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WEEK: 6
TOPIC: LOGIC CIRCUIT (II)
CONTENT: Alternative logic gates:
- Description of alternative logic gate
- Types of alternative logic gates NAND, NOR
- Symbols of each logic gate
- Input/output signals for: NAND, NOR gates.
Sub-Topic 1: DESCRIPTION OF ALTERNATIVE LOGIC GATE
Although OR, AND, and NOT gate combinations are good enough for the construction of any logic operation, there are alternative methods that could be used. The alternative logic gates are NAND and the NOR logic gates. Alternative methods using NAND and NOR gates are more economical and therefore more widely used.
All logic operations can be performed with combinations of just NANDs or NORs, but a combination of NORs with NANDs is usually preferable to one provided just by NANDs.
TYPES OF ALTERNATIVE LOGIC GATES
Basic digital logic gates perform logical operations using the following standard single logic gates:
- Logic “NAND” gate
- Logic “NOR” gate
THE LOGIC “NAND” GATE
The logic NAND gate is a combination of the digital logic AND gate with that of an inverter, or NOT gate connected together in series. The NAND (Not AND) gate has an output that is normally at logic level “1” and only goes “LOW” to logic level “0” when ALL of its inputs are at logic level “1”. It is the reverse of the ‘complementary’ form of AND gate.
The Boolean expression for a logic NAND gate, is denoted by a single dot or full stop symbol (.) with a line or overline (-) over the expression to signify the NOT or logical negation of the NAND gate, giving us the Boolean expression of A.B=Q.
Sub-Topic 2: SYMBOLS OF NAND LOGIC GATE
THE LOGIC “NOR” GATE
The logic NOR or inclusive NOR gate is a combination of the digital logic OR gate with that of an inverter or NOT gate connected together in series. The NOT (Not OR) gate has an output that is normally at logic “1” and only goes “LOW” to level of “0” when ANY of its inputs are at logic level “1”. The logic NOR gate is the reverse or “complementary” from of the OR gate.
The Boolean expression for a logic NOR gate is denoted by a plus sign (+) with a line or overline (-) over the expression to signify the NOT or logical negation of NOT gate giving us the Boolean expression of A+B=Q
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