  
 
Reprinted from MACHINE DESIGN October 7,1999 FIELDBUS Technology
With Interbus Low-speed network boasts high efficiency Staff Report To meet future challenges of the seamless information flow and open and flexible automation systems, many experts agree that bus systems should provide the user with a universal solution to automation tasks. And for applications in open-control architectures, it must feature independence and neutrality toward major PLC suppliers. Today, according to the Interbus Club, most networks fail to meet these two requirements. That's because for the most part, each system on the market is only supported by one of the major PLC manufacturers - for example, Profibus by Siemens, DeviceNet by Rockwell Automation, and FIP by Telemecanique. Interbus is not sponsored by a major PLC manufacturer but, rather, by an independent network supplier as well as more than 1000 device manufacturers. According to proponents, this bus system is particularly suitable for use with industrial sensors and actuators. The attraction as a control standard is twofold. First, multiple vendors offer products adhering to the standard, and these range from simple I/O to intelligent motion-control modules and operator terminals. Second, Interbus can be more reliable and efficient than alternative approaches, say its adherents. Though they run at a relatively low 500 kbps, Interbus networks provide greater data throughput than some that use a higher clock rate. This lower speed lessens their susceptibility to electrical noise. At first glance, a low data-transmission rate seems at odds with the need for short transmission times. But, according to Interbus supporters, a high data-transmission speed, say 12 Mbps, is not necessarily advantageous. It is not the transmission speed that is decisive, they say, but the amount of data transmitted, also called protocol efficiency. Interbus is a total summation frame protocol that is known for being completely synchronous and deterministic, effective over networks encompassing distances of up to eight miles. All data from sensors and actuators are summarized in one message. The message periodically goes to all sensors and actuators simultaneously. Thus there is message overhead only once per cycle, resulting in a high message efficiency compared to multimaster or peer-to-peer protocols. Moreover, faults in Interbus networks are easily diagnosed, particularly during installation, say vendors. If an Interbus network cable is accidentally damaged, the affected modules send back a message to the master controller indicating the exact location of the problem, based on the transmission errors. Users can go directly to the segment between two modules and fix the problem. And that applies even for the intermittent problems of electrical noise. Standard diagnostic software can time stamp incidents of trouble. Personnel can then correlate this with some event in the plant such as the switching on of a welding station. This deterministic, diagnostic process contrasts sharply with the procedure for finding shorts or opens in multidrop networks. The line impedances of other multidrop networks tend to vary widely, so technicians typically are forced to do a lot of trial-and-error checking with expensive line analyzers, according to users. There has, however, been a major misconception about Interbus: That one bad module could take down the entire Interbus network. This arises because all nodes handle all messages in the total summation frame protocol as used by Interbus. However, vendors have addressed this difficulty with controllers that can subsegment a network and keep it working if any given module goes off-line. When a remote bus node dies or bus lines become open or shorted, nodes physically before and after the fault automatically reconfigure the network and continue to operate. Also not widely known is that bus master modules are available from vendors in addition to Phoenix Contact, the original developer of the Interbus protocol. That firm now has a daughtercard that lets any suitably equipped controller provide Interbus master functions. Newly approved as a European standard (EN 50254), Interbus will be able to run TCP/IP services as available over Ethernet networks. The new standard serves as the backbone for open-system connections to industrial I/O and motion-control axes. Each Interbus network uses a bus master module that cyclically polls all stations on the network. The bus master can take several forms. For example, it frequently exists as a plug-in ISA board within an industrial PC, or as a daughterboard. It can also be had as a PCMCIA card for interfacing to laptop computers or other suitably equipped controllers. In addition, Interbus allows for what are called remote-field controllers (RFCs). These are typically small self-contained controllers that often take the form of a DIN-rail-mounted module. RFCs make it possible to set up Interbus subnetworks which, in turn, can exchange data with high-level networks. RFCs handling such subnetworks can be implemented either as slaves or as bus masters. These remote controllers can be programmed with standard IEC 61131 languages such as ladder logic or structured text. Some also provide a TCP/IP interface for connecting Interbus networks to factory wide intranets. I/O modules install near the control and signaling units. These convert sensor and actuator signals into bus signals and vice versa. Software for developing Interbus applications handles tasks that include configuration, programming, and graphic display of status. For example, one such package called PC Worx, is an IEC 61131 edition and uses a single database to store program variables and configuration data. This common database helps simplify problem diagnosis during startup and operation
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