Designing a Logic Circuit to Route a Common Input Data Line onto 1 Out of 4 Output Lines Using Active Low Output

When dealing with digital circuits, the task of routing a common data input line onto one out of multiple output lines often arises. This problem can be addressed using a 1-to-4 Decoder/Demultiplexer, a fundamental logic circuit used for data routing. In this scenario, the output lines operate on active low logic, where 0 represents a high signal and 1 represents a low signal.

In this article, we will discuss the design and implementation of such a logic circuit, focusing on the use of active low output. We will explore the principles behind the demultiplexer, the role of gates, and the steps to achieve the desired functionality.

Introduction to 1-to-4 Decoder/Demultiplexer

A 1-to-4 decoder/demultiplexer is a critical component in digital systems. Its primary function is to selectively route a single input signal to one of its four outputs based on a selection input. This is particularly useful in scenarios where a single data line needs to be distributed to multiple output lines, enabling efficient data manipulation and routing.

Understanding Active Low Logic

In digital electronics, active low logic is a system wherein the active state is represented by a low output signal (0), while the inactive state is represented by a high output signal (1). This is in contrast to active high logic, where the active state is represented by a high output signal (1).

Designing the Logic Circuit

To design the logic circuit required for routing a common input data line D to one of the four output lines using active low output, follow these steps:

Selection Inputs: Define the selection inputs that determine which output line will receive the input signal. In this case, we will use a 2-bit selection input (SEL) to control which of the four outputs will receive the input D. Logic Gates: Use logic gates to implement the functionality of the demultiplexer. Typically, a combination of AND gates, OR gates, and inverters is used to achieve the desired output behavior. Active Low Output: Ensure that the outputs operate on active low logic. This means that the asserted (high) state on the selection inputs will deactivate (set to 0) the corresponding output.

Logic Circuit Design

To begin, we will lay out the basic structure of the circuit:

Assume the inputs are: D (common data input) and SEL (2-bit selection inputs). The outputs are: OUT0, OUT1, OUT2, OUT3 (the four output lines).

The desired behavior is that only one of the outputs, determined by SEL, will be active (0) at any given time, while the others will be inactive (1).

The logic for this can be described as follows:

OUT0 D !SEL0 !SEL1

OUT1 D SEL0 !SEL1

OUT2 D !SEL0 SEL1

OUT3 D SEL0 SEL1

Where !SEL0 and !SEL1 are the inverted selection inputs.

Implementing the Logic Circuit

The implementation involves the use of logic gates as follows:

AND Gates: Use AND gates to implement the selection logic. Each AND gate will have one input from D and the corresponding inverted selection inputs. NOT Gates (Inverters): Use NOT gates to invert the selection inputs as required. Or Gate (If Required): In some instances, you might need an OR gate to combine signals if all outputs need to be active simultaneously.

Example Circuit Diagram

Example 1-to-4 Decoder/Demultiplexer Circuit

In the above diagram, the common input D is connected to the AND gates. The 2-bit selection inputs (SEL0 and SEL1) are inverted using NOT gates and then used in conjunction with the AND gates to determine which output will be active.

Conclusion

Designing a logic circuit to route a common input data line onto one out of four output lines using active low outputs involves understanding the principles of decoders, implementing the necessary logic gates, and properly configuring the selection inputs. By following the steps outlined above, you can create a functional and efficient 1-to-4 demultiplexer, ensuring that the active low output requirement is met.

For further exploration, consider diving into more advanced topics such as Karnaugh maps, which can help in simplifying complex logic designs, or exploring modern digital design software tools that can automate much of the design process.

Remember, the key to successful digital design is a solid understanding of the underlying principles and the ability to apply them correctly.