Combining Ethernet and fieldbus

A number of different standards have developed in field communication in recent years, while at the office level Ethernet with TCP/IP has established its dominance. Martin Muller and Hermann Mohlenbein from Phoenix Contact consider whether Ethernet could also take on the tasks of fieldbus systems, enabling users to have only one communication system company-wide the main arguments which have so far opposed the use of Ethernet in applications around industrial processes are the non-deterministic bus access pattern, the higher connection costs as compared to fieldbus systems and the more complex installation. There are, however, also protocol-related drawbacks which become evident, particularly in cyclical operations.

For example, it is the case with the message-oriented protocols such as Ethernet with TCP/IP or some fieldbus systems that each device has to be addressed with its own message. If, for example, Ethernet TCP/IP were used to connect an individual sensor to a control system, then in order to transmit the ON/OFF status, at least 74 bytes of overhead information of the TCP/IP protocol would have to be transmitted over the medium used.

With a system with, for example, 32 field devices connected to it, this corresponds to an efficiency of less than 1%. In other words, less than 1% of the information transmitted is useful information. By comparison, in the total-frame protocol, with which the Interbus fieldbus system from Phoenix Contact operates, there is an efficiency of more than 50%. In the use of communications systems in automation applications, the cycle time, above all, plays a decisive role. This is the time needed to read in or read out the process data of all connected devices. It particularly includes the transmission time over the communication medium as well as software run-times in the individual devices.

Cycle time

While the efficiency level can still be compensated for by a faster transmission speed - 100Mbit/s with Ethernet as against 2Mbit/s with Interbus - the software run-time when a large number of devices are connected has a considerable impact on the cycle time. With a typical run-time of 1.5ms per device, in a 100Mbit/s Ethernet network comprising 256 devices each with 8 bits of I/O information, only a few milliseconds are used for the transmission of data. Almost 99% of the cycle time is used up by the software run-time.

The effectiveness of the transmission protocols show clearly that there is no point in forcing Ethernet into the role of a new and superior fieldbus. On the contrary, what is needed is to combine the complementary characteristics of Ethernet and fieldbus technologies into a universal communication system.

The advantages of Interbus in field wiring also become evident at the commissioning and system operation stages. An Interbus system is put into operation without adjustments to the participating devices or programming of their software. What is important here are simple controls on the installation and robust components. System-related basic training is not required, nor are specialists such as programmers or electrical engineers needed to operate the installation.

Fault detection

When a system is running and numerous devices are interlinked in the field, a system-specific diagnostic feature is needed in order to react quickly to faults. Maintenance personnel can ensure high installation availability only if sporadic voltage dips or intermittent contacts resulting from defective cables or plug connectors are detected and the precise location of the fault is identified. Here, Interbus offers significantly more than is provided for office communication systems.

In the future, many devices requiring low numbers of bits of information will be the most significant components in an installation, and will thus dominate the process of automation. Modular installation systems with digital terminals for 2, 4, 8 or 16 channels will allow low-cost installation. Self-creating buses are an essential component of the automation terminals for Interbus.

At the technical level, such systems with potential loops and vibrator contacts with 100Mbaud, for example for the use of Ethernet, have not yet been explored. The Interbus protocol, with its transmission and diagnostic characteristics, is designed precisely for the interlinking of large numbers of simple devices each requiring small amounts of information. In less than 4ms, 4096 input and output signals are transmitted. At the same time, the transmission rate of 500KBaud does not require any special installation. With an increase to 2MBaud, cycle times of 1ms can be achieved for the same quantities of signals. This rate of data throughput requires powerful microprocessors, as otherwise the software run-time during data carriage is of the same order of magnitude.

Building on the advantageous features of Interbus, this technology also forms a basis for further development stages. Higher transmission rates for larger and faster systems and Interbus Safety for transmission of safety-related data are examples of enhancements coming in the short term. For optimum matching of the communications structures, the transmission of larger data blocks will be performed using the TCP/IP protocol. Essential features of the fieldbus technology and of the Ethernet technology will be retained, and the implementation of 'web-ready' sensors/actuators is possible without any break in the technological approach. TCP/IP protocols will be transferred transparently through Interbus to intelligent field devices.

Seamless communication from the sensor right through to the management can be implemented simply using Ethernet and Interbus. Ethernet is the network which ties parts of the installation and the components of the machines, control systems and intelligent technology experts all together, creating the link between office and factory. Interbus is the installation system in the switch cabinet and the field, used to implement rapid machinery control systems and for synchronisation of drives. On both systems TCP/IP will be used as the communications protocol, and using standards such as FTP or HTTP it will be possible to communicate seamlessly with all intelligent field devices.

• Phoenix Contact
  Email c175@industrialnetworking.co.uk