1
1
1
0
1
The objective of this lab is to design, build and use decoders.
Complete the following table for a 2-to-4 decoder. Design and implement a 2-to-4 decoder
1. Complete the following table
EN S1 S0 Q0 Q1 Q2 Q3 x
0 x
0
0
1
1
1
0
1
2. Write Boolean expressions for the outputs
3. From the Boolean expressions, draw and implement the logical circuit
Procedure
1.
Install the IC 74LS138 on the trainer board.
2.
IC 74LS138 is a 3 to 8 decoder. Make connections to the IC as shown in Fig (a).
3.
Use logic switches S2, S1 and S0 for decoder select inputs C, B and A respectively.
4.
Connect outputs Y0 – Y7 to LEDs L0-L7 respectively
5.
Ground pin G2A and G2B and connect G1 to switch A0.
6.
Bias IC by applying +5 volts at pin 16 and ground to pin 8 .
7.
Apply different combinations of inputs and fill the truth table below.

E nable
G1(A0)
1
1
1
1
1
0
1
1
1
Select Inputs
S2 S1 S0
0
1
1
1
1
X
0
0
0
1
0
0
1
1
X
0
0
1
1
0
1
0
1
X
0
1
0
Decoder Outputs
L
0
L
1
L
2
L
3
L
4
L
5
L
6
L
7
0
1
1
1
1
1
1
1
1
Implement the following functions using the above circuit and any additional gates you may require. Draw the circuit diagram.
F(S2, S1, S0) =
 m(2,4)
G(S2,S1,S0) =
 m(3,7)
Fill the following truth table with outputs from your circuit
E nable
G1(A0)
Select Inputs
S2 S1 S0
X
0
0
0
0
1
1
1
1
X
0
0
1
1
0
0
1
1
X
0
1
0
1
0
1
0
1
F G