Siemens ST PCS 7 manual Profibus PA

© Siemens AG 2007

Communication

PROFIBUS PA

Introduction

■Overview

Direct interfacing of the devices in the field, especially in the hazardous area, together with the information content of the communication, are of significant importance in the process in- dustry. PROFIBUS PA, which permits both digital data transmis- sion and the power supply on a two-wire line with the intrinsi- cally-safe MBP transmission technology (Manchester Coded; Bus Powered) is tailored to these requirements. It is optimally suitable for direct integration of the pneumatic actuators, sole- noid valves and sensors positioned in operating environments up to Ex zone 1 or 0 into the process control system.

The typical response time of a transmitter of approx. 10 ms indi- cates that short cycle times can be achieved with the PROFIBUS PA even in the case of a segment configuration with up to 31 devices. Practically all typical applications of the pro- cess industry can be implemented - both with small or large plant distances. Bidirectional communication and high informa- tion content allow enhanced diagnostics for fast and exact fault detection and elimination. The standardized communications services guarantee interoperability and replaceability between multi-vendor field devices and remote parameterization of the field devices during operation.

The PROFIsafe profile allows seamless integration of safety communication into the PROFIBUS PA. You need not configure

aseparate safety bus for your safety-related applications. The PROFIBUS PA with the PROFIsafe profile is incorporated in "Safety Integrated for Process Automation". This comprehensive range of products and services from Siemens for failsafe, fault- tolerant applications in the process industry offers you attractive and cost-effective alternatives to separate safety systems.

You can define the degree of redundancy separately for the con- troller, fieldbus and I/O levels of your plant depending on the au- tomation task and the derived safety requirements, and match them to the field instrumentation (Flexible Modular Redundancy, FMR). You can find an overview of the redundant architectures of PROFIBUS PA under "Design".

The PROFIBUS PA is based on electrical transmission compo- nents. A shielded two-wire cable is used for digital data trans- mission and for the power supply. With line, tree and ring topol- ogies, it is possible to configure networks with spur lines up to 120 m and bus segments up to approx. 1.9 km for operating en- vironments up to Ex zone 2. With bus segments for hazardous areas in line with zone 1, the max. possible length per spur line is reduced to 30 m and per bus segment to 1 km. The line lengths actually achievable in practice depend on the number of PA devices and their current consumption. Bus segments are terminated either automatically (with active field distributors AFD in the case of ring topologies) or with the passive terminating el- ement for PROFIBUS PA (SpliTConnect terminator).

The DP/PA Link is preferred for the gateway from PROFIBUS PA to PROFIBUS DP. When using the DP/PA Link, the transmission rate on the PROFIBUS DP is independent of the subordinate PROFIBUS PA segments. The configuration of the DP/PA Link depends on the operating environment (Ex zone) or the selected redundancy architecture. The types of coupler described in the Section "Gateways - DP/PA link and DP/PA coupler" are used for the configuration. With a small amount of data (small quantity framework) and low timing requirements, the DP/PA coupler can also be operated in stand-alone mode as a gateway.

■Benefits

Advantages provided by distributed field automation with appli-

cation of the PROFIBUS PA profile included low hardware over- 9 head, cost-effective engineering, increased operational safety

and problem-free maintenance. These advantages are under- lined by the following features:

•Modularity and uniformity from the sensor up to the control level permit new plant concepts

•Implementation of intrinsically-safe applications through use of the fieldbus in hazardous areas

•Flexible Modular Redundancy (FMR) from the automation sys- tem (controller) down to the PROFIBUS PA field device with re- dundant architectures for environments up to Ex zone 2 (ring and line topologies with coupler redundancy)

•Safety-related and fault-tolerant applications with low device and cabling overheads

•Reduced configuration costs through simple, central engi- neering of the field devices (PROFIBUS PA and HART with SIMATIC PDM, also cross-vendor)

•Simple installation using two wire cable for common power supply and data transmission

•Reduced commissioning costs through simplified loop check

•Low servicing costs thanks to simple wiring and comprehen- sive diagnostics facilities

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