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An RS latch (Reset-Set latch) is a simple digital circuit that serves as a basic memory element. It consists of two cross-coupled NOR gates or NAND gates. The RS latch has two inputs: Set (S) and Reset (R), and two outputs: Q and Q' (the complement of Q).
Here's how an RS latch works:
Set Input (S): When the Set input (S) is activated (high), it forces the output Q to be set to 1, regardless of the state of the Reset input.
Reset Input (R): Conversely, when the Reset input (R) is activated (high), it forces the output Q to be reset to 0, regardless of the state of the Set input.
Feedback Loop: The outputs of the latch are connected to the inputs of the opposite gate, creating a feedback loop. This feedback ensures that once the latch is set or reset, it remains in that state until the opposite input is activated.
Now, let's look at the truth table for an RS latch:
S | R | Q(t) | Q(t+1) |
0 | 0 | Q | Q |
0 | 1 | Q | 0 |
1 | 0 | Q | 1 |
1 | 1 | X | X |
Q(t) represents the previous state of the output Q.
Q(t+1) represents the next state of the output Q.
X indicates that the state is indeterminate.
An RS latch can be implemented using NAND gates. Here's how:
Set Input (S): Connected to one input of one NAND gate.
Reset Input (R): Connected to one input of the other NAND gate.
Cross-Connections: The outputs of the NAND gates are connected to each other's inputs.
Feedback Loop: The output of each NAND gate is connected to the input of the other NAND gate.
Truth table for NAND gate RS latch:
S | R | Q(t) | Q(t+1) |
0 | 0 | Q | Q |
0 | 1 | Q | 0 |
1 | 0 | Q | 1 |
1 | 1 | X | X |
Implementation of RS latch using NOR gates:
Similarly, an RS latch can be implemented using NOR gates. The logic is the same, but instead of using NAND gates, NOR gates are employed.
Truth table for NOR gate RS latch:
S | R | Q(t) | Q(t+1) |
0 | 0 | Q | Q |
0 | 1 | 0 | Q |
1 | 0 | Q | 1 |
1 | 1 | X | X |
Overall, an RS latch is a fundamental building block in digital electronics, providing memory storage and basic sequential functionality. Its simplicity and versatility make it useful in various applications.
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