Experiment 8 (a)
1 Objective
Investigate the operation of Encoders.
2 Purpose
Having completed this experiment you will be able to:
• Understand the operating principles of encoder circuits..
3 Apparatus
• Logic Trainer (SMT-8004)
• Logic Trainer (SMT-800)
• Power Supply
• Connecting wires
4 Theory
An encoder is combinational logic gates that accept one or multiple inputs and generates a specific output
code. Only one input is triggered at a time. An encoder with n-bit inputs and n-bit outputs is shown in
Fig. 1. When one of the inputs is triggered there will be an n-bit code at the output.
Figure 1: Encoder with n-inputs and n-outputs.
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Figure 2: Octal to binary encoder.
4.1 Octal to Binary Encoder
An octal to binary encoder is shown in Fig. 2. There are 8 octal inputs A1 A7 or (0 7); and three binary
outputs Q0, Q1, Q2 or (000 111). If input A0 = 0 the corresponding output Q2, Q1, Q0 is equal to 000.
Actually, A0 is not connected to the gate input. If A1 = 1 then Q2Q1Q0 = 001. When A2 = 1 the output
Q2Q1Q0 = 010. There cant be more than one 1 among the inputs. For example, if A2 = 1 and A3 = 1
simultaneously, Q2Q1Q0 = 011. If A3, A4 both are 1 at the same time, Q2Q1Q0 = 111. Both outputs are incorrect.
4.2 Constructing—a −4 to2 Encoder with Basic Gates (Module− SMT
8004
block Encoder 1)
4.2.1 Procedure
• Insert connection clips according to Fig. 3.
Figure 3: 4 − 2 Encoder with basic gates.
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• Connect Vcc to +5V.
• Connect inputs A D to Date Switches SW0 SW3 respectively; Outputs F8 and F9 to Logic Indicator
L0 and L1.
• Follow the input sequences for D, C, B, A; in Table 1 and record the output states.
Table 1: Truth Table of 4− 2 Encoder with basic gates.
D C B A F8 F9
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 0 0
1 1 0 1
1 1 1 0
1 1 1 1
• Remove the connection clip between A and A1; insert it between A1 and F1 as shown in Fig. 4. All
other connections remain the same. Follow the input sequences in Table 2 and record output states.
Figure 4: 4 − 2 Encoder with basic gates.
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Table 2: Truth Table of 4− 2 Encoder with basic gates.
D C B A F8 F9
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 0 0
1 1 0 1
1 1 1 0
1 1 1 1
• Compare the outputs states in Table 1 and 2.
• What is the difference between them?
5 Conclusion
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