4.1 Name TWO methods to display information in digital systems - NSC Electrical Technology Digital - Question 4 - 2021 - Paper 1

In a common anode seven-segment display, all the anodes of the LEDs are connected together to the positive voltage supply. Each segment is illuminated by applying a low signal (ground) to its cathode, enabling control over which segments are on or off.

Transistor coupling in LED seven-segment displays is often used to increase the current-driving capability. In the circuit, a transistor acts as a switch that controls the flow of current through the LEDs when activated.

Polarisation of light refers to the direction in which light waves oscillate. In an LCD, light travels through polarizing filters, allowing light aligned in a specific direction to pass while blocking other orientations. This is essential for displaying images, as it controls light transmission.

The circuit in FIGURE 4.5 is identified as a full adder, which combines multiple binary inputs to produce a sum and carry output.

The logic circuit of a full adder consists of:

• Two exclusive OR gates for computing the sum output:
  • The first XOR takes inputs A and B.
  • The second XOR takes the output from the first XOR and Cin.
• Two AND gates for calculating the carry output:
  • The first AND takes A and B.
  • The second AND takes the output from the first XOR and Cin.
• An OR gate to combine the outputs of the two AND gates, resulting in Cout.

The waveforms for the clocked RS flip-flop will show changes in output Q and Q’ based on the S and R inputs during the clock cycle. The outputs should reflect the state based on the timing of the CLK pulse.

A three-bit parallel adder will consist of three full adders connected in a cascade form:

• Each full adder will take two bits from the respective three-bit numbers plus a carry input from the previous adder.
• The outputs will provide a three-bit sum and a carry output.

1. Asynchronous Counter: Also known as ripple counters, these types change state in response to clock pulses at different times.

2. Synchronous Counter: All flip-flops are driven by the same clock signal, ensuring they change state simultaneously.

Combinational logic circuits output results based solely on current inputs, without memory elements. Sequential logic circuits, however, utilize feedback and memory elements (like flip-flops), meaning their output depends on current and past inputs.

The counter in FIGURE 4.12 is identified as a three-bit synchronous down counter.

1. Parallel-in: Serial-out (PISO) Shift Register: This type takes multiple parallel inputs and converts them into a serial output.

2. Serial-in: Parallel-out (SIPO) Shift Register: This type accepts serial data and converts it into parallel output.

A = Serial data B = Clock

In this shift register, data bits are shifted in sequentially on the clock edge. The serial input data is loaded into the register, shifting to the next state with each clock pulse. This allows multiple bits to be organized into a parallel form for output.