6.1 Explain the term program when referring to a PLC - NSC Electrical Technology Power Systems - Question 6 - 2017 - Paper 1

PLC stands for Programmable Logic Controller.

1. User interface (on the PLC unit - screen & buttons).
2. Computer or laptop with interface cable.
3. Handheld programming device.

The OR-gate circuit can be represented as follows:

      Supply
         |\n      Switch A
        / \
       /   \
      /     \
     +-------+
     |       |
     |       |
  Lamp  Switch B
     |       |
     |       |
     +-------+
         |
       Output

In this diagram, when either Switch A or Switch B is closed (on), the lamp will be illuminated, simulating the operation of an OR-gate.

The ladder logic representation for the OR-gate can be illustrated as follows:

|   Switch A   |  +----( )----+   Lamp   
|              |  |           |          
|   Switch B   |  +-----------+          

In this diagram, either switch can energize the lamp, indicating the OR-operation.

1. Ladder Logic (IL).
2. Instruction List (IL).
3. Structured Text (ST).

To simplify the expression, we can apply Boolean algebra rules:

1. Start with the original expression:
   \overline{A}BC + ABC + \overline{A} + ABC
2. Combine like terms: \overline{A}BC + ABC + \overline{A} = \overline{A} + BC
3. Therefore, the simplified expression is:
   \overline{A} + BC

The maintenance of a PLC system is typically less than that of a relay system due to fewer components involved, such as contactors, which are subject to wear and require periodic replacement. Additionally, PLCs often come with built-in diagnostic functions, making it easier to identify and address issues.

The Boolean equation for the control system can be expressed as:
F = A \lor B \lor C
This means the output F is true (1) when at least one of the sensors A, B, or C is activated.

To simplify the Boolean expression using a Karnaugh map, you would place the values from the truth table into the map and group the ones accordingly. This would allow for visual simplification of the equation. The resulting simplified equation, depending on the configuration and grouping, might look like:
F = A \lor B (assuming overlaps are minimized).

The gate network of the simplified system can be illustrated as follows:

 A  --->
          \
           OR -----> F
          / \
 B  --->/   

This diagram represents an OR gate that combines the outputs of the sensors A and B, providing an output F.