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.

Three basic devices used in programming a PLC include:

1. User interface (on the PLC Unit - Screen & Buttons)
2. Computer or laptop with interface cable
3. Handheld programming device.

6.4.1 The circuit diagram:

    Supply
      |
    +-----+
    |     |
    |     |----> Lamp Output (OR Gate)
    +-----+
      |     
    Switch A   Switch B

6.4.2 The ladder logic diagram:

|  Switch A |      |  Switch B |  ---> Lamp Output
|-----------|      |-----------|
|    OR     |      |     OR    |
|-----------|      |-----------|

Three commonly used programming languages for PLCs are:

1. Ladder Logic (LL)
2. Instruction List (IL)
3. Structured Text Sequential Flow / Function Chart.

To simplify the expression [A'BC + ABC' + AB'C + ABC]:

1. Combine like terms:
   [= BC(A + A') + AB'C + ABC] [= BC (1) + AB'C + ABC]

2. Further simplify:
   [= B C + AB'C + ABC]
   [= B(C + A') + BC]
   [= B + C]

The maintenance of a PLC system is generally less than that of a relay system because PLCs have fewer components, such as contactors that are subject to wear over time. In addition, PLCs often include built-in diagnostic functions that help identify issues more readily.

The Boolean equation for the control system, given that it requires an output of 1 when two or more sensors are activated, is: [F = A \cdot B + A \cdot C + B \cdot C]

To simplify the equation using a Karnaugh map, the layout would be as follows:

   | BC  
  ----------------
A | 00 | 01 | 11 |
   | 0  | 1  | 1  |
   | 1  | 1  | 1  |

This gives us the simplified output of: [F = A + B \cdot C]

The gate network for the simplified equation can be represented as:

     +--A----+
     |        |
     |        +--- F 
     |        |
     +--B - C  

Here, with A and the combination of B and C generating the output F.