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 an interface cable
3. Handheld programming device.

In this diagram, two switches (A and B) are connected to an OR-gate. If either switch A or switch B is closed, the lamp will light up, indicating the output of the OR operation. The circuit can be represented as follows:

    Supply
       |
      / \
    Switch A   Switch B
      \ /
       |  
      OR-GATE
       |  
     Lamp  

The ladder logic diagram for the OR-gate operation can be illustrated with the following representation:

|---[ ]---|  
|  A      |--( )--  
|       Lamp 
|---[ ]---|  
|  B      |  
|----------|  

Here, each rung of the ladder represents a condition that must be true for the output (lamp) to be activated.

Three programming languages commonly used for programming PLCs are:

1. Ladder Logic (LL)
2. Structured Text (ST)
3. Function Block Diagram (FBD).

To simplify the expression $\overline{A}BC + ABC + \overline{A} + ABC + ABC$, we can follow these steps:

1. Combine like terms:
   $\overline{A}BC + ABC + \overline{A} = \overline{A}BC + \overline{A} + AB$
2. Apply the Absorption Law:
   $\overline{A} + B (C + A) = \overline{A} + B$. Thus, the simplified expression is $B + \overline{A}$.

The maintenance of a PLC system is generally less than that of a relay system due to fewer components that are subject to wear, such as contactors. PLCs offer built-in diagnostic functions that can aid in troubleshooting and identification of faults, reducing time and effort in maintenance compared to a hard-wired relay system.

The Boolean equation for the control system can be represented as:
$F = A ext{ OR } B ext{ OR } C$
The output F is true (1) when at least two of the sensors A, B, or C are activated.

Using a Karnaugh map for the expression $F = A + B + C$ will show the groupings needed to simplify the logical output. The grouped values will help to identify common terms and determine the simplest form of the expression.

The gate network of the simplified system can be illustrated using AND and OR gates based on the simplified expression from the Karnaugh map. The configuration would focus on ensuring that the output F is 1 when the conditions from the sensing devices are met.