6.1 Identify THREE programming methods used in programmable logic controllers (PLCs) - NSC Electrical Technology Power Systems - Question 6 - 2016 - Paper 1

1. Push Buttons - Often used for manual input to start or stop machines.
2. Proximity Sensors - Used to detect the presence of an object within a certain range.

1. Lack of Flexibility - Changes in a hardwired system require physical rewiring, whereas PLCs can be easily programmed.
2. Increased Complexity - A hardwired system can become complex as more devices are added, making troubleshooting difficult.
3. Higher Maintenance Costs - The physical nature of hardwired systems often results in greater maintenance needs due to wear and tear.

Low-current devices are activated by using the output relay of the PLC. The relay functions as an interface that translates the low power PLC signal to a higher power signal that can drive the transistor, effectively switching it on while protecting the PLC from higher currents.

The simplified Boolean equation for the expression $X = A'BCD + AB'CD + ABCD + ABC'D + AB'C'D$ is: $X = AB + A'C$

Starting with the equation $Q = AB'C + ABC' + AB + A$, we apply Boolean algebra:

1. Combine terms: $Q = AB + A(B'C + C')$
2. Apply the consensus theorem: $= AB + A(C + B')$
3. Final expression: $Q = A + B'C$.

To find $W$, consider the inputs A and B. If A and B are high ($1$), then the output $W = A + B = 1$.

For $X$, it is the OR of inputs A and C. Therefore, $X = A + C$. If either A or C is high ($1$), then $X$ will be high.

The output $Y$ is determined by the AND operation of A and B. Thus, $Y = A.B$. It will be high only if both A and B are high.

For $Z$, the equation is $Z = (A + B)(A + C)(A.B)$. This will be high if A is high and either B or C are high.

6.8.1 - Normally Open Contact: Represented by a single vertical line.

6.8.2 - Normally Closed Contact: Represented by a curved line indicating the contact remains closed until activated.

6.8.3 - Coil Output: Represented as a small circle that activates an output device.

The ladder logic diagram will consist of a normally open switch in series with the coil output and a holding contact parallel to the start switch, ensuring continuity once the circuit is closed.

An example where this circuit could be used is in a motor starter circuit, where the PLC controls the start and stop of an electric motor.