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Assertion (A) : A multiplexer (MUX) is a device which selects one of the many inputs to a single output. Reason (R): A digital multiplexer is a sequential circuit that selects binary information from one of the many input line sand directs it to a single output line

Assertion (A) : A multiplexer (MUX) is a device which selects one of the many inputs to a single output. Reason (R): A digital multiplexer is a sequential circuit that selects binary information from one of the many input line sand directs it to a single output line Correct Answer (A) is true, but (R) is false

Multiplexers:

  • A multiplexer is a combinational circuit.
  • A multiplexer is Many to one data selector.
  • A multiplexer selects one of the many data available at its input depending on the bits on the select line.
  • For 2n inputs, there are n select lines that determine, which input is to be connected to the output.

 

A multiplexer is used for Parallel to serial data conversion with the help of a counter as shown:


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  • The Q-outputs of the modulus-8 counter is connected to the data select inputs of an 8- multiplexer.
  • The first byte (8-bit) of parallel data is applied to the multiplexer inputs.
  • As the counter goes through a binary sequence from 0 to 7, each bit beginning with D0 is sequentially selected and passed through the multiplexer to the output line.
  • After 8 clock pulses, the data byte is converted to a serial format and sent out on the transmission line.

Hence, (A) is true, but (R) is false.

Example:

A 4-input multiplexer is as shown below.

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Output, Y = I0S̅1S̅0 + I1S̅1S0 + I2S1S̅0 + I3S1S0

The output of a 4-input multiplexer represents one combinational functions of 3-variables each.

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Sequential circuits:

Sequential circuits produce output based on the current input and previous outputs, i.e. sequential circuits include memory elements that are capable of storing binary information.

 A sequential circuit performs this transformation by means of logic gates and a memory element.

The block diagram shown below explains this:

 

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COMBINATIONAL CIRCUIT

SEQUENTIAL CIRCUIT

Its output is determined by the present values of its input only.

Its output is determined by the present input as well as past output values

It does not have a memory and feedback path from output to input

It has a memory element and a feedback path from output to input

Its circuit is simpler than that of a sequential circuit.

Its circuit is more complex than that of combinational circuits.

Ex- Adder, Subtractor, Multiplexer, Demux, Decoder etc.

Ex- Flip flops, counters, shift registers

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