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Centralized control systems, DCS, and SCADA
Published in Raymond F. Gardner, Introduction to Plant Automation and Controls, 2020
Figure 17.12 and 17.15 shows a Modbus communication system using RS 485 cabling for inter-PLC connectivity and Figure 17.16 shows a radial communication system for Ethernet IP. Modbus uses serial data transmission in a master-slave arrangement, with all devices wired in series. The ends are terminated with resistors that match the cable impedance to prevent voltage reflections that might corrupt the signals. As an alternative, a looped network can be provided to allow continued operation of all devices in the event of a wire break, like the NFPA Class A fire-alarm systems, which provides a redundant path back to the control panel. When the controller detects an open wire in the primary string, it automatically switches to back feed all devices beyond the break in continuity.
Digital recording and transmission principles
Published in John Watkinson, The Art of Digital Audio, 2013
Transmission lines which transport energy in this way have a characteristic impedance caused by the interplay of the inductance along the conductors with the parallel capacitance. One consequence of that transmission mode is that correct termination or matching is required between the line and both the driver and the receiver. When a line is correctly matched, the rolling energy rolls straight out of the line into the load and the maximum energy is available. If the impedance presented by the load is incorrect, there will be reflections from the mismatch. An open circuit will reflect all the energy back in the same polarity as the original, whereas a short circuit will reflect all the energy back in the opposite polarity. Thus impedances above or below the correct value will have a tendency towards reflections whose magnitude depends upon the degree of mismatch and whose polarity depends upon whether the load is too high or too low. In practice it is the need to avoid reflections which is the most important reason to terminate correctly.
Digital coding principles
Published in John Watkinson, An Introduction to Digital Video, 2012
Transmission lines which transport energy in this way have a characteristic impedance caused by the interplay of the inductance along the conductors with the parallel capacitance. One consequence of that transmission mode is that correct termination or matching is required between the line and both the driver and the receiver. When a line is correctly matched, the rolling energy rolls straight out of the line into the load and the maximum energy is available. If the impedance presented by the load is incorrect, there will be reflections from the mismatch. An open circuit will reflect all the energy back in the same polarity as the original, whereas a short circuit will reflect all the energy back in the opposite polarity. Thus impedances above or below the correct value will have a tendency towards reflections whose magnitude depends upon the degree of mismatch and whose polarity depends upon whether the load is too high or too low. In practice it is the need to avoid reflections which is the most important reason to terminate correctly.
Transfer function models for distributed-parameter systems with impedance boundary conditions
Published in International Journal of Control, 2018
Rudolf Rabenstein, Maximilian Schäfer, Christian Strobl
A scenario for simple boundary conditions is shown in Figure 5. The transmission line is driven by an ideal current source and is terminated by a short cut. This circuit leads to the following rules at the boundaries, Referring to the description of simple boundary conditions in Section 3.1, the simple boundary conditions can be reformulated in vector form, and the vectors of boundary excitations The variable Φs1(s) is zero, according to the boundary condition for the voltage at x = ℓ. The variable is necessary for the expression of complex boundary conditions by the simple conditions in Section 6.5.
Low-cost leaky feeder communication for mines rescue
Published in Mining Technology, 2020
Michael D. Bedford, Angel J. A. Rodríguez López, Patrick J. Foster
The transmitter was a Yaesu FT-857D amateur radio transceiver. It was configured to transmit a CW (continuous) signal with an output power of 5W. The cable was hard wired to the transmitter and the end of the cable distant from the transmitter was terminated with a 75 ohm ‘dummy load’ resistor, this being the characteristic impedance of the cables. This corresponds to normal practice with leaky feeders, and is necessary to provide an acceptable match between the transmitter and the cable. The receiver was an Aeroflex 9103 spectrum analyser which was used with a 1.5 m whip antenna in a vertical configuration. The effective noise floor of this configuration, taking into account background noise, varied from −65 dB at 7 MHz to −73 dB at 144 MHz.
Impulse-voltage Measurement of Distribution-class Surge Arresters by D-dot Probes
Published in Electric Power Components and Systems, 2011
The D-dot “electric field-coupled” probe can also serve as a capacitive divider, if the high-voltage and low-voltage capacitances (C1 and C2) and the connections have a negligible inductance [5]. According to the nature of the low-voltage capacitance, two kinds of electric-field probes are distinguished. A low-voltage capacitance (C2) of some tens of nanofarads can be realized. The measurement of fast-front voltages can be easily performed with electric field probes [5, 17, 22]. The probe can be terminated by the characteristic impedance of the cable (Rt = Zo) or by a high impedance (Rt ≫, i.e., in the order of MΩ). For the first case (where Rt = Zo), the capacitive current is much lower than the resistive one. Therefore, D-dot probes measure the time derivative of the electric-flux density (dD/dt = [Ddot]) at the surface of a conductor [5, 17, 22, 23]. This is equivalent to the measurement of the time rate of change of the surface charge density. The basic theory follows from the fact that the displacement current density (Jd) is equal to the time rate of change of the electric flux density, i.e., Jd = dD/dt = [Ddot]. To obtain the electric field, the measuring signal has to be integrated by, e.g., a passive resistive-capacitive (RC) integrator. The upper frequency limit is proportional to the reciprocal value of the time constant RtC2 [5, 17, 22, 23].