Lab 10: Multiplexer/Demultiplexer

Introduction

A multiplexer/Encoder (MUX) is a selector device that has 2i data inputs, and i select inputs. The value on the output of such a device is the value nth data input, where n is the binary number on the select inputs.

A demultiplexer/Decoder (DEMUX) is a selector device that has some i select inputs and 2i outputs. Interpreting the inputs sa an i bit binary number, n, the decoder will make its nth output line TRUE, and all the FALSE. These devices often have one (or more) enable inputs.

In the previous labs we have used gate that are considered SSI (Small Scale Integration). In this lab we will begin working with chips that have more complex circuits in them that perform more complex operations than simple logic. These type of devices are called MSI (Medium Scale Integration).

Equipment needed

Procedure

Part 1 (MUX)

  1. Construct the circuit shown in figure 1. Be sure to connect the +5 volts and ground to pins 14 and 7 respectively.

  2. Use the Oscilloscope to assure that a +5 volts, 1 Hz clock (CLK) signal is applied to pin 9. Refer to the earlier labs on the proper use of this equipment.

  3. Apply power to circuit 1 and set SW1 to binary 1 and note the circuit output at pin 11 with the logic probe. Note the effect of switching SW2 high and low.

  4. Set SW1 to binary 0. Note the effect of switching SW2 high and low.

    What is the fuction of SW1? What type of circuit is this? What are the two signal sources? Draw the device that this circuit mimics in your notebook.

Part 2 (MSI-MUX)

  1. Wire the Quad 2-input multiplexer (74LS157) as shown in fiqure 2.

  2. Create a truth table for one of the four 2-input multiplexers. Refer to your lecture notes to verify the truth table.

    What is the difference between circuit 1 and circuit 2? How many MUX's are on this chip? How many select inputs? What is the input E for? Relate the inputs and outputs from the 7400 (circuit 1) to those in the 74157 (circuit 2).

Part 3 (DEMUX)

  1. Construct the circuit shown in figure 3. The data switches SW1- SW4 will p[rovide the input. Be sure to connect both power and ground to each chip.

  2. Apply the 16 states 0000 - 1111 to the circuit and observe the output (create a truth table). What is the binary input state where the output is high? What is the decimal equivalent?

  3. Construct the circuit shown in fiqure 4. Recheck your wiring for errors. Create a truth table for circuit 4.

  4. Remove the output inverters and redo the truth table in step 3. (Look at the outputs of the nand gates directly)

    Compare the outputs in your truth table from steps 3 and 4 above. What type of circuits are these? What is their function?

Part 4 (MSI-DEMUX)
  1. Remove circuit 4 from the breadboad and mount the 74LS42 IC on the breadboard. Use the TTL handbook to assure the correct power and ground connection.

  2. Wire the circuit as shown in figure 5. Connect the inputs to the switches SW1 - SW4 and monitor the outputs one at a time with the logic probe.

  3. Create a truth table for circuit 5. (4-inputs and 10-outputs)

    This chip is a one of ten decoder. Decoders are often called "minterm generators". Verify this by writing the output expression in cononical form from the truth table in step 3 above. Notice this circuit ignores six state. What are these input st ates?

    Figure 2

    Figure 3

    Figure 4

    Figure 5