Practical TCP/IP and Ethernet Networking- P52 pot

Process automation 237 architectures for Internet and ATM, and is easy to troubleshoot with tools such as TELNET. Considering all the advantages of TCP/IP, the overhead of 40 bytes per packet is a small price to pay. 15.3.2 Application layer data structures An ideal solution for the implementation of the application layer is the MODBUS, a vendor neutral data representation protocol. MODBUS is referred to as ‘every vendor’s second choice but every integrator’s first choice’. Reasons for this include the fact that the specification is open and published and that the minimal implementation involves only two messages. It is easy to adapt existing serial interface software for MODBUS and also very easy to perform automatic protocol translation. Although most implementations of the MODBUS protocol are used on low-speed point-to-point serial links or twisted pair multidrop networks, it has been adapted successfully for radio, microwave, public switch telephone, infrared, and almost any other communication mechanism conceivable. Despite the simplicity of MODBUS, it performs its intended function extremely well. 15.3.3 Embedded web servers Once all computers and control devices are connected via a seamless Internet-compatible network, it becomes possible to use web servers to make plant information available to operators. This can be done in two different ways. Firstly, a control device can incorporate its own local web server. This means that information, accessible only to that device, can be reported in a legible form as a set of web pages, and therefore displayed on any computer on the intranet, extranet, or Internet by means of a web browser. Alternatively, a general-purpose computer can act as a web server, and gather data for individual web page requests by generating the native MODBUS requests used by the control devices. These requests can then be sent out over the MODBUS/TCP network, and will interrogate either the control devices directly (if they have TCP/IP interfaces) or via simple protocol converters (such as a MODBUS gateway) to convert requests into a form that legacy equipment would understand. In both cases, there are two specific obstacles, namely that of reconfiguring a computer that has replaced a defective one, and maintaining the data directory. In order to solve the problem of reconfiguring a computer after replacing a defective one, a network computer can be used as a web server, which means that the latter would be self-configuring on installation. A network computer installs itself from a server elsewhere on the network when it is powered up. This means that if such a computer were ever to fail, a new computer could be installed in its place, powered up, and it would immediately take on the same identity as its predecessor. Another problem is how to present and maintain the directory, which stores and maintains the attributes of all data items. Despite a variety of proprietary solutions to this problem, there is an emerging standard called LDAP (lightweight directory access protocol), which was originally intended for keeping a registry of e-mail addresses for an organization. Under this scheme, LDAP maintains a hierarchical ‘picture’ of plant points within machines, machines within locations, and areas within an organization. LDAP makes it easy to reorganize the directory if the organization of the physical machines and data points need to be modified. 238 Practical TCP/IP and Ethernet Networking Each plant point could have attributes such as: • Tag name • Data type, scale, input limits, units • Reference number, size, orientation • Physical machine name In addition to this, each physical machine has attributes such as: • Network address • Node number This concept of enabling a device with a web server, and then controlling/supervising it with a browser, was dealt with in more detail in Chapter 9. 15.3.4 Routers and switches The advantage of using Ethernet for the enterprise, device and ‘fieldbus’ networks, is that these levels can be interlinked by means of standard Ethernet compatible products such as routers and switches. Switches The use of switching hubs is the key to high performance coupling between the different plant network layers since it becomes easy to intermix stations of different speeds. Inserting switches between sub-networks requires no change to hardware or software and effectively isolates the traffic on the two network layers joined by it (i.e. the traffic on the subnets connected via the switch does not ‘leak’ across the switch). In order to preserve bandwidth, it is imperative not to use broadcast techniques. Routers In terms of inter-layer connection, routers can augment the speed adaptation function by being deployed in series with a switching hub. A throttling router, connected in series with the switch, can impose delays in order to achieve flow control in a situation where the destination network cannot cope with the data flow. 15.3.5 Network access Ethernet is becoming the de facto standard for the implementation of the network access layer because of its scalability and low cost. The following paragraphs will briefly deal with the factors that until recently have been Ethernet shortcomings namely throughput, determinism and redundancy. Throughput concerns The entry-level Ethernet standard is10BaseT (IEEE 802.3) but this can be upgraded with little effort to 100BaseT (IEEE 802.3u) and even 1000BaseT (IEEE 802.3z) providing the network wiring has been done with CAT 5 UTP as per specifications. It is therefore one of the fastest network standards today. Determinism (response time) Until recently, it has been argued that Ethernet does not possess sufficient determinism. This problem had been solved by IEEE 802.1p – ‘traffic class expediting’ or ‘message prioritization’. This specification addresses the need to deliver time critical messages in a deterministic fashion. Initially designed for multimedia applications, it directly impacts Process automation 239 Ethernet as a control network by allowing system designers to prioritize messages, guaranteeing the delivery of time critical data with deterministic response times. This ability has been used by companies such as HOST engineering and think & do software to produce an Ethernet bus that can provide deterministic scan time in the 2 to 3 millisecond range for one I/O rack with 128 points. Redundancy The IEEE 802.12d standard provides the ability to add redundant links to a network device. This facilitates automatic recovery of network connectivity when there is a link or repeater failure anywhere in the network path. This standard obviates the need for custom solutions when redundancy is required as part of the control solution. 15.3.6 Thin servers Universal thin servers A universal thin server is an appliance that network-enables any serial device such as a printer or weighbridge, which has an RS-232 port. In addition to the operating system and protocol independence of general thin servers, a universal thin server is application independent by virtue of its ability to network any serial device. The universal thin server is a product developed primarily for environments in which machinery, instruments, sensors and other discrete ‘devices’ generate data that was previously inaccessible through enterprise networks. They allow nearly any device to be connected, managed and controlled over a network or the Internet. Thin server applications One of the pioneers in the field of universal thin servers is the US-based company Lantronix, that manufactures the MSS family of thin servers. In general, thin servers can be used for data acquisition, factory floor automation, security systems, scanning devices and medical devices. One of the more unusual thin server applications regulates cattle feed in stock yards. Cattle wear radio frequency ID tags in their ears that relay data over a TCP/IP network as they step on to a scale. By the time the cattle put their heads in a trough to eat, the system has distributed the proper mix of feed. Thin servers control video cameras used by the California Department of Transportation to monitor highway traffic. The US Border Patrol uses similar cameras to spot illegal border crossings. Food processing companies use the technology to track inventory in a warehouse, or the weight of consumable items rolling off an assembly line. Here is another list of devices, which are being connected to Ethernet LANs via thin servers: • Blood analyzers • LAN security devices • PBX accounting systems • Card readers in debit systems • Remote power management controllers • Telecommunications equipment • Displays in call centers • Security alarms • Time and attendance clocks and terminals • Badge access control 240 Practical TCP/IP and Ethernet Networking • Customer traffic measurement • UPS management devices • High-end fax machines • Electronic key systems • Radiation equipment • Marine equipment aboard ships • Video cameras for ATM surveillance • Vending machines • Data loggers • Static control boards • Postal equipment • CNC machines in machine shops • Electronic signboards • Temperature monitoring devices • Chemical and gas chromatography instrumentation • Oil rig monitors • Satellite receivers • Serial devices on wireless station adapters • ATM machines on cruise ships • Warehouse inventory tracking devices • Unix workstations console port • Refrigeration and heating controls • Bar code scanners • Heart monitors • Electronic maps • Power meter measurement devices • Oil and gas automation • Battery monitors • Robotic controls • Chemical monitors in pools • Modems (character-mode) • Weather stations • Rocket launch pad telemetry equipment 15.3.7 Network capable application processors (NCAPs) With the recently approved IEEE 1451.2 network independent standard, sensors and actuators can be easily interfaced onto control networks. By allowing the sensor to communicate directly on the network, complete distributed control can be achieved. This IEEE specification will act as a catalyst to sensor manufacturers who otherwise would have to support multiple protocols, hereby driving their costs up. The IEEE 1451.2 activity is only one-half of the overall IEEE 1451 activity. IEEE 1451 is actually composed of two components, each of which is managed by its own working group. P1451.1 targets the interface between the smart device and the network, while the 1451.2 focuses on the interface between the sensor/transducer and the on-board microprocessor within the smart device. IEEE 1451.1 defines a ‘Network Capable Application Processor (NCAP) Information Model’ that allows smart sensors and actuators to interface to many networks including Process automation 241 Ethernet. The standard strives to achieve this goal by means of a common network object model and use of a standard API, but it specifically does not define device algorithms or message content. IEEE 1451.2 is concerned with ‘Transducer To Microprocessor Communication Protocols and Transducer Electronic Data Sheet Formats’. This standard provides an interface that sensor and actuator suppliers can use to connect transducers to microprocessors within their smart device without worrying about what kind of microprocessor is on-board. A second part of the P1451.2 activity is the specification of electronic data sheets and their formats. These electronic data sheets, which amount to physically placing the device descriptions inside of the smart sensor, provide a standard means for describing smart devices to other systems. These transducer electronic data sheets, dubbed TEDS, also allow for self-identification of the device on the network. One of the early adopters of this technology is Hewlett Packard. Using the standard and combining it with microprocessor-based technology with embedded Java software, HP’s Ventera product line allows for seamless integration of any compatible IEEE 1451.2 sensors directly onto Ethernet. Using standard Web browser technology, users can obtain or modify sensor information by communicating with the sensor as if it were an URL address on the Web. 15.3.8 Ethernet compatible PLCs There are several models available, one of the more popular ones being the series manufactured by KOYO in China, and marketed by companies such as Siemens, PLC Direct and Allen-Bradley under their own brand names. An example of such a PLC is Allen-Bradley’s PLC-5 Ethernet compatible PLC. This is a modular PLC, which accepts either a 10BaseT or a 10BaseF (fiber) communications module. The fiber option enables the PLC to operate in very (electrically) noisy environments, yet still retain their Ethernet connectivity. The PLC-5 processors have TCP/IP and SNMP (simple network management protocol) built in, which enables them to be managed via the network using commercially available network management software. 15.3.9 Ethernet compatible SCADA systems One of the industry’s first Java-based SCADA systems is WIZNET, manufactured by Conlab. WIZNET allows a PLC to be integrated with the Plant Intranet (typically at the device layer level) and includes a web server. This allows operators and managers to monitor and control the plant through a standard Web browser, and view both factory data and corporate information through a common interface and from any desktop or mobile computer. The web server provides security by allowing user access according to IP address or via selected web pages only. 15.4 References 15.4.1 Automation trends ARC (Automation Research Corporation): http://www.arc.com . Displays in call centers • Security alarms • Time and attendance clocks and terminals • Badge access control 240 Practical TCP/IP and Ethernet Networking • Customer traffic measurement •. device layer level) and includes a web server. This allows operators and managers to monitor and control the plant through a standard Web browser, and view both factory data and corporate information. sensors and actuators to interface to many networks including Process automation 241 Ethernet. The standard strives to achieve this goal by means of a common network object model and use of a standard