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

PLC stands for Programmable Logic Controller.

1. User interface (such as a computer or laptop with software).
2. Programming cable for connection to the PLC.
3. Handheld programming device.

To represent the OR-gate operation:

1. Connect two switches (A and B) to the input terminals of an OR-gate.
2. The output terminal of the OR-gate should be connected to a lamp (output).

This configuration will ensure the lamp lights up when either switch A, switch B, or both are activated.

In the ladder logic diagram:

• The left side will show two rungs.
• The first rung represents Switch A in parallel with Switch B, leading to the output denoted as the lamp.
• Each switch is represented by its respective symbol, ensuring the output activates with either switch being closed.

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

To simplify the expression:

1. Combine similar terms:
   • ar{A}BC + ABC simplifies to BC(ar{A} + A) = BC.
2. The expression now is: $BC + ABC'$.
3. Factor out common terms: $B(C + AC')$.
4. Use absorption law: This gives $B(C + A)$. Hence, the final simplified expression is $B(C + A)$.

The maintenance of a PLC system should be less because:

1. PLCs are programmable and can be reconfigured without needing physical changes to wiring, making adjustments easier.
2. PLCs have built-in diagnostic functions that help in troubleshooting, whereas relay systems require manual checks and replacements of individual components.

The Boolean equation can be expressed as:

$F = A ext{ OR } B ext{ OR } C$

Where F is the output that goes high (1) when two or more of the inputs are active.

Using a Karnaugh map for three variables A, B, and C, plot the 1s based on the conditions:

• Mark the positions where the output F should be 1.
• Group adjacent 1s to simplify the expression. The simplified form can be derived based on the clusters identified.

To draw the gate network:

• Use AND gates for the conditions where two sensors must be activated.
• An OR gate combines the outputs of these AND gates to generate the final output F. Each sensor corresponds to one input into the AND gates, thereby checking the minimum count requirement.