Communication Protocols in DCS

 Communication Protocols in DCS

In Distributed control systems there are multiple communication protocols are used based on instruments, controller and environmental Zone. So here we will see some of very important communication protocols which are widely used to establish DCS network.

So these are the communication protocols used in Industrial networking.

•         Profinet/ Industrial Ethernet

•         Profibus

•         AS-I

•         Modbus

•         HARTc

•         CAN

•          Fieldbus

So now we will see basic overview of every protocols and its features.

1Profinet /Industrial Ethernet :- PROFINET IO uses traditional Ethernet hardware and software to define a network that structures the task of exchanging data, alarms and diagnostics with Programmable Controllers and other automation controllers. PROFINET is communicating fastest up to 100MBPS speed.

Figure 1. Profinet/Industrial Ethernet Cable

Industrial Ethernet is just as it sounds – Ethernet applied to an industrial setting, which often requires more rugged connectors, cables and – most importantly – better determinism. Industrial Ethernet uses specialized protocols in conjunction with Ethernet. The more popular Industrial Ethernet protocols are PROFINET, EtherNet/IP, EtherCAT, SERCOS III and POWERLINK. With Industrial Ethernet, data transmission rates range from10 Mbits/s to 1GBPS. However, 100 Mbit/s is the most popular speed used in Industrial Ethernet applications.

2 Profibus :-  The PROFIBUS (Process Field Bus) is qualified for the networking of complex devices with its multi master protocol . The PROFIBUS is named after DIN 19245, where it extends its user range from the field level to the process control level. In principle it is applicable with its protocol profile PROFIBUS-DP (Distributed I/O) down to the sensor/actor level.

 PROFIBUS has two type protocols

Profibus DP (Decentralize Peripherals) 9.6Kbps to 12 Mbps Speed

Profibas PA (Process Automation) 32 Kbps

Figure 2. Profibus DP cable

3 AS-I : The AS-I (Actuator Sensor Interface) is matched to the requirements in the lowest level . AS-I operates actors and sensors with the first control level and replaces them with cable harnesses, distributor cabinets and connecting terminal plates.  Since then, the AS-I is an open standard.  In the meantime, many manufacturers offer intelligent, AS-I compatible actors and sensors in order to be able to transfer more information than only 1/0.

AS-I is especially easy in data manipulation. Field devices are simply clamped into cut terminal technology on an unprotected 2 way conductive flat cable.  As a result, the installation can then be accomplished by people without any expertise.

4 Modbus :- MODBUS is a Master/Slave   communications protocol . The protocol provides for one master device and up to 247 slave devices on a common line. Each device is assigned an address to distinguish it from all other connected devices.

The MODBUS protocol comes in 2 versions

·         ASCII transmission mod- Each eight-bit byte in a message is sent as 2 ASCII characters.

·         RTU transmission mode - Each eight-bit byte in a message is sent as two four-bit hexadecimal characters

Figure 3. Modbus Cable

The main advantage of the RTU mode is that it achieves higher throughput. ASCII mode allows time intervals of up to 1 second to occur between characters without causing an error.

      HART :- Highway Addressable Remote Transducer. HART is a hybrid communications technology in which a modulated, two-way digital signal is imposed on the industry-standard 4–20 mA analog signal carrying the primary process variable.

Figure 4. HART Modem Cable

The digital signal conveys additional process variables, device status and diagnostics information that can be routed to asset management, process control and safety systems. This means that HART provides two simultaneous communication channels on the same wire–the industry standard 4–20 mA channel for fast, reliable and robust control (PV) and a digital channel for real-time communication of additional process/device information.  HART (Hiway Addressable Remote Transducer) protocol enhances these operations by transmitting digital data along with the 4-20 ma signal – without interfering with it.

  CAN :- The CAN bus system (Controller Area Network) was primarily developed by Bosch in cooperation with Intel, in order to reduce cable harnesses in automobile building. CAN is therefore very well suited for the networking of intelligent sensors and actors within machines. In the CAN physical layer uses differential transmission on a twisted pair wire.

Figure 5. CAN Cable

7Field Bus :- Field Bus is a bi-directional digital communication that interconnects smart field devices to  control system or to instrument located in the control room.  Field Bus is based on the OSI (Open System Interconnect), which was developed by the ISO (International Standard Organization) to represent the various functions required in any Communication network.

It was discovered by Indian Engineer Mr. Ram Ramchandran (M.S in Com Tech- Texas) The OSI model consists of seven layers. However for real time application layers 3 to 6 are not considered since they deal with transference of data among networks. For such application following layers are used:

·         LAYER 1 - PHYSICAL LAYER- Defines the type of signal, transmitting medium, data transmission speed, etc.

·         LAYER 2 – DATALINK LAYER- Define the interface between the physical layer and the application layer. It establishes  how the messages shall be structured and normalizes the use of multiple masters.

·         LAYER 3 – APPLICATION LAYER- Defines how data is specified, its addresses and its representation

Figure 6. Fieldbus Cable

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