Everything on one bus

Dipl. Ing. Claude Hestroffer

Over the years, user demands in specific industry sectors as well as automation suppliers with strategic presence in these sectors have developed several different fieldbus technology solutions, each with its own characteristics and limits of applicability. In spite of the differences in these fieldbus technologies, the IEC has compiled them in IEC 61158 and IEC 61784-1. Profibus, however, is unique among this group since all sectors of the manufacturing and process industries and all applications from inbound to quality check have been taken into account from the very beginning of its development. This allows for its universal use in all kinds of automation applications and it is for this reason that the frequently emphasized distinction between factory automation and process automation has little relevance for Profibus.

Profibus enjoys this unique position thanks to its modular design which combines uniform communication technology with the relevant application profiles and appropriately adapted physical transmission media to form integrated solutions. This universality, combined with modern technologies and capable manufacturers, has made Profibus, with over 10 million devices installed to date, the clear market leader in industrial communciation.

The universal appeal of Profibus is particularly evident in the hybrid production plants that are common in the process industry. In addition to the main continuous or batch manufacturing process, ancillary Inbound and Outbound logistics processes are almost always present and need automation. Examples of typical Inbound logi-stics processes are supply, raw materials warehousing or proportioning systems while Outbound logistics processes are product weighing systems and packaging machines. In numbers, these discrete applications are often in the majority. In addition, it is also necessary to consider hazardous areas with a demand for intrinsic safety as well as safety-relevant applications. And the variety of devices used includes everything from a simple switch or conventional 4…20 mA device to complex process devices such as drives and frequency converters to safety equipment such as light arrays or scanners.

This market segment, the hybrid industry, is fully covered by Profibus: Profibus DP serves as uniform high-speed backbone with a transmission rate of up to 12 Mbit/s to which all devices respectively device types, from inbound logistics over the main process to outbound logistics, are connected. The intrinsically safe field devices are combined into segments, and in these segments they fulfill the special demands of process engineering for an intrinsically safe power supply over the bus and hot swapping. This adapted physical transmission medium and the PA Devices application profile, which ensures that the process devices all show the same performance characteristics, is referred to as Profibus PA (Process Automation). Couplers with a transmission speed of 31.5 Kbit/s, which handle the physical conversion of Profibus DP to the intrinsically safe MBP (Manchester Coded, Bus Powered) interface, which is suitable for powering devices over the bus, are used for the connection to Profibus DP. The communications protocol remains unchanged. The transition from Profibus DP to Profibus PA is thus seamless, and there is neither a system break nor a protocol conversion, as would be required by other fieldbus systems.

Intrinsically safe devices are an indispensable prerequisite for plant operation in electrically hazardous explosive zones, and Profibus supports these in two different ways: With the MBP-IS interface and the combination of Profibus DP and Profibus PA, the solution is available in two-wire technology to the IEC standard. Particularly when remote I/O stations are involved, however, interest is also being shown toward using the RS485 interface in hazardous areas because of its high data rate. Profibus has specified the RS485-IS version with EEx i protection, which also allows easy replacement.

Significant savings in the time the user needs for planning and implementing intrinsically safe segments are made pos-sible thanks to models with definitions of device data, the observance of which guarantees the intrinsic safety of an installation (in its initial configuration as well as in the event of later modifications) without expensive, time-consuming calculations and approval procedures. For the MBP-IS it is the Fisco model, which is based on the assumption, as far as intrinsic safety is concerned, that there is only one active source (the supply unit) per segment. For RS485-IS, the corresponding marginal conditions are stipulated in Profibus International’s Guideline 2.262. In contrast to Fisco, all bus nodes represent active sources in this guideline.

Profibus makes the implementation of intrinsically safe fieldbus segments easy and time-efficient for the user. The choice between two interfaces provides flexibility in problem-solving, and the definitions to Fisco in the IEC standard resp. in Profibus Guideline 2.262 save costs and make everything from the planning to the commissioning of intrinsically safe installations much easier.

A fundamental dilemma exists between the current limit required for intrinsic safety and the user’s wish to have the largest possible number of devices in a segment. In Profibus, this dilemma is resolved using the fieldbarrier technique. This makes use of a hybrid concept that is somewhere between increased safety and intrinsic safety and which takes into consideration the fact that intrinsic safety in a plant serves a practical purpose only where modifications need to be made or maintenance done while the plant is in operation. The fieldbarrier installed in the field is therefore supplied with a high supply current of e.g. 400 mA in EEx e, which is then distributed by the barrier to the EEx i connection cables for field devices with e.g. 40 mA. The use of several such barriers on the same supply device makes it possible to use the maximum number of devices allowed per segment.

Along with the Simatic PCS7 process control system, Siemens also offers a complete product environment of fieldbus devices for process automation for specific applications. These components are seamlessly integrated into PCS7 and are based on the Totally Integrated Automation platform. The Simatic ET 200iSP intrinsically-safe and modular distributed I/O system, for example, is a remote I/O that is connected direct to Profibus DP. It is especially flexible, simple to engineer, install and wire, and can also be operated redundantly. With ET 200iSP, existing investments can be secured because Hart devices can be seamlessly integrated into Profibus. Sitrans F, on the other hand, represents a multi-variable process instrument that can cover the functionality of three conventional 4…20 mA encoders. Actual throughput, total throughput (forward, back, reset) and temperature are measured in a single device and transferred to the control system via Profibus PA.

In the past, safety-relevant solutions were hardware-based, with separate wiring and special components on a second level or interfaced via special buses, a costly and time-consuming solution. Profibus signalled a decisive turning point with the integration of safety functions in the devices themselves. The software-based Profisafe application profile is also implemented in the otherwise unchanged standard devices, thus enabling the operation of standard and fail-safe applications up to SIL 3 on one and the same bus; a separate safety bus is not longer necessary. It is easier to install and can be more easily modified, expanded and documented. Profisafe is open for Profibus DP and Profibus PA.

Innovations in the process industry are often incorporated in the modernization of existing installations stage by stage. The reasons for this are the long service life of an industrial plant and the size of the investment involved in its creation, which normally rule out a complete overhaul in the short-term. Don’t discard the proven, introduce the new with care, an adage of plant operators which Profibus has taken to heart. The large majority of industrial plants are still equipped with 4…20 mA devices, including Hart functionality. The development of remote I/O technology as well as, and above all, of the “HART on Profibus“ and “RIO for PA“ Profibus application profiles have created the prerequisites for using these devices in Profibus systems and thus for introducing fieldbus technology without endangering the investment which has already been made. User benefits consist of the cost-optimized, step-by-step modernization or expansion of existing plants with fieldbus technology. Plant sections that are still operational will remain unchanged while new plant sections are equipped with fieldbus technology. No system barriers will arise as a result.

Profibus in the process industry is only one chapter, although a very successful chapter, in the Profibus success story. Just as Profibus, with a total of 10 million installed nodes to date, is today’s world leader among fieldbus systems, this is also the case in the process industry, where 1.6 million nodes represent the sum of all process applications with Profibus PA as a subset for potentially explosive areas. And rightly so because the wide range of user benefits resulting from Profibus integration, technology and innovative power is so impressive.

Hall 9, Booth A72

cpp 432

Siemens Process Automation Portal

Profibus in the overview