers as part without switching off the other devices Ethernet redundancy with peer-to- of asset management tools for device on the bus. FOUNDATION fieldbus peer communication and therefore a management. H1 and HART cable can run long redundant digital automation system Together, device management distances to the devices mounted in controller pair has a total of four software and fieldbus device func- the field, far from the control room. Ethernet ports. tionality typically enable: H2: Remote-I/O Level The middle tier in the process control system is remote I/O subsystems where conventional sensors and actuators connect to I/O cards using conventional wiring. The remote I/O subsystems connect to the digital automation system or PLC controller HART/Classic I/O using a bus that may also be used for drives. Since I/O subsystems and drives are separately powered, no bus power is required, and since control is not done at this level, peer-to- H1 Fieldbus AS-i/DP/DeviceNet peer communication is not required. Modbus/RTU and PROFIBUS-DP are based on RS485 and are used at this level where long wire distance can be achieved by adjusting to lower speed. Digital automation system architecture. 42 industrial automation asia | April 2007 www.iaasiaonline.com SOFTWARE & NETWORKS • Identification & information • Diagnostics, performance analysis , Emerson & operational statistics • Parameterisation, ranging, recon- ciliation, & audit trail • Simulation and override • Calibration trim and log • Document access • Device event capture & monitoring • Commissioning • Maintenance log & service notes • Device listing • Maintenance & calibration scheduling HART was designed specifically for use in process control instrumentation. Buses And Organisations a handheld communicator is available Device management software allows A wide range of industry consortia were to work on HART devices in the field. diagnostics and performance analysis to be formed to manage the development of HART is a hybrid, superimposing easily carried out. different open fieldbus protocols, ensure digital communications on top of interoperability between products from 4-20 mA signals, and is usually used in a Without a bus supporting acyclic different vendors, and promote their point-to-point scheme, not multi-drop. communication, these features are use in the different industries through not possible. FOUNDATION fieldbus, marketing and training. The members AS-International: AS-Interface PROFIBUS-DPv1, and DeviceNet provide are typically a mix of manufacturers, The actuator sensor interface technology acyclic access to device information. endusers, engineering companies, as is managed by the AS-International HART is typically purely acyclic com- well as academic institutions. By now