As discussed earlier, a digital circuit represents and manipulates information encoded as electric signals that can assume one of two voltages – logic-high voltage (or Vdd) and logic-low voltage (or GND). A digital circuit requires a power supply that can produce these two voltages, and these same supply voltages are also used to encode information in the form of two-state, or binary signals. Thus,if a given circuit node is at Vdd, then that signal is said to carry a logic ‘1’; if the node is at GND, then the node carries a logic ‘0’. The components in digital circuits are simple on/off switches that can pass logic ‘1’ and logic ‘0’ signals from one circuit node to another. Most typically, these switches are arranged to combine input signals to produce an output signal according to basic logic relationships. For example, one well-known logic circuit is an AND gate that combines two input signals to produce an output that is the logic AND of the inputs (i.e., if both input1 and input2 are a ‘1’, then the output is a ‘1’).
Wiring diagrams use standard symbols for wiring devices, usually different from those used on schematic diagrams. The electrical symbols not only show where something is to be installed, but also what type of device is being installed. For example, a surface ceiling light is shown by one symbol, a recessed ceiling light has a different symbol, and a surface fluorescent light has another symbol. Each type of switch has a different symbol and so do the various outlets. There are symbols that show the location of smoke detectors, the doorbell chime, and thermostat. On large projects symbols may be numbered to show, for example, the panel board and circuit to which the device connects, and also to identify which of several types of fixture are to be installed at that location.
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