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Figure 9.40 illustrates the principle of serial data transmission. Only one line is available for serial data transmission. This means that an
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FIGURE 9.40
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The principle of serial transmission.
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8-bit piece of data, for example, must be sent bit by bit from the send register via the data line to the receiver register. A distinction is made between synchronous and asynchronous data transmission. With asynchronous data transmission, data can be sent at any time; pulse recovery is achieved through the start and stop bits. In the synchronous process a transmitter and receiver have the same time pulse at their disposal. Control lines are necessary for this purpose. Functional, electrical, and mechanical characteristics of serial data transmission can also be found in several standards. The most widely used standard versions are the V-24 and RS-232 interfaces (these interfaces are almost identical). With standards, it is guaranteed that different manufacturers devices are able to exchange data. The V-24 is an asynchronous interface. Synchronization is achieved by ready signals. The transmitter may, for example, pass data on to the receiver only when the receiver has previously signaled its readiness to receive. The transmission sequence of the data bits determines the data format (Fig. 9.41). Before transmission of the first data bit, a start bit is sent at the logic zero level. Then, 7 or 8 data bits follow, the one with the lowest value coming first. Afterward, a test bit (parity bit) is transmitted, followed by one or two stop bits.
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FIGURE 9.41 The transmission sequence of the data bits determines the data format.
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FIGURE 9.42 Electrical characteristics determine the signal level.
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Transmission speeds are freely selectable within a specified framework. Transmitter and receiver must be set to the same speed, or baud rate. Common rates are 75, 110, 135, 150, 300, 600, 1200, 2400, 4800, 9600, and 19,200 bit/s (baud). The electrical characteristics determine the signal level (Fig. 9.42). If the voltage of a signal on a data line opposite the signal ground is greater than 3 V and is negative, signal status 1 applies, corresponding to the binary character 1. If the voltage of a signal on a data line opposite the signal ground is greater than 3 V and is positive, signal status 0 applies, corresponding to the binary character 0. Signal status is undefined in the transmission range +3 to 3 V. Connection of the interface line between two devices is effected with a 24-pin D plug. Figure 9.43 shows a V-24 plug with the most important signal lines. This is one of the oldest serial interfaces. It is used to drive teleprinters [teletypewriters (TTY)]. Transmission takes place along a pair of send and receive lines. Logic 0 is realized via a 20-mA current, and logic 1, via the lack of this current. The functional characteristics of the V-24 interface may also apply to the 20-mA current loop interface.
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FIGURE 9.43 V-24 plug.
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9.13 The Collection and Generation of Process Signals in Decentralized Manufacturing Systems
Not long ago, it was customary to centrally collect all process signals. The processor and control elements were housed in a central control cabinet (Fig. 9.44). Today, with automation systems, extensive control tasks can be solved effectively if these tasks are distributed among several decentralized self-sufficient systems. A decentralized structure is illustrated in Fig. 9.45. The sensing, actuating, and processing sections of subsystems are connected to a master computer that evaluates signals applied by each subprocess and sends back control signals. These subsystems can be realized in different ways for example, a PLC or single-board computer could be used as the control system for the subprocesses. A process computer, personal computer, or mainframe can be used as a master computer.
FIGURE 9.44 A centralized control system.
FIGURE 9.45 A decentralized control system.
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FIGURE 9.46 Networking of various systems in a liquid mixing operation.
Figure 9.46 shows a simple control task from chemical process technology as an example of the networking of various systems. In a liquid mixing plant, three different liquids A, B, and C are mixed in a specified proportion. Filling is effected by three valves A, B, and C and the mixture is extracted via one valve, D. Before the mixture is removed, it is heated to a prespecified reference temperature T. A PLC controls the valves and the mixer. The temperature of the mixer is adjusted to the reference temperature by a two-step controller.
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