Download Logic Gates 1 - Simple AND Logic Function

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LOGIC GATES 1: Simple AND Logic Function.
The AND function requires both switches to be closed for the lamp to become lit.
Opening either removes the lamp voltage supply. Therefore, both S1 AND S2 have to remain closed to
complete the circuit
LOGIC GATES 1: AND Gate Function.
This diagram shows the logic symbol for an AND gate along with two pull down resistors. If the
switches are open the inputs are held low, closing either pulls up the voltage on the selected input.
For the AND gate there is only one condition which produces a logic 'high' output. Which is it?
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LOGIC GATES 1 Simple OR logic function
The OR logic function can be clearly demonstrated using two push switches connect in parallel. Press
'S1' OR 'S2' and the lamp is lit.
If both switches are closed the lamp, as you would expect remains on. The only state, which
extinguishes the lamp, is when both switches are open. Replace logic '0' for an open switch and logic '1'
for a closed switch and you could produce an OR gate truth table.
LOGIC GATES 1: OR Gate Function.
Here we use the same basic circuit to demonstrate the OR logic function. Examine the truth table and
determine the input conditions which produce a logic 'high' output.
Go back and compare this with the parallel switches we looked at earlier.
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NAND gate function
The NAND gate is simply an AND gate with an inverted output. Compare the truth tables for each
type.
Could you draw a circuit diagram which would demonstrate the NAND function? Use whatever
components you like, not just switches.
LOGIC GATES 1: NOR Gate Function.
Again the NOR logic gate is an OR function with an inverted output. Compare the truth tables.
This is probably one of the most useful gates available. As we explore later this gate can be configured
as an AND function or logic inverter.
Compare the truth tables for the two functions and note the differences.
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LOGIC GATES 1: NOT Gate Function.
The NOT gate is simply an inverter. The output will always be opposite to the input.
Place two NOT gates in series and you have your original input logic state. This diagram shows a
NAND gate connected as inverter by strapping the inputs together. Also the conventional schematic
symbols for logic inverters.
LOGIC GATES 1: Exclusive OR Gate Function.
The EXCLUSIVE OR logic function will output a logic '1' only if either of the inputs is 'high' not both.
Familiarise yourself with the truth table. The next topic shows how this function is derived.
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LOGIC GATES 1: OR Function from NAND Gates.
This is an example of how a simple OR Gate can be constructed from three NAND gates.
Also shown are the Boolean expressions for each input and output.
LOGIC GATES 1: Mixed Logic Analysis 1.
This circuit uses mixed logic gates.
Try constructing a truth table and see if the circuit could be simplified, and therfore replaced by just
one or two equivalent gates.
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LOGIC GATES 1: Logic Analysis 2.
A second example uses six NAND gates connected to achieve a particular function for the combination
of 'A' and 'B' input logic.
Try working out what it is. Then by introducing other gate types can you simplify the diagram?
Remember the tip given earlier. Change the gate type (AND / OR) and invert the inputs and outputs.
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