Communication

Industrial Communication

First of all, communication is considered to be the exchanging of information between two or more parties. For this, the information is transferred from one party to another party, which then receives it, processes it, and stores / discards it depending on its relevance. When the industry component is added, it is possible to speak of “industrial communication”. The definition becomes considerably more difficult, when the purpose, i.e. the data transmission between devices of a system, is formulated clearly from the start. A real explanation of the meaning only becomes apparent when the functions of this communication are listed. A comparison with the previous procedure, i.e. the wiring, facilitates our initial approach into this extensive topic.

The advantage of using industrial networks, as opposed to the previous wiring, can be found quickly: Modernization makes it possible to save considerable amounts of money and to lower the costs. In addition, communication via industrial units has advantages for functionality. However, the means that is used in the field of “industrial communication” differs strongly according to the location. As such internal office communication mostly works with systems based on the Ethernet TCP/IP standard, while automation technology uses many communication systems, which are however compatible with each other. However in recent years, there has been an increase in the industrial networks that are employed, regardless of the particular field that requires rapid and unimpeded communication. Based on the expansion of the automobile industry, the components of the industrial Ethernet particularly show an upward tendency. In fields controlled by highly efficient technology, the industrial communication is found in sectors such as serial machine building. In spite of all modernization and specification, it is mainly the fieldbus which, in the field of industrial communication, represents an important link.



For example, based on information concerning the trend in 2010, there was growth of 50 % (in comparison to the values from the preceding year). The development could also be seen in industrial Ethernet as well as in fieldbuses. For networks, it was mainly ModBus-TCP, EtherCat, EtherNet/IP and PROFINET that indicated growth. For fieldbuses, PROFIBUS, CC-Link and DeviceNet indicated growth. This tendency has not ended over the past two years, even though the transition from the installed fieldbus nodes to the Ethernet that was predicted during the 90s, is taking place considerably slower than was initially expected and hoped for. As such, range of applications related to the “industrial communication” medium has still got further to go and space for expansion. After all, modernization of the networks brings additional functions with it:

  • Energy control – lowering of the operating expenses, especially in the sector of complex installations, through controls and the checking of the energy is an advantage of the industrial Ethernet and with it the “industrial communication” sector. The functions of all components of a complex automation system can be controlled via a programmable logic controller (PLC) which in turn, leads to saving in operating expenses.
  • I/O signals – the industrial communication in the form of the industrial Ethernet can transmit I/O signals in real-time. As transmission in real-time is possible, this is also defined as real-time Ethernet. One and the same cable also can transmit IT data, e.g. files, notes for websites, or e-mails and their contents. Here also, the positive effect of cost minimizing can be seen in the case of extensive installations.

The Industrial Communication as the Basis for the Cost-efficient Modern Age

The theme of industrial communication is as wide-ranging and diverse as the general term for technology that it implies. The special feature of this form of communication which is illustrated by a clear expansion especially in connection with the required modernization of many sectors of industry, is the return of tried and tested techniques in order to bring about specification and progress. In all of its aspects, industrial communication already makes use of specified standards. Through optimization, the ability is found to establish new, improved standards in the field of communication technology. Use of this standard creates a basis on which to realize powerful networks for data transmission, which can demonstrate this performance consistently. This brings the decisive advantage of cost minimization, as all devices can communicate with each other by connection of a single sensor, and as in the past individual cables are not needed to realize a work process. This also facilitates the registration of all data within a system, so that finally, industrial communication brings the gain of integrating all parts to all technological sectors of a company.

Industrial communication network

An industrial communication network is a backbone for any automation system architecture as it has been providing a powerful means of data exchange, data controllability, and flexibility to connect various devices. With the use of proprietary digital communication networks in industries over the past decade led to improve end-to-end digital signal accuracy and integrity.

These networks, which can be either LAN (Local Area Network, which is used in a limited area) or WAN (Wide Area Network which is used as global system) enabled to communicate vast amounts of data using a limited number of channels. Industrial networking also led to the implementation of various communication protocols between digital controllers, field devices, various automation related software tools and also to external systems.

As the industrial automation systems become complex and large with more number of automation devices on control floor, today, the trend is toward Open Systems Interconnection (OSI) standards that permits to interconnect and communicate any pair of automation devices reliably irrespective of the manufacturer.

With the advancements in digital technology, fieldbus technology is now ruling the automation field as it provides multidrop communication facility that results cost effective and cable saving communication. The following is an overview of some industrial communication networks that play a significant role in today’s industrial control systems.

What is an Industrial Communication Network?

Data communication refers to the transformation of information or data, mostly in digital format from a transmitter to a receiver through a link (which can be copper wire, coaxial cable, optical fiber, or any other medium) connecting these two.

Traditional communication networks are used to enable data communication between computers, computers and its peripherals and other devices. On the other hand, industrial communication network is a special type of network made to handle real-time control and data integrity in harsh environments over large installations.

The examples of industrial communication networks include Ethernet, DeviceNet, Modbus, ControlNet, and so on.

The three significant control mechanisms used in industrial automation field include Programmable Logic Controllers (PLCs), Supervisory Control and Data Acquisition (SCADA) and Distributed Control System (DCS). All these elements deals with field instruments, smart field devices, supervisory control PCs, distributed I/O controllers and HMI suits.

In order to provide an interconnection between these devices and also to enable communication in between them, a powerful and more effective communication network or scheme is needed. They differ quite significantly from traditional enterprise networks. These industrial networks form a communication path among field devices, controllers and PCs.

The transmission media for passing the data and control signals can be either wired or wireless. In case of wired transmission, a cable is used that can be a twisted pair, coaxial cable or fiber optics. Each network cable has its own electrical characteristics that may be less or more suitable to a type of network or specific environment. In case of wireless transmission, communication is performed through radio waves.

A fieldbus is another local control area network used for real-time distributed control systems in complex automated industrial systems. It is a digital two-way multidrop communication link between controllers and intelligent field devices such as smart sensors/actuators/transducers. It replaces the conventional point-to-point communication system that consists of as many wire pairs as the number of field devices.

In case of fieldbus system two wires are enough for many devices that belong to the same segment. This result, enormous cable saving thereby, cost effective. Profibus and Foundation Field Bus are the two most dominant fieldbus technologies used in process automation field.

Hierarchical Levels in Industrial Communication Networks

Earlier we have discussed the hierarchical arrangement of the automation system depending on their functionality and information flow in Industrial Automation article. In a manufacturing or process industry, the information or data flows from field level to enterprise level (bottom-to-top) and vice-versa.

Different levels have to handle different requirements of a particular level. So it is obvious that no single communication network address requirements needed by each level. Hence different levels may use different network based on the requirements such as data volume, data transmission, data security, etc. Based on the functionality, industrial communication networks are classified into three general levels which are discussed below.

Device Level:

This lowest level consists of field devices such as sensors and actuators of processes and machines. The task of this level is to transfer the information between these devices and technical process elements such as PLCs. The information transfer can be digital, analog or hybrid. The measured values may stay for longer periods or over a short period.

In order to provide field level communication, 4-20 mA current loop, serial point-to-point communication methods are widely used. These networks consist of parallel, multiwire cables as transmission medium. The common serial communication protocol standards used in this level include RS232, RS422 and RS485. There are many other field level communication networks available which are characterized different factors such as response time, message size, etc.

Nowadays, fieldbus technology is the most sophisticated communication network used in field level as it facilitates distributed control among various smart field devices and controller. This is a bidirectional communication system in which many variables are taken care by single transmission. Different types of fieldbuses include HART, ControlNet, DeviceNet, CAN Bus, Profibus, and Foundation Field Bus.

Control Level:

This level consists of industrial controllers such as PLCs, distributed control units, and computer systems. The tasks of this level include configuring automation devices, loading of program data and process variables data, adjusting set variables, supervising control, displaying variables data on HMIs, historical archiving, etc. So this level requires characteristics like short response time, high speed transmission, short data lengths, machine synchronization, constant use of critical data, etc.

Local Area Networks (LANs) are widely used as communication networks in this level to achieve desired characteristics. The Ethernet with TCP/IP protocol is mostly used as control level network to connect control units with computers. In addition, this network acts as a control bus to coordinate and synchronize between various controller units. Some fieldbuses are also used in this level as control buses such as Profibus and ControlNet.

Information Level:

This is the top level of the industrial automation system which gathers the information from its lower level i.e., control level. It deals with large volumes of data that are neither in constant use or time critical. Large scale networks are exists in this level. So Ethernet WANs are commonly used as information level networks for factory planning and management information exchange.  Sometimes these networks may connect to other industrial networks via gateways.

Commonly used Industrial Networks

There exist many different communication networks designed to interconnect industrial field devices and various I/O modules. These are described based on certain protocols. A protocol is a set of rules that are used in communication between two or more devices. Based on these protocols, communication networks are classified into many types. Some common and popular industrial communication standards are described below.

Serial Communication

Serial communication is the basic communication system provided for every controller such as a PLC. This communication is implemented by using protocol standards such as RS232, RS422 and RS485. The acronym RS stands for Recommended Standard which specifies serial communication characteristics in terms of electrical, mechanical and functional features.

Serial communication interfaces are either built into the CPU or process module (consider, for a Programmable Logic Controller) or it can be a separate communication module. These RS interfaces are mainly used to transfer the data reasonably at high data rate between a PLC and the remote device. Barcode readers, operator terminals and vision systems are the examples of these interfaces.

RS-232 serial communication is designed to support one transmitter and one receiver and hence it offers communication between one controller and one computer. The maximum cable length should be up to 50 feet. RS 422 (1Tx, 10 Rx) and RS485 (32Tx, 32 Rx) serial communication standards are designed to communicate between one computer and many controllers. These standards are limited to lengths of 1650 feet (in case of RS422) and 650 feet (in case of RS485).

HART:

It is an acronym for Highway Addressable Remote Transducer. It is an open process control network protocol that superimposes digital communication signal on the top of 4-20mA signals using the Bell 202 Frequency Shift Keying (FSK) technique.

It is the only communication network that facilitates both analog and bidirectional digital communication at the same time by the same wiring, and hence these networks are also called as hybrid networks. This digital signal is called as HART signal which carries diagnostic information, device configuration, calibration and other additional process measurements.

HART networks operate in either point-to-point or multidrop mode. In point-to-point mode, 4-20 mA current signal is used to control the process while HART signal remains unaffected. Multidrop HART networks are used when devices are widely spaced. HART-compatible multivariable smart field devices are widely used in many industries. HART communication network is mainly used in SCADA applications.

DeviceNet

It is an open-device level network based on CAN technology. It is designed to interface field level devices (such as sensors, switches, barcode readers, panel displays, etc.) with higher level controller (such as a PLC) with a unique adoption of basic CAN protocol. It can support up to 64 nodes and supporting up to 2048 total devices.

It reduces the network cost by integrating all devices on a four-wire cable that carries both data and power conductors. The power on the network allows the devices to be powered up directly from the network and hence it reduces the physical connection points. This network is popularly used in automotive and semiconductor industries.

ControlNet

It is an open control network, which uses Common Industrial Protocol (CIP) in order to combine the functionality of peer-to-peer network and an I/O network by providing high speed performance. This network is the combination of Data Highway Plus (DH+) and remote I/O. It is used for real-time data transfer of time-critical as well as non time-critical data between I/Os or processors on the same network.

It can communicate up to a maximum of 99 nodes with a data transfer rate of five million bits per second.  It was designed to be used on both device and field level of industrial automation system. It provides media and communication redundancy at all the nodes of the network.

Modbus

It is an open system protocol that can run on a variety of physical layers. It is the most widely used protocol in industrial control applications. It is a serial communication technique which provides master/slave relationship to communicate between devices connected on network. It can be implemented on any transmission medium, but most commonly used with RS232 and RS485.

Serial Modbus with RS232 or RS485 (as physical layers) facilitates the connection of Modbus devices to the controller (such as a PLC) in a bus structure. It can communicate between one master and a number of slaves up to 247 with a data transmission rate of 19.2 kbits/s.

A newer version of Modbus TCP/IP uses Ethernet as physical layer that facilitates the data exchange between PLCs in different networks. Irrespective of the type of physical network, it facilitates a method of access and control of one device by another.

Profibus

It is one of the well known and widely implemented open-field networks. These networks are mainly used in process automation and factory automation fields. It is most suitable for complex communication tasks and time-critical applications. There are three different versions of Profibus namely, Profibus-DP (Decentralized Periphery), Profibus-PA (Process Automation) and Profibus-FMS (Fieldbus Message Specification).

Profibus-DP is an open fieldbus communication standard that utilizes master/slave communication between network devices. It uses RS485 or fiber optic transmission technologies as physical layer media. It is mainly used to provide communication between controllers and distributed I/Os at device level.

Profibus-PA is specially designed for process automation. Profibus-PA networks are recommended to use in intrinsically safe areas. These networks permit sensors, actuators and controllers to connect to a single common bus, which provides data communication and power over bus. These networks use the Manchester Bus Powered (MBP) physical layer based on international standard IEC 61158-2.

Profibus-FMS is a multimaster or peer-to-peer messaging format which allows master units to communicate with one another. It is a general purpose solution that performs communication tasks in control level, especially in cell sublevel to facilitate communication between Master PCs.

Most commonly FMS and DP are used simultaneously in COMBI mode in situations where a PLC is being used in conjunction with a PC. In such case primary master communicates with secondary master via FMS while DP transfers control data on same network to I/O devices.

Foundation Field Bus

It is an open fieldbus standard designed specially to meet the mission-critical demands in intrinsically safe environments. It is a type of LAN for foundation fieldbus compatible instruments and controllers used in manufacturing and process industries. It is a two-way, digital protocol standard defined by intrinsically safe IEC 61158-2 physical layer (for FF H1) and compatible with Ethernet equipment (in case of FF HSE). The three kinds of FF networks include low speed H1, high speed H2 and High Speed Ethernet HSE.

H1 network supports a speed of 31.25 kbps. There are two speeds supported by H2 network, which are 1.0 Mbps and 2.5 Mbps. HSE network supports 10 or 100 Mbps speed as it uses Ethernet protocol.