6.1 Identify THREE programming methods used in programmable logic controllers (PLCs) - NSC Electrical Technology Power Systems - Question 6 - 2016 - Paper 1
Question 6
6.1 Identify THREE programming methods used in programmable logic controllers (PLCs).
6.2 Name TWO INPUT devices that may be connected to the input stage of a PLC.
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Worked Solution & Example Answer:6.1 Identify THREE programming methods used in programmable logic controllers (PLCs) - NSC Electrical Technology Power Systems - Question 6 - 2016 - Paper 1
Step 1
Identify THREE programming methods used in programmable logic controllers (PLCs).
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Answer
Ladder Logic
Structured Text
Function Block Diagram
Step 2
Name TWO INPUT devices that may be connected to the input stage of a PLC.
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Answer
Push Buttons
Proximity Sensors
Step 3
State THREE disadvantages of a hardwired system in comparison with a PLC system.
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Lack of flexibility for modifications and expansions.
Difficulty in troubleshooting and maintenance.
Higher installation costs due to extensive wiring.
Step 4
Explain how low-current devices, such as transistors, may be activated by the output of a PLC.
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Answer
Low-current devices can be activated by connecting them via the PLC output relay. The PLC delivers an output voltage through a transistor, allowing it to control devices that require lower currents.
Step 5
Write the simplified Boolean equation for the expression below. Use a four-variable Karnaugh map.
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Answer
Using the Karnaugh map, the simplified expression for
X = A'BCD + AB'CD + ABCD + AB'CD + AB'C'D
is
X = A'B + AC + BCD.
Step 6
Simplify the following Boolean equation by using Boolean algebra.
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Answer
Starting with
Q = AB'C + ABC + AB + A + B,
the simplified form is
Q = A + B.
Step 7
Determine the output at the following points: 6.7.1 W
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Answer
W = A + B
Step 8
Determine the output at the following points: 6.7.2 X
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Answer
X = A + C
Step 9
Determine the output at the following points: 6.7.3 Y
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Answer
Y = A . B
Step 10
Determine the output at the following points: 6.7.4 Z
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Answer
Z = (A + B)(A + C)(A . B)
Step 11
Name the following ladder logic symbols: 6.8.1
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Answer
Normally open contact / switch (Input)
Step 12
Name the following ladder logic symbols: 6.8.2
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Normally closed contact (Input)
Step 13
Name the following ladder logic symbols: 6.8.3
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(Coil) output
Step 14
Draw the ladder logic diagram that will execute the same function in a PLC system.
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Answer
The ladder logic diagram should illustrate the control logic with the START and STOP push buttons, and the motor control relay, MC1.
Step 15
Give ONE example where the circuit in FIGURE 6.2 may be used in an electrical application.
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An example is its use in a motor starter for controlling electric motors.
