Text-only reference. Published from the recorded official FAA General Chapter 12 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
12-122 Load AC DC Ref DC Control autopilot engage. [Figure 12-257] The OR Gate The OR gate has two or more inputs and one output and is normally represented by the standard logic symbol and truth table. [Figure 12-258] Note that the OR gate can have any number of inputs as long as it is greater than one. The operation of the OR gate is such that a high on any one of the inputs produces a high on the output. The only time that a low is produced on the output is if there are no high levels on any input. Figure 12-259 is a simplified circuit that illustrates the OR logic. The example used is a “DOOR UNSAFE” annunciator. Let’s say in this case that the plane has one cabin door and a baggage door. In order for the annunciator light on the master warning panel to extinguish, both doors must be closed and locked. If any one of the doors is not secured properly, the baggage door OR the cabin door, then the “DOOR UNSAFE” annunciator illuminates. In this case, two switches are in parallel with each other. If either one of the two switches is closed, the light bulb lights up. The lamp is off only when both switches are open.
The NAND Gate The term NAND is a combination of the NOT-AND gate and indicates an AND function with an inverted output. A standard logic symbol for a two input NAND gate is shown in Figure 12-260. Notice that an equivalent AND gate with an inverter is also shown. The logical operation of the NAND gate is such that a low output occurs only if all inputs are high. If any of the inputs are low, the output is high. An example of a two input NAND gate and its corresponding truth table are shown in Figure 12-261. The NOR Gate The term NOR is a combination of the NOT and OR and indicates an OR function with an inverted output. The standard logic symbol for a two-inputs NOR gate is shown in Figure 12-263. Notice that an equivalent AND gate with an inverter is also shown. The logical operation of the NOR gate is such that a low output happens when any of its inputs are high. Only when all of its inputs are low is the output high. The logic of this gate produces resultant outputs that are the opposite of the OR gate. In the NOR gate, the low output is the active output level. Figure 12-263 illustrates the logical operation of a two-input NOR gate for all of its possible combinations and the truth table.
Exclusive OR Gate The exclusive OR gate is a modified OR gate that produces a 1 output when only one of the inputs is a 1. The abbreviation often used is X-OR. It is different from the standard OR gate in that when both inputs are a 1, then the output remains at a 0. The standard symbol and truth table for the X-OR gate are shown in Figure 12-264. Exclusive NOR Gate The exclusive NOR (X-NOR) gate is nothing more than an X-OR gate with an inverted output. It produces a 1 output when all inputs are 1s and also when all inputs are 0s. The standard symbol is shown in Figure 12-265. The Integrated Circuit All of the logic functions so far discussed plus many other components are available in some form of an integrated circuit. The digital systems found in today’s aircraft owe their existence to a large extent to the design of the integrated circuit (IC). In most cases, the IC has an advantage over the use of discrete components in that they are smaller, consume less power, are very reliable, and are inexpensive. The most noticeable characteristic of the IC is its size and in comparison to the discrete semiconductor component, can easily be on the order of thousands of times smaller. [Figure 12-266]\ A monolithic integrated circuit is an electronic circuit that is constructed entirely on a single chip or wafer of semiconductor material. All of the discrete components, such as resistors, transistors, diodes, and capacitors, can be constructed on these small pieces of semiconductor material and are an integral part of the chip. There are a number of levels of integration. Those levels are: small-scale integration, medium-scale integration, large- scale integration, and microprocessors. The small-scale integration is considered the least complex design of the digital ICs. These ICs contain the basic components, such as the AND, OR, NOT, NOR and NAND gates.
[Figure 12-267] The medium-scale integration can contain 12-123 High Leading edge Low Trailing edge Positive logic pulse Negative logic pulse Trailing edge High Leading edge Low Pulse width Pulse characteristic from a databus Rise time 50% 5 V Pulse amplitude (10 volts) 90% 9 V 10% 1 V Fall time Input = 1 (High) Output = 0 (Low) A X B A B A B C D Input Output High Low High the same components as found in the small-scale design but in larger numbers ranging from 12 to 100. The medium- scale designs are house circuits that are more complex, such as encoders, decoders, registers, counters, multiplexers, smaller memories, and arithmetic circuits. [Figure 12-268] The large-scale integrated circuits contain even more logic gates, larger memories than the medium-scale circuits, and in some cases microprocessors.
Microprocessors The microprocessor is a device that can be programmed to perform arithmetic and logical operations and other functions in a preordered sequence. The microprocessor is usually used as the central processing unit (CPU) in today’s computer systems when it is connected to other components, such as memory chips and input/output circuits. The basic arrangement and design of the circuits residing in the microprocessor is called the architecture.
DC Generators
Theory of Operation In the study of alternating current, basic generator principles were introduced to explain the generation of an AC voltage by a coil rotating in a magnetic field. Since this is the basis for all generator operation, it is necessary to review the principles of generation of electrical energy. When lines of magnetic force are cut by a conductor passing through them, voltage is induced in the conductor. The strength of the induced voltage is dependent upon the speed of the conductor and the strength of the magnetic field. If the ends of the conductor are connected to form a complete circuit, a current is induced in the conductor. The conductor and the magnetic field make up an elementary generator.
This simple generator is illustrated in Figure 12-269 , together with the components of an external generator