Data Acquisition

• The communications link (twisted-pair cable, coaxial cable, radio, telephonenetwork etc), which transfers the message to the receiver at the other end• The receiver of the data where the signal is converted back into a form that can beused by the local electronics circuitryBoth the receiver and the transmitter must agree on a number of different factors to allowsuccessful communications between them, the most important being:• The type of electrical signals used to transmit the data• The type of codes used for each symbol being transmitted• The meaning of the characters• How the flow of data is controlled• How errors are detected and correctedThe hardware rules that apply to the physical interface and its connections are known asinterface standards, while the software rules which apply to the format and control of dataflow and the detection and correction of errors are generally referred to as the protocol.All data communication is based on the same binary system used in computers. Each basicunit of information is called a BIT (BInary digIT), and it can be one of two values: ‘0’ or ‘1’.These are termed logic 0 and logic 1. Inside a computer, a bit is usually represented by avoltage; typically, 5 V is logic 1, and 0 V is logic 0. In data communications, a logic 1 is alsoreferred to as a mark and a logic 0 as a space.In a data communications link, the 1s and 0s may be indicated by a +/– voltage. Othermethods can involve audio tones, with one frequency for logic 1 and a different frequency forlogic 0. Some advanced techniques allow several bits to be encoded in a single voltagechange: this is how you can use a 14.4 kbps modem on a telephone line with a nominal bandwidthof only around 3 kHz.A string of bits is a binary number. It can be interpreted simply as a numerical value (inbinary, hexadecimal or some other numbering system) or it may be translated into a characteraccording to an agreed code. For example, in the ASCII system the binary value 1000100represents ‘D’.6.1.1 Transmission modes – simplex and duplexIn any communication system connecting two devices, data can be sent in one direction only,or in both directions. A simplex system is one that is designed for sending messages in onedirection only. This is shown in Figure 6.1.Figure 6.1Simplex communications