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Implementation

This chapter contains instructions on how to implement the communications protocol and the interfaces in automation/field devices.

For the device development or implementation of the PROFIBUS protocol, a broad spectrum of standard components and development tools (PROFIBUS ASICs, PROFIBUS stacks, monitor and commissioning tools) as well as services are available that enable device manufacturers to realize cost-effective development. A corresponding overview is available in the product catalog of the PROFIBUS User Organization (www.profibus.com/productguide.htm). For further details please use technical literature and, for expert advice, contact one of the PROFIBUS Competence Centers.

During implementation of a PROFIBUS interface, please note that the certification refers to the overall device. Standard components are not subject to the certification process as this does not provide a guarantee for the end product device. However the quality of the standard components also plays an important role in the successful certification of devices.

Example for the implementation of a PROFIBUS slaveFig. 33: Example for the implementation of a PROFIBUS slave


9.1 Standard Components

Interface Modules

The use of a complete PROFIBUS interface module is ideal for a low/medium number of devices. These credit card size modules implement the entire bus protocol. They are fitted on the master board of the device as an additional module.

Protocol Chips

In the case of high numbers of devices, an individual implementation on the basis of commercially available PROFIBUS basic technology components is recommendable, whereby a distinction is made between

  • Single chips, in which all protocol functions are integrated on the chip and which do not require an additional controller,
  • Communications chips, which implement smaller or larger parts of the protocol on the chip and also require an additional controller and
  • Protocol chips with integrated microcontroller. The type of implementation version largely depends on the complexity of the field device, and the performance and functionality required. The following offers some examples.

Implementation of Simple Slaves

The implementation of single-chip ASICs is ideal for simple I/O devices. All protocol functions are already integrated on the ASIC. No microprocessors or software are required. Only the bus interface driver, the quartz and the power electronics are required as external components.

Implementation of Intelligent Slaves

In this form of implementation, the essential layer-2 parts of the PROFIBUS protocol are implemented on a protocol chip and the remaining protocol parts implemented as software on a microcontroller. In most of the ASICS available on the market all cyclic protocol parts have been implemented, which are responsible for transmission of time-critical data. Alternatively, protocol chips with interpreted controllers may be used, where part of the protocol for less time-critical data transmission can be realized.

These ASICs offer a universal interface and operate together with common microcontrollers. A further option is offered by microprocessors with an integrated PROFIBUS core.

Implementation of Complex Masters

In this form of implementation, the time-critical parts of the PROFIBUS protocol are also implemented on a protocol chip and the remaining protocol parts implemented as software on a microcontroller. Various ASICs of different suppliers are currently available for the implementation of complex master devices. They can be operated in combination with many common microprocessors.

A corresponding protocol chip overview is available on the PROFIBUS website. For further information please contact the suppliers directly.

PROFIBUS Stacks

Frequently, chips and the respective protocol software (PROFIBUS stacks) may originate from different suppliers, positively increasing the variety of solutions available in the market.

Based on that, technically optimized and cost effective products can be developed, which meet application specific market requirements, in accordance with the respective commitment of the PROFIBUS User Organization. This also proves the openness and multi-vendor capability of PROFIBUS which are not limited to specifications but also comprise product implementations.

Pure software solutions are rare because of their unfavorable cost/performance relation compared with chip-oriented implementations. Therefore, they are used only for specific applications.

A corresponding overview of PROFIBUS stacks that are available on the market is presented on the PROFIBUS website. For further information please contact the respective suppliers.

 

9.2 Implementation of Interfaces

MBP transmission technology

When implementing a bus-powered field device with MBP transmission technology, particular attention must be paid to low power consumption.

As a rule, only a feed current of 10-15 mA over the bus cable is available for these devices, which must supply the overall device, including the bus interface and the measuring electronics.

Special modem chips are available to meet these requirements. These modems take the required operating energy for the overall device from the MBP bus connection and make it available as feed voltage for the other electronic components of the device. At the same time, the digital signals of the connected protocol chip are converted into the bus signal of the MBP connection modulated to the energy supply. A typical configuration with a commercially available round-board is shown in Fig. 36.

For further details on how to implement the bus connection for field devices with MBP transmission technology, please refer to the technical PNO guideline No. 2.092.

RS485 Transmission Technology

For field devices that cannot be powered over the bus, it is possible to use the standard RS485 interface. This increases flexibility when implementing the device as this can then be connected to a PROFIBUS DP segment without a coupler or link.

Key features of RS485 Technology are its low interface costs and ruggedness. Data rates of 9.6 Kbit/s to12 Mbit/s are supported without the need to implement any changes.

As a further enhancement, the RS485 IS has been developed, which offers an intrinsically safe version of the RS485.

The RS485 modules are available from various manufacturers and proven in millions of applications.

Go back to the header article PROFIBUS Technology and Application


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