PLC/DCS SYSTEM DESIGN (Networking, Process Interfacing, Programming, Man Machine Interface )
The Programmable Logic Controller (PLC) was initially developed in the early 1970's for applications within the manufacturing industries, mainly the automotive industry. The PLC soon proved to be useful for implementing discrete logic. The earliest PLC's were limited to discrete I/O, basic logic functions (AND, OR, NOT), timers, and counters. However, they were soon used for analog I/O, math functions, PID control algorithms, and other process control functions. Since the initial target market was electrical, the earliest PLC's were programmed using the same ladder logic diagrams used to represent logic implemented with hard-wired relays. Alternatives to ladder logic are now available for programming PLC's, but ladder logic is the only accepted (although not always popular) means for implementing this discrete logic. Regardless of its disfavor, ladder logic is still the most efficient means of processing discrete logic, because the PLC's are able to scan it very rapidly. Consequently, for applications loaded with discrete logic, even DCS suppliers will incorporate one or more PLC's into their system.
HV&L Industries is very experienced with the programming of PLC's, especially Allen-Bradley and Modicon. We have used a variety of software programs specifically designed for use with these products, including Rockwell Software and ModSoft. In our recent Fieldbus efforts, we have had extensive work with SMAR PLC's, using their configuration software, Conf700. 		  PROFIBus PROTOCOL
Technology Overview
Communication in automation is becoming increasingly direct, horizontally at field level as well as vertically through all hierarchy levels. Depending on the application and the price, graduated, matching industrial communication systems such as the Ethernet-based PROFInet, the fieldbus PROFIBUS and other systems like the sensor/actuator bus AS-Interface offer the ideal preconditions for transparent networking in all areas and levels of the automation process.

 

At sensor/actuator level signals of the binary sensors and actuators are transmitted via a sensor/athectuator bus. Here, a particularly simple, low-cost installation technique, through which data and a 24-volt power supply for the end devices are transmitted using a common medium, is an important requirement. The data are transmitted purely cyclically. AS-Interface is a suitable bus system for this field of applications.

At field level the distributed peripherals, such as I/O modules, measuring transducers, drive units, valves and operator terminals communicate with the automation systems via an efficient, real-time communication system. The transmission of the process data is effected cyclically, while alarms, parameters and diagnostic data also have to be transmitted acyclically if necessary. PROFIBUS meets these requirements and offers a transparent solution for manufacturing as well as for process automation.

At cell level, the programmable controllers such as PLC and IPC communicate with each other. The information flow requires large data packets and a large number of powerful communication functions. Smooth integration into company-wide communication systems, such as Intranet and Internet via TCP/IP and Ethernet are important requirements.

The IT revolution in automation technology is opening up new savings potentials in the optimization of system processes and makes an important contribution towards improved use of resources. Industrial communication systems have assumed a key function in this respect.

A Detailed Typical Profibus System

PROFIBUS is a vendor-independent, open field bus standard for a wide range of applications in manufacturing and process automation. Vendor-independence and openness are ensured by the international standards EN 50170, EN 50254 and IEC 61158.PROFIBUS allows communication between devices of different manufacturers without any special interface adjustment. PROFIBUS can be used for both high-speed time critical applications and complex communication tasks. Through its continuing further technical developments, PROFIBUS is still the industrial communication system prepared for the future.

PROFIBUS offers functionally graduated communication protocols (Communication Profiles): DP and FMS. Depending on the application, the transmission technologies (Physical Profiles) RS-485, IEC 1158-2 or fiber optics are available. In the course of further technical development, the PROFIBUS User Organization is currently working on the implementation of universal concepts for vertical integration on the basis of Ethernet TCP/IP. Application Profiles define the options of protocol and transmission technology required in the respective application area for the individual device types. These profiles also define vendor-independent device behavior.

PROFIBUS defines the technical characteristics of a serial field bus system with which distributed digital programmable controllers can be networked, from field level to cell level. PROFIBUS is a multi-master system and thus allows the joint operation of several automation, engineering or visualization systems with their distributed peripherals on one bus.
What are the benefits of Fieldbus?

• The benefits of Fieldbus are many and some were already described.
• Lower cost of purchase and ownership.
• Productivity and quality increase.
• Higher integrity and accuracy.
• Access to more information and diagnosis.
• Easier installation and start-up.
• Freedom of component choice.
• Easier to configure.
• Easy for having single consistent database.

How is Fieldbus more distributed than a "Distributed Control System"?

Unlike the "semi-distributed" DCS system, the Fieldbus system is a completely distributed. In Fieldbus the process control functions are distributed to equipment in the field, while still allowing operation and tuning from the control room using the digital communication. The field devices are typically multi-dropped, several connected to each other and the operator console, drastically reducing wiring with subsequent savings in cost of purchase and installation.

In the legacy DCS of the 1970 the control functions for several loops was centralized to one or more "unit controllers" which contains control, input and output cards.

Since in Fieldbus communication is completely digital no input or output cards are required, and since the control functions are performed by the devices in the field, no control cards are required either. Field devices may be connected directly to the operator console, hence a data highway connecting control cards is not required either. Since there are no cards and no data highway, they need not be made redundant either. All that remains of the classic DCS architecture is the field devices and the operator console. Obviously a Fieldbus system means a tremendous hardware and subsequent cost reduction.