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What Are Combinational Logic Circuits?

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  • Written By: Geisha A. Legazpi
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 08 September 2016
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Combinational logic circuits are electronic circuits that produce outputs based on the states of the inputs. Unlike in sequential logic circuits, the previous outputs do not partly determine the next outputs. Combinational logic circuits are used in various electronic equipment to implement limited intelligence in all sorts of applications.

The transistor-transistor logic (TTL) family of circuits also implements combinational logic circuits. Voltage levels are either 0 volts direct current (VDC,) or low voltage level, and +5 VDC, or high voltage level. A buffer is a TTL circuit that outputs a level that is the same as its input, while an inverter outputs a level that is the opposite of its input. Combinational logic circuits like the buffer and the inverter are single-input circuits.

There are also two-input logic circuits known as logic gates. A common logic gate is the OR gate, which may have two or more inputs. Given the first input is input 1 and the second input is input 2, a two-input OR gate will output a high if input 1 or input 2 is high. A two-input AND gate will output a high if input 1 and input 2 are a high. The combinations of input levels determine the output.

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Combinational logic circuits may be used in practical circuits, such as a simple alarm. Given two access doors in a house, a signal called “Door1_Open” and “Door2_Open” may have a high level when the specific door is open. A buzzer may then be controlled by the output of an OR gate that sounds off an alarm when one or both doors are open.

Sequential logic circuits rely on the combination of inputs as well as the current states of outputs. In the door alarm example, the alarm goes off when all doors are shut, but using a sequential circuit, the alarm may stay on until a reset signal is applied. In this case, the purely combinational logic circuit may not be ideal. People want the alarm sound to remain until the authorized user explicitly acknowledges the alarm with a signal, such as an "acknowledge alarm" command.

Combinational logic schematics are made up of buffers, inverters, OR gates, AND gates, NOR gates, NAND gates, and XOR gates. NOR gates are OR gates with an inverter at the output, while an XOR gate is an exclusive OR gate that outputs a high only if one input is high. NAND gates are AND gates with inverters at the output. Combinational logic electronics may use TTL levels using 0 to 5 VDC. Other levels may be 0 to 3.3 VDC, as long as two states are distinct.

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