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

The term common anode refers to a configuration in a seven-segment LED display where all the anodes (positive connections) of the LED segments are connected together to a common positive voltage supply. In this arrangement, the individual segments are activated by grounding their respective cathodes (negative connections). This means that to light up a segment, a LOW signal (0V) is applied, allowing current to flow through the segment from the common anode to the cathode.

Transistor coupling in LED seven-segment displays is used to control the current flowing to each segment. When an input signal is applied to the base of the transistor, it allows current to flow from collector to emitter, effectively activating the connected LED segment. In FIGURE 4.3, the circuit likely involves a transistor that acts as a switch for each segment of the display, ensuring that only the desired segments light up based on the input signals.

The term polarisation of light refers to the direction in which light waves oscillate. In the context of liquid crystal displays (LCDs), polarisation is crucial for the functioning of the display. Light travels naturally in all directions, but when it passes through a polariser, only light waves oscillating in a specific direction can pass through. In an LCD, polarised light is manipulated using liquid crystals that can change the orientation of light, allowing or blocking it, which creates images on the display.

The circuit in FIGURE 4.5 is identified as a decoder. This circuit takes binary inputs and activates a specific output based on the combination of those inputs.

The truth table for the decoder in FIGURE 4.5 can be constructed as follows:

To construct a full adder logic circuit, the following components are needed:

1. Two inputs (A and B) representing the two bits to be added.
2. One Carry-In input (Cin) which represents the carry from a previous addition.
3. Outputs:
   • Sum output
   • Carry-Out output (Cout)

The connections are as follows:

• The Sum output can be produced by combining the results of two XOR gates:
  • XOR(A, B) → First XOR gate
  • XOR(Sum_from_previous, Cin) → Second XOR gate to produce the final Sum.
• The Carry output is produced by using AND gates and OR gates:
  • Carry from inputs A and B: AND(A, B)
  • Carry from Sum and Cin: AND(Sum_from_previous, Cin)
  • These carry results are combined using an OR gate to produce the final Cout.

In a clocked RS flip-flop, the output waveforms Q and Q' will be based on the inputs S (Set) and R (Reset), synchronized with the CLK (clock) signal. The output behavior is as follows:

• When S is high (1) and R is low (0), Q is set to 1.
• When S is low (0) and R is high (1), Q is reset to 0.
• When both S and R are low, Q retains its previous state.
• When both S and R are high, it's considered an invalid state for an RS flip-flop.

The truth table for the clocked J-K flip-flop can be constructed as:

A three-bit parallel adder consists of three full adders, each responsible for adding corresponding bits from two three-bit numbers, including carry from the previous addition. The connections are as follows:

• Let A = A2A1A0 and B = B2B1B0 be the two three-bit numbers to be added.
• The output will be S = S2S1S0 and a carry-out Cout.
• Each full adder uses:
  • A to add the bits A0 + B0 with Carry-In (Cin for the least significant bit).
  • The next bits A1 + B1 will have the Carry-Out from the previous full adder as their Carry-In, and so forth for A2 + B2.

Two types of counters commonly used in digital electronics include:

1. Synchronous counters: All flip-flops are driven by a common clock signal, allowing for simultaneous counting.
2. Asynchronous counters (Ripple counters): The flip-flops are not clocked simultaneously; instead, the output of one flip-flop acts as the clock input for the next.

The difference between combinational logic circuits and sequential logic circuits lies in how they function based on input:

• Combinational Logic Circuits: These circuits produce output based solely on the current inputs without any memory of past inputs. They utilize basic elements like AND, OR, and NOT gates.
• Sequential Logic Circuits: These circuits have memory elements (like flip-flops) that store the state, and their outputs depend not only on the present inputs but also on past states. They can be used to create memory, counters, and more complex behaviors.

The counter in FIGURE 4.12 is identified as a three-bit synchronous down counter. This counter decreases its count with each clock pulse.

The truth table for a three-bit down counter, which counts from 7 to 0, is as follows:

TWO types of shift registers used in digital circuits include:

1. Parallel-in: Parallel-out Shift Register (PIPO): This type allows data to be loaded and output in parallel simultaneously.
2. Serial-in: Parallel-out Shift Register (SIPO): Data is shifted in serially and can be output in parallel.

In FIGURE 4.14:

• A is labeled as Serial data.
• B is labeled as Clock.

The operation of the serial-in: serial-out shift register involves data being shifted in one bit at a time, usually with a clock pulse. The data enters from the left, moving from one flip-flop to the next on each clock edge. This allows for a sequence of bits to be loaded through a single input line and output through a single output line, effectively allowing for data serialization and deserialization.