Ethernet devices have protocol rules to obey, which are related to the needs of the Ethernet, so as to allow variety of possible devices that may be connected to an Ethernet network similar to frame relay. These are local area based but can be applied to wide area when from network to network since we have MB and Gigabit Ethernet routers which can do this type of transfer 2. Broadband is a form of network modulation in which multiple channels are formed by dividing the transmission medium such as fiberopotic cable in to discrete frequency segments This is a global standard.
For integrated, high speed broadband networks that can handle any kind of information including data, voice, and video as stated above.
Networking these computers together was desirable and beneficial. When linking these computers over a long distance, the existing voice-optimized WANs were used.
Because computers send data instead of voice and data has different characteristics and so these WANs did not send computer data very efficiently. To address these concerns, ITU-T International Telecommunication Union— Telecommunications is an agency of United Nations formally CCITT and other standards groups started work in the s to establish a series of recommendations for the transmission, switching, signaling and control techniques.
These are required to implement an intelligent fiber-based network that could solve current limitations and would allow networks to be able to efficiently carry services of the future.
These can be used for long distance distributed SCADA system transmission of data by private or public switched networks at speeds of TCP is a protocols developed in late 70s by Department of Defense for providing interoperability among several equipment vendors. Internet is a collection of independent Packet Switched Networks that are loosely interconnected to act as a Coordinated network. It is not to be confused with a server: Servers and client workstations are all IP hosts.
The hosts are connected together by one or more networks. The IP address of any host consists of its network address plus its own host address on the network. IP addressing uses one address for both network and host addresses.
How much of the address is used for the network portion and how much for the host portion varies from network to network An IP address is 32 bits wide, and it is composed of two parts: the network number, and the host number [1, 2, and 3].
By convention, it is expressed as four decimal numbers separated by periods, such as " Valid addresses thus range from 0. Leveraging the availability of the Internet and wireless technology, SCADA vendors found remote access solutions, which allow virtually every telemetry and monitoring device to be connected to the Internet and thereby to your terminal of choice.
It enables you to be connected to your network or device, without being confined to the office. Broader ranges require some form of public network. All these are being done now with the SCADA systems using off the shelf hardware and software products available on the market today.
The cost of operating this local and wide area networks is to be taken in to consideration when designing a SCADA system. For example, a typical cost of leasing a 1. But to get n equivalent leased line, you can see that it will be cost prohibitive and can not be compared on equal footing.
All these now overlap each other with respect to functionality, protocols, hardware and software. The main requirement for all these networks is, they have to operate in real time deterministic mode so that the process and production data can be received and sent in a timely manner to all required places on the local and wide area networks.
SCADA systems, whether of the latest design or systems that have been in operation for years, can usually be migrated to the high-speed networks and open non- proprietary protocols with a little bit of planning and study of available options. System designers must be familiar with issues of functionality, transmission delays, polling cycles, data processing requirements and available technologies. The two-wire connection is used to connect field instruments like limit switches, proximity switches, motor run feedback, Pump start permissive, etc.
When we need additional protection measures to identify the short circuit and open circuit of digital inputs then we use this configuration. Generally, a pair of resistors are used at the terminations of field instruments or in some cases at marshalling cabinets. This works on the principle of change in resistance.
The input cards are designed such that it will identify the signal status and differentiates these conditions, alerts the system engineers with alarms. This configuration is used when we need an additional power supply for the field instruments, motor control centers, etc.
Generally, in marshalling cabinets, we make the necessary wiring arrangements as per the application requirements. We are using the additional power supply in the marshallaling cabinet or at the control room side , so we are calling this a WET Contact. When the field instrument is in Close state then the relay will be energized thus DI card has the return signal, so it will consider as ON state. When there is some activity in the digital inputs like limit switch activation, then there will be a change in input state from open to close or close to open depends on the type of contact used in the limit switch.
The PLC reads these signals and takes the action as per its logic. This voltage is called as interrogation voltage.
We are NOT using any additional power supply in the marshallaling cabinet or at control room side , so we are calling this a DRY Contact. When the field instrument is in Close state then the relay will be energized thus DI card have the return signal, so it will consider as ON state.
This may be carried out through existing LANs, private telephone systems or existing radio systems used for mobile vehicle communications. Figure 9. These features depend on the hardware to be implemented.
The system must be easily modifiable to allow for the requirements changing and expanding as the task grows - in other words the system must use a scaleable architecture. There have been two main approaches to follow in designing the SCADA system: x Centralized, where a single computer or mainframe performs all plant monitoring and all plant data is stored on one database which resides on this computer. An effective solution is to examine the type of data required for each task and then to structure the system appropriately.
A client server approach also makes for a more effective system. Each of these tasks performs its own separate processing. This program is the interface between the control and monitoring system and the plant floor. This manages all alarms by detecting digital alarm points and comparing the values of analog alarm points to alarm thresholds.
The trends task collects data to be monitored over time. Reports are produced from plant data. These reports can be periodic, event triggered or activated by the operator. This manages all data to be monitored by the operator and all control actions requested by the operator.
System Response Times These should be carefully specified for the following events. Introduction to Protocols The transmission of information both directions between the master station and RTUs using time division multiplexing techniques requires the use of serial digital messages. These messages must be efficient, secure, flexible, and easily implemented in hardware and software.
Efficiency is defined as: Information Bits Transmitted y Total Bits Transmitted Security is the ability to detect errors in the original information transmitted, caused by noise on the communication channel. Flexibility allows different amounts and types of information to be transmitted upon command by the master station. Implementation in hardware and software requires the minimum in complicated logic, memory storage, and speed of operation.
All messages are divided into three basic parts as follows: x Message Establishment; which provides the signals to synchronize the receiver and transmitter. Message security checks consist of logical operations on the data which result in a predefined number of check bits transmitted with the message.
At the receiver the same operations are performed on the data and compared with the received check bits. If they are identical, the message is accepted; otherwise, a retransmission of the original message is requested. A typical example of commonly used asynchronous message format is shown in Figure 9. Information Transfer Master to Remote Data Transfer: Information transmitted from master to remote is for the purpose of device control, set point control, or batch data transfer.
Due to the possible severe consequences of operating the wrong device or receiving a bad control message, additional security is required for control. This is provided in the form of a sequence of messages, commonly called a select-before-operate sequence, as shown in Figure 9. Remote to Master Data Transfer: All remote to master data transfer is accomplished with one basic message sequence by using variations in the field definitions to accommodate different types of data. The basic sequence is shown in Figure 9.
HDLC is a bit based protocol. Asynchronous Balanced Mode ABM : In this mode each node has equal status and can act as either a secondary or primary node. The standard format is indicated in Figure 9. In order to ensure that the receiver always knows that the character it receives is a unique flag character rather than merely some other character in the sequence ; a procedure called zero insertion is followed. This requires the transmitter to insert a '0' after a sequence of five 1's in the text i.
This then indicates that there is another byte to follow in the address field. The control field is indicated in Figure 9. Note that the send and receive sequence numbers are important to detect and correct errors in the messages. Protocol Operation: A typical sequence of operations is given below.
Alternatively if the receiving node is unable to accept the set up command a disconnected mode frame is returned. The data frame consists of a 48 bit source and destination address, 16 bits of length or type fields, data and a 32 bit CRC field. The minimum and maximum sizes of the data frames are 64 bytes and bytes respectively. Table 9. Distributed Network Protocol The Distributed Network Protocol is a data acquisition protocol used mostly in the electrical and utility industries.
It is designed as an open, interoperable and simple protocol specifically for SCADA controls systems. The physical layer is generally designed around RS V. It is designed as a data acquisition protocol with smart devices in mind. These devices can be coupled as a multi-drop fieldbus system. DNP does not specify a single physical layer for the Serial bus multi-mode topology.
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