Digital Logic Design


In this lecture we will complete viewing the standard combinational circuits. 
Specifically, we will study:

Multiplexer is a combinational circuit that selects one of the inputs and directs it to a single output line.
  1. It is abbreviated as a MUX. 
  2. It is also called data selector and a digital switch. 
  3. The operation performed by a MUX is called election. 
  4. The selection of the inputs is controlled by a selection lines or inputs.
In general a MUX has 2ninputs, n selection lines (implied from the inputs so they are not counted with the inputs), and exactly 1 output.

Each input of the multiplexer is 1-bit only.
The internal implementation of the MUX is an AND-OR circuit.
Each input is fed into one AND gate in addition to one input from each selection line.
The selection lines are very similar to the enable line in the decoder. If their values are 1 then the AND gate is enabled, otherwise the AND gate is disabled so the input has no effect on the output.




2.1 Procedure:

2.1.1 Multiplexer:

Note: In this experiment we use the Multiplexer As one of integrated circuit

We used the 74151 MSI devices which routes one of eight input line to a single output line:

By selecting A, B and C as inputs and connecting them to switches, then connecting (D0 - D3 ) to the ground, and connecting (D4 - D7) to Vcc. Then by connecting the strobe with zero or with one of the off switches, and the output with the LED as shown in the schematic diagram, we got the following truth table:

2.1.2 Decoder:
Note: In this experiment we use the Decoder As one of integrated circuit

In this experiment we used the 74LS138 IC, which is a 1 of 8 decoder or 3 line to 8 line decoder.

We connected inputs A, B and C to switches and all data outputs to LEDs. Then we connected G2A ,G2B with the ground, G1 with Vcc as shown in the following schematic diagram, we get the truth table below:

2.2 Discussion:

§  Strobe in a multiplexer and the enable in the decoder are used to control the device. In multiplexer the strobe must be 0 to be active (active low). The enable in the decoder consist of 3 terminal the terminal number 6 (G1) must be equal to 1 and (G2) equal 0 to be active.
§  To construct a 16 channel multiplexer using SSI devices we need: 16 AND gate , 6 inverter gates, and 1 OR gate.
§  To construct a 14 of 16 decoder using SSI devices we need: 16 NAND gates, 9 inverter gates, and 1 NOR gate.
§  The multiplexer we used in this experiment had 3 select lines, 8 inputs and one output. The select lines decided the value of the output depending on the input they chose. For example, in our experiment  (I0 – I3) = 0 , (I4 – I7) = 1. If the select lines chose an input between  (I0 – I3), the output = 0. If they chose an input between (I4 – I7), then the output=1.

2.3 Conclusion:

1.     MSI is short for “Medium scale Integrated circuit” devices.
2.     A multiplexer selects one of many analog or digital input signals and forwards the selected input into a single line.
3.     A decoder is a device which does the reverse of an encoder. A decoder takes the form of a multiple-input, multiple-output logic circuit that converts coded inputs into coded outputs.
4.     An enable must be high for the device to work properly.


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