When you have completed studying this post you will be able to:
• Describe the basic principles of all communication systems
• Describe the historical background and evolution of data communications
• Explain the role of standards and protocols
• Describe the OSI model of communication layers
• Describe four important physical standards
• Explain the purpose of instrumentation and control system
• Describe the four most important control devices:
– DCS
– PLCs
– Smart instruments
– PCs
Introduction:
Data communications is the transfer of information from one point to another. In this blog, we are specifically concerned with digital data communication. In this context,‘data’ refers to information that is represented by a sequence of zeros and ones; the same sort of data that is handled by computers. Many communications systems handle analog data; examples are the telephone system, radio, and television. Modern instrumentation is almost wholly concerned with the transfer of digital data.
Any communications system requires a transmitter to send information, a receiver to accept it and a link between the two. Types of link include copper wire, optical fiber,radio, and microwave. Some short distance links use parallel connections; meaning that several wires are required to carry a signal. This sort of connection is confined to devices such as local printers. Virtually all modern data communication use serial links, in which the data is transmitted in sequence over a single circuit.
The digital data is sometimes transferred using a system that is primarily designed for analog communication. A modem, for example, works by using a digital data stream to modulate an analog signal that is sent over a telephone line. At the receiving end, another modem demodulates the signal to reproduce the original digital data. The word ‘modem’ comes from modulator and demodulator.
There must be mutual agreement on how data is to be encoded, that is, the receiver must be able to understand what the transmitter is sending. The structure in which devices communicate is known as a protocol.
History:
In the past decade many standards and protocols have been established which allow data communications technology to be used more effectively in industry. Designers and users are beginning to realize the tremendous economic and productivity gains possible with the integration of discrete systems that are already in operation.
Although there were many early systems (such as the French chain of semaphore stations) data communications in its modern electronic form started with the invention of the telegraph. The first systems used several parallel wires, but it soon became obvious that for long distances a serial method, over a single pair of wires, was the most economical.
The first practical telegraph system is generally attributed to Samuel Morse. At each end of a link, there was an operator with a sending key and sounder. A message was sent as an encoded series of ‘dots’ (short pulses) and ‘dashes’ (longer pulses). This became known as the Morse code and comprised of about 40 characters including the complete alphabet, numbers, and some punctuation. In operation, a sender would first transmit a starting sequence, which would be acknowledged by a receiver. The sender would then transmit the message and wait for a final acknowledgment. Signals could only be transmitted in one direction at a time.
Manual encoding and decoding limited transmission speeds and attempts were soon made to automate the process. The first development was ‘teleprinting’ in which the dots and dashes were recorded directly onto a rotating drum and could be decoded later by the operator.
The next stage was a machine that could decode the signal and print the actual characters by means of a wheel carrying the typefaces. Although this system persisted for many years, it suffered from synchronization problems. Perhaps the most severe limitation of Morse code is its use of a variable number of elements to represent the different characters. This can vary from a single dot or dash, to up to six dots and/or dashes, and made it unsuitable for an automated system. An alternative ‘code’ was invented, in the late 1800s, by the French telegraphic engineer Maurice Emile Baudot. The Baudot code was the first uniform-length binary code. Each character was represented by a standard 5-bit character size. It encoded 32 (25) characters,which included all the letters of the alphabet, but no numerals.
The International Telecommunications Union (ITU) later adopted the code as the standard for telegraph communications and incorporated a ‘shift’ function to accommodate a further set of 32 characters. The term ‘baud’ was coined in Baudot’s honor and used to indicate the rate at which a signal changes state. For example, 100 baud means 100 possible signal changes per second.
The telegraph system used electromechanical devices at each end of a link to encode and decode a message. Later machines allowed a user to encode a message off-line onto punched paper tape, and then transmit the message automatically via a tape eader. At the receiving end, an electric typewriter mechanism printed the text. Facsimile transmission using computer technology, more sophisticated encoding and communications systems,has almost replaced telegraph transmissions.
The steady evolution of data communications has led to the modern era of very high speed systems, built on the sound theoretical and practical foundations established by the early pioneers.
Protocols are the structures used within a communications system so that, for xample, a computer can talk to a printer. Traditionally, developers of software and hardware platforms have developed protocols, which only their products can use. In order to develop more integrated instrumentation and control systems, standardization of these communication protocols is required.
Standards may evolve from the wide use of one manufacturer’s protocol (a de facto standard) or may be specifically developed by bodies that represent an industry. Standards allow manufacturers to develop products that will communicate with equipment already in use, which for the customer simplifies the integration of products from different sources.
The OSI model, developed by the International Standards Organization (ISO), is apidly gaining industry support. The OSI model reduces every design and communication problem into a number of layers as shown in Figure . A physical interface standard such as RS-232 would fit into the ‘physical layer’, while the other layers relate to various other protocols.
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