Question 1

Exclusive-NOR as an equality detector Why is an exclusive-NOR (XNOR) gate also referred to as an equality gate?

Accepted Answer

Introduction / Context: In digital logic, comparators detect when two signals match. The exclusive-NOR (XNOR) gate implements the simplest 1-bit equality function, making it a building block for multi-bit equality comparators. Given Data / Assumptions: XNOR truth table yields 1 when inputs are the same (00 or 11). Opposite inputs (01 or 10) produce 0. Inputs are single-bit logical values. Concept / Approach: Equality means A equals B. The Boolean function for equality is X = A XNOR B. This can also be written as X = (A AND B) OR (NOT A AND NOT B). Step-by-Step Solution: List pairs: (0,0) → 1, (0,1) → 0, (1,0) → 0, (1,1) → 1.Observe that output is 1 exactly when inputs match.Therefore, the device directly tests for equality of two bits.This property generalizes: cascading bitwise XNORs and ANDing the results yields n-bit equality. Verification / Alternative check: Algebraic form X = A*B + A'*B' matches the equality condition because each product term covers one of the equal-input cases. Why Other Options Are Wrong: The output is false if the inputs are equal: This describes XOR, not XNOR. The output is true if the inputs are opposite: Also describes XOR, not XNOR. Common Pitfalls: Confusing XOR and XNOR symbols or names, and forgetting that the bar (or bubble) on XOR indicates inversion of the XOR result. Final Answer: The output is true if the inputs are equal.

Question 2

Odd parity assignment for data words For each 7- or 8-bit data word, determine the required parity bit P to achieve odd parity: • 1011101 • 11110111 • 1001101

Accepted Answer

Introduction / Context: Parity bits provide simple error detection over a set of data bits. For odd parity, the total number of 1s—including the parity bit—must be odd. This question asks you to compute the parity bit P for three separate data words. Given Data / Assumptions: Words: 1011101, 11110111, 1001101. Odd parity required: total count of 1s (data + P) must be odd. We append one bit P to each word independently. Concept / Approach: Count the number of 1s in each data word. If the count is already odd, set P = 0. If the count is even, set P = 1 to make the total odd. This follows directly from parity definition and requires only a popcount of each word. Step-by-Step Solution: Word 1: 1011101 has 1s at positions 1,3,4,5,7 → 5 ones (odd) → P = 0.Word 2: 11110111 has ones count 7 (odd) → P = 0.Word 3: 1001101 has ones count 4 (even) → P = 1.Therefore results: P = 0, P = 0, P = 1. Verification / Alternative check: Add P and recount: Word 1 total ones 5, Word 2 total ones 7, Word 3 total ones 5 → all odd, confirming correctness. Why Other Options Are Wrong: P = 1, 1, 0 and P = 1, 1, 1: Do not satisfy odd parity for the words with already-odd one counts. P = 0, 0, 0: Fails for the third word which has an even number of ones and requires P = 1. Common Pitfalls: Miscounting ones, assuming the same parity bit for all words, or forgetting that each word’s parity is computed independently. Final Answer: P = 0, P = 0, P = 1

Question 3

Identify the equality gate Which logic gate outputs HIGH when both inputs are HIGH or both inputs are LOW (i.e., when the inputs are equal)?

Accepted Answer

Introduction / Context: Digital systems frequently need to compare two signals. The gate that outputs HIGH when the inputs are the same implements equality at the 1-bit level and is commonly used in comparators and parity circuits. Given Data / Assumptions: Two binary inputs A and B. We seek a gate that asserts when A equals B (00 or 11). Concept / Approach: The exclusive-NOR (XNOR) function is 1 exactly when inputs match. Boolean form: X = A*B + A'B'. This is the complement of XOR, which is 1 when inputs differ. Step-by-Step Solution: Evaluate truth table pairs: 00 → 1, 01 → 0, 10 → 0, 11 → 1.This matches the equality condition.Therefore, the equality gate is XNOR.Other common gates (OR, NAND) do not meet this pattern. Verification / Alternative check: Construct XNOR from simpler gates: two ANDs (AB and A'*B') feeding an OR implements equality, confirming the behavior. Why Other Options Are Wrong: OR gate: Produces 0 only for 00; does not assert for 11 exclusively. Ex-OR gate: Asserts when inputs differ (01 or 10). NAND gate: Produces 0 only for 11; not an equality detector. Common Pitfalls: Confusing XOR with XNOR because of similar names; remember XNOR is the complemented XOR and corresponds to equality. Final Answer: Ex-NOR gate

Question 4

Parity system basics Parity systems are defined as either ________ or ________, and they add an extra ________ to the digital information being transmitted.

Accepted Answer

Introduction / Context: Parity is a simple and widely used error-detection technique. Depending on whether the system uses odd or even parity, one extra bit is appended to ensure the total number of 1s in a word is odd or even, respectively. Given Data / Assumptions: Digital data words are sequences of bits. We add exactly one parity bit per word. Two common schemes: odd parity and even parity. Concept / Approach: In even parity, the parity bit is chosen so that data+parity contains an even count of 1s. In odd parity, it is chosen so the count is odd. This single-bit redundancy allows detection of all single-bit errors within a word (and some multi-bit patterns depending on the error model). Step-by-Step Solution: Compute the number of 1s (popcount) in the data word.If even parity: set P = 0 for even counts; P = 1 for odd counts.If odd parity: set P = 1 for even counts; P = 0 for odd counts.Append this parity bit to the data and transmit/stored the composite word. Verification / Alternative check: After adding parity, recount the ones. For even parity, the total must be even; for odd parity, total must be odd. Any single-bit flip will toggle the total's parity and thus be detected. Why Other Options Are Wrong: positive, negative, byte: Parity is not described by polarities and does not add a whole byte. upper, lower, digit: Not relevant terminology for parity. on, off, decimal: Mixed and incorrect concepts unrelated to parity bits. Common Pitfalls: Confusing parity with checksums or CRCs, or believing parity can correct (it only detects) single-bit errors. Final Answer: odd, even, bit

Question 5

Generating odd vs even parity How is an odd-parity bit generated differently from an even-parity bit for the same data word?

Accepted Answer

Introduction / Context: Odd and even parity differ only in the chosen value of the parity bit for a given data word. Understanding the simple relationship between them helps when designing parity-generation hardware using XOR networks. Given Data / Assumptions: Parity bit is a single bit appended to the data word. Even parity requires an even total count of 1s; odd parity requires an odd total. Even-parity bit can be formed as the XOR (mod-2 sum) of all data bits. Concept / Approach: Because the even-parity bit equals XOR of the data bits, the odd-parity bit is simply the inversion of that result. Hardware-wise, you can generate even parity with a tree of XOR gates and then pass it through a NOT gate to obtain odd parity. Step-by-Step Solution: Let Peven = XOR of all data bits.For odd parity, set Podd = NOT(Peven).Append the chosen parity bit to the data word as required by the protocol.Verify by counting ones: data + Peven → even total; data + Podd → odd total. Verification / Alternative check: Try a sample word with an odd number of 1s. Even parity produces 1 to make the total even; inverting yields 0 for odd parity, preserving the odd total. The complement relationship holds for all words. Why Other Options Are Wrong: The first output is inverted: There is no notion of a “first” output; parity is a single bit. Use AND in place of XOR: AND does not produce a parity (mod-2) sum. Double the parity field: Parity uses one bit, not two. Common Pitfalls: Confusing parity generation with majority or checksum logic, or forgetting that XOR implements addition modulo 2 across bits. Final Answer: The parity bit is the logical NOT of the even-parity bit (invert the even-parity output).

Question 6

Using XOR as a controlled inverter Show from the truth table how an exclusive-OR (XOR) gate can invert the data on one input when the other input acts as a control.

Accepted Answer

Introduction / Context: The XOR function is invaluable for conditional inversion. Treating one input as a control line turns XOR into a 1-bit multiplexer between “pass-through” and “invert” behaviors, used in arithmetic units and data encoding. Given Data / Assumptions: XOR definition: X = A XOR B = 1 when inputs differ, 0 when equal. Let A be the control input; B is the data input. We want X to equal B when A = 0, and X to equal NOT B when A = 1. Concept / Approach: From the XOR truth table: X = A XOR B = (NOT A AND B) OR (A AND NOT B). This directly encodes pass-through for A = 0 and inversion for A = 1. Step-by-Step Solution: If A = 0: X = 0 XOR B = B → pass-through.If A = 1: X = 1 XOR B = NOT B → inversion.Truth-table rows: (A,B) = (0,0)→0, (0,1)→1 (same as B); (1,0)→1, (1,1)→0 (inverted).Therefore XOR acts as a controlled inverter with A as the control. Verification / Alternative check: Algebraic rewrite: X = B XOR A = B when A = 0; X = NOT B when A = 1. Hardware implementations of adders use this to conditionally complement operands. Why Other Options Are Wrong: Same as B for both A values: Ignores XOR behavior; that would be a buffer. Inverse when A = 0 and same when A = 1: Reversed from XOR’s property. Inverse for both A values: That would be a pure inverter, not controlled. Common Pitfalls: Swapping the role of control and data, or confusing XOR with XNOR (which is equality, not inversion control). Final Answer: Using A as the control, when A = 0, X is the same as B. When A = 1, X is the inverse of B.

Question 7

Identify “one-or-the-other” logic Which logic circuit outputs HIGH if one input or the other input, but not both, is HIGH?

Accepted Answer

Introduction / Context: Sometimes we require a signal that indicates exclusivity: exactly one of two conditions is true. The exclusive-OR (XOR) gate implements this function at the 1-bit level and is widely used in arithmetic and parity generation. Given Data / Assumptions: Two inputs A and B. Desired behavior: X = 1 when A ≠ B; X = 0 when A = B. Concept / Approach: The XOR truth table is 1 for input pairs 01 and 10, and 0 for 00 and 11. This matches “one or the other, but not both.” Step-by-Step Solution: List cases: A=0,B=0 → X=0; A=0,B=1 → X=1.A=1,B=0 → X=1; A=1,B=1 → X=0.Therefore XOR provides the exclusive condition.Other common gates (OR, NAND, XNOR) do not satisfy exclusivity. Verification / Alternative check: Boolean identity: X = A XOR B = (A + B) * (NOT(A*B)). It is OR gated with the complement of AND, ensuring not both. Why Other Options Are Wrong: Ex-NOR: True for equality (both same), opposite of exclusivity. OR: True for 11 as well, violating “not both.” NAND: False only when both are 1; not the exclusive function. Common Pitfalls: Assuming OR implies exclusivity; OR includes the both-true case, whereas XOR excludes it. Final Answer: Ex-OR gate

Question 8

Error detection in digital communication — understanding parity checking Which of the following statements best describes the parity method of error detection used during code transmission between two locations?

Accepted Answer

Introduction / Context: Parity checking is a classic, low-cost error-detection technique widely used in memory systems and serial links. It appends one extra bit (parity bit) to a data word so that the total number of 1s becomes either even (even parity) or odd (odd parity). This question tests whether you know what kind of errors parity can reliably detect and what it cannot. Given Data / Assumptions: The channel might flip bits during transmission. Only a single parity bit is added to each data word. No additional redundancy (like CRC or ECC) is present. Concept / Approach: Parity adds one bit to enforce a parity rule. If exactly one bit in the data word flips, the total number of 1s no longer satisfies the rule and the receiver flags an error. However, if two bits flip, the parity may still appear correct (an even number of errors can cancel out), so parity cannot reliably detect double-bit errors, nor can it locate or correct any error by itself. Step-by-Step Solution: 1) Define parity: add 1 bit so that total count of 1s is even (even parity) or odd (odd parity).2) Consider a single-bit error: parity changes from valid to invalid → detection succeeds.3) Consider a double-bit error: two flips can keep parity valid → detection may fail.4) Correction requires locating the flipped bit(s); a single parity bit cannot do this. Verification / Alternative check: Try a concrete example: with even parity, data 1011 (three 1s) gets parity 1 to make total 4 (even). If one bit flips, total becomes odd → error detected. If two bits flip, total returns to even → error can slip through. Why Other Options Are Wrong: Double-bit detection (option a): parity cannot guarantee detection of two errors. Not suitable for single-bit errors (option b): the opposite is true; it is ideal for single-bit error detection. Detecting and correcting (option d): correction needs codes like Hamming; parity only detects. Common Pitfalls: Confusing detection with correction; assuming parity works for all multi-bit errors; forgetting that parity gives zero information about which bit flipped. Final Answer: Parity checking is best suited for detecting single-bit errors in transmitted codes.

Question 9

Naming the EX-NOR logic function The Exclusive-NOR (EX-NOR) gate is sometimes also referred to by which descriptive names in digital logic?

Accepted Answer

Introduction / Context: Logic functions often have descriptive aliases that highlight their behavior. The EX-NOR gate outputs HIGH when its inputs are the same, making it valuable in comparators and parity checking. Recognizing these common names helps when reading datasheets and design notes from different vendors. Given Data / Assumptions: We are discussing the EX-NOR (Exclusive-NOR) operation. Two-input case is the most common reference for naming. Terminology should reflect the function's behavior. Concept / Approach: For a two-input EX-NOR, the output is 1 if inputs are equal (00 or 11) and 0 if they differ (01 or 10). Because it reports equality, it is often called an equality gate. In parity circuits, an EX-NOR outputs 1 when an even number of inputs are 1s (for two inputs, 0 or 2 ones), matching an even-parity detector—hence the name parity gate. Step-by-Step Solution: 1) Write truth table pairs: 00 → 1, 01 → 0, 10 → 0, 11 → 1.2) Observe that output is HIGH when inputs are the same → equality behavior.3) Note that the count of ones is even at HIGH outputs → parity relation.4) Therefore, common aliases are equality gate and parity gate. Verification / Alternative check: Compare to XOR (EX-OR), which outputs 1 when inputs differ; EX-NOR is exactly the complement, reinforcing the equality interpretation. Why Other Options Are Wrong: Inverted OR: that is a NOR gate, not EX-NOR. Singular names (only parity or only equality) are partially correct, but the most complete name includes both common aliases. Common Pitfalls: Confusing EX-NOR with NOR because of the “NOR” syllable; remember EX-NOR is the complement of XOR, not of OR. Final Answer: parity gate or the equality gate

Question 10

Adding two single bits using logic Which type of logic gate directly produces the sum output when adding two one-bit operands (ignoring the carry)?

Accepted Answer

Introduction / Context: Binary addition of two single bits (A and B) produces a sum and a carry. In half adders and full adders, distinct logic functions generate these outputs. Knowing which gate corresponds to the sum output is a foundational digital design skill. Given Data / Assumptions: Operands: A and B are 1-bit values. We are asking only about the sum output, not the carry. Standard gate-level realization is assumed. Concept / Approach: The truth table for the binary sum without carry is: A B → Sum: 00→0, 01→1, 10→1, 11→0. This exactly matches the Exclusive-OR (XOR, written Ex-OR) function. The carry-out for a half adder is A * B (AND), but that is not requested here. Step-by-Step Solution: 1) Write sum truth table: 00→0, 01→1, 10→1, 11→0.2) Compare with XOR truth table: identical behavior.3) Therefore, the gate that outputs the bitwise sum is Ex-OR.4) Recognize carry is separate (AND), confirming XOR is specific to sum. Verification / Alternative check: Construct a half adder: Sum = A XOR B, Carry = A AND B. Simulation or logic-equation derivation yields the same mapping. Why Other Options Are Wrong: Ex-NOR: outputs 1 when inputs are equal (00 or 11) → not the sum. Ex-NAND: not a standard primitive function for addition. NOR: outputs 1 only when both inputs are 0. Common Pitfalls: Confusing the roles of XOR and AND in a half adder; remember XOR is for sum, AND for carry. Final Answer: Ex-OR

Question 11

Exclusive-NOR (XNOR) gate terminology: The statement “The Ex-NOR is sometimes called the equality gate” is ________.

Accepted Answer

Introduction / Context: Logic gate nicknames often reflect their truth-table behavior. The exclusive-NOR (XNOR) gate outputs a HIGH when its inputs are equal (both 0 or both 1). Because equality is exactly the condition it detects, XNOR is widely referred to as the “equality gate.” This question verifies recognition of that standard terminology. Given Data / Assumptions: Two-input XNOR considered (extensions to more inputs follow parity-like rules). Logic HIGH indicates a match between inputs. Standard positive-logic conventions apply. Concept / Approach: XNOR is the logical complement of XOR. XOR is HIGH when inputs differ; therefore, XNOR is HIGH when inputs are the same. This is equivalent to a digital equality test for two bits, thus the nickname “equality gate.” Step-by-Step Solution: Write XOR truth table: 00→0, 01→1, 10→1, 11→0.Complement to get XNOR: 00→1, 01→0, 10→0, 11→1.Observe that outputs are HIGH for equal pairs (00 and 11).Conclude that calling XNOR the equality gate is accurate. Verification / Alternative check: Implement equality test using basic gates: (A AND B) OR (not A AND not B) equals XNOR. This composite also returns HIGH when A equals B, reinforcing the equality behavior. Why Other Options Are Wrong: Incorrect: Conflicts with the truth-table definition. Ambiguous/only for three-input/only for CMOS: The equality interpretation is independent of logic family and holds for the canonical two-input case. Common Pitfalls: Confusing XOR with XNOR; overlooking that “exclusive” in XOR means “one or the other but not both.” Remember: XNOR = equal → HIGH; XOR = different → HIGH. Final Answer: Correct

Question 12

Parity overhead: “The odd/even parity system would require a sixth bit to be added to a 4-bit data word.” This statement is ________.

Accepted Answer

Introduction / Context: Parity adds a single bit to a data word to enable simple single-bit error detection. The item claims that a 4-bit word would need two extra bits (making “a sixth bit”), which overstates the overhead. Understanding how parity is formed clarifies the correct number of added bits. Given Data / Assumptions: Data word length: 4 bits. Parity scheme: either odd or even parity (both use one parity bit). No mention of multi-dimensional parity or longitudinal redundancy checks. Concept / Approach: In classic parity systems, exactly one parity bit is appended to any length of data word to force the total count of 1s to be odd (odd parity) or even (even parity). Thus, an n-bit word becomes n+1 bits. For a 4-bit word, total length becomes 5 bits, not 6. Step-by-Step Solution: Start with 4 data bits: d3 d2 d1 d0.Compute parity bit p so that count(1s in d3..d0 plus p) matches the selected parity (odd/even).Transmit/store 5 bits total: p d3 d2 d1 d0 (order may vary by system).Note that only one extra bit is used. Verification / Alternative check: Look at standard UART framing: a user may enable 1 parity bit in addition to data bits. There is never a doubling to two parity bits for basic odd/even parity in such systems. Why Other Options Are Wrong: Correct/only for odd parity: Both odd and even parity use one additional bit. Two stop bits: Stop bits are framing elements, not parity; they do not change the single parity-bit rule. LRC instead of parity: LRC is a different checksum mechanism; the claim concerns simple parity only. Common Pitfalls: Confusing parity with checksums/CRCs; assuming one parity bit per nibble or per byte; misinterpreting stop bits as parity bits. Final Answer: Incorrect

Question 13

Exclusive-OR (XOR) behavior: “In an exclusive-OR gate, both inputs cannot be HIGH if the output is to be HIGH.” This statement is ________.

Accepted Answer

Introduction / Context: An XOR gate outputs HIGH when its inputs are different and LOW when they are the same. Therefore, if both inputs are HIGH simultaneously, the XOR output is LOW. This question checks accurate recall of the XOR truth table and the meaning of “exclusive.” Given Data / Assumptions: Two-input XOR under positive logic. Ideal logic behavior (no timing hazards considered). Interest in the condition for a HIGH output. Concept / Approach: By definition, XOR = 1 when A ≠ B and 0 when A = B. Hence, for A = 1 and B = 1, A = B, so output is 0. The statement “both inputs cannot be HIGH to get a HIGH output” aligns with this rule, and is therefore correct. Step-by-Step Solution: Write XOR truth table: 00→0, 01→1, 10→1, 11→0.Identify HIGH-output rows: 01 and 10 (inputs differ).Check 11: outputs 0, so both HIGH does not yield HIGH.Conclude the statement is correct. Verification / Alternative check: Boolean identity: XOR can be written as (A AND not B) OR (not A AND B). Plugging A = 1, B = 1 yields 0, confirming the truth table result. Why Other Options Are Wrong: Incorrect/conditional variants: XOR’s behavior is logic-family independent and frequency independent in the ideal truth table sense. Common Pitfalls: Confusing XOR with OR; misremembering that OR is HIGH for 11 while XOR is LOW for 11; overlooking that “exclusive” means “one or the other, but not both.” Final Answer: Correct

Question 14

CPLD design workflow: “Using a CPLD (Complex Programmable Logic Device) design environment, we can simulate arbitrary input combinations and observe the resulting outputs to verify correct operation.” This statement is ________.

Accepted Answer

Introduction / Context: Modern digital design flows rely on simulation before hardware programming. CPLD and FPGA toolchains include functional and timing simulators that let engineers test logic with exhaustive or targeted input vectors. The question checks understanding that simulation is a standard, integral step of CPLD design verification. Given Data / Assumptions: Design described in HDL or schematic. Design environment includes a simulator. Goal: verify logical correctness prior to device programming. Concept / Approach: Simulation lets you apply input stimuli and analyze expected outputs using waveforms, truth tables, and assertions. You can perform both functional simulations (logic only) and, with device models, timing simulations (including propagation delays). This process reduces hardware debug time and catches logic errors early. Step-by-Step Solution: Create or import the CPLD design (e.g., VHDL/Verilog/AHDL/schematic).Write a testbench to generate comprehensive input vectors.Run the simulator and inspect output waveforms and assertions.Refine the design until outputs match specifications, then synthesize and program the device. Verification / Alternative check: Most vendor tools (e.g., from Intel/Altera, Lattice, Microchip/Microsemi) include simulators or integrate with third-party tools. Simulation-based signoff is a standard milestone before hardware bring-up. Why Other Options Are Wrong: Incorrect/Only for PALs/Only after programming: Simulation occurs in software without needing hardware programmed. Only without timing models: Timing simulation is also common when device-specific models are available. Common Pitfalls: Skipping simulation to “test on hardware,” leading to longer debug cycles; failing to include corner cases in the testbench; ignoring timing closure and constraints which can change behavior post-synthesis. Final Answer: Correct

Question 15

Parity systems: “In a parity generator circuit, an error is signaled on an error indicator.” This statement is ________.

Accepted Answer

Introduction / Context: Parity subsystems often include both generation and checking functions, sometimes within the same integrated circuit. However, the roles are distinct: the generator creates a parity bit from data; the checker compares received data and parity to detect errors. This question probes whether you can distinguish those responsibilities. Given Data / Assumptions: Parity generator: Computes parity bit from local data (transmit side). Parity checker: Examines incoming data plus parity to detect mismatches. Some ICs combine both functions, but the logic blocks remain conceptually separate. Concept / Approach: Only the checker asserts an error indicator because it performs a comparison between expected parity (recomputed from received data) and the received parity bit. A pure generator does not perform comparison and therefore has no basis to assert an error flag. Confusion arises because catalog parts may be titled “parity generator/checker.” Step-by-Step Solution: On transmit: generator computes p so data+p has the required parity.On receive: checker recomputes parity p′ from data and compares against received p.If p′ ≠ p, checker raises an error indicator.Therefore, a generator alone has nothing to compare and cannot signal an error. Verification / Alternative check: Examine block diagrams of common parts (e.g., parity generator/checker devices): separate logic blocks are shown for generation and checking, with the error output present on the checker side only. Why Other Options Are Wrong: Correct/only for even parity/with specific frames/with CRC: All imply a generator signals errors, which it does not. CRC is unrelated to a simple parity generator’s function. Common Pitfalls: Assuming “generator” and “checker” are interchangeable; overlooking that error detection requires comparison between expected and actual parity, which a generator does not perform. Final Answer: Incorrect

Question 16

Availability of standard parts: “Parity generator and parity checker circuits are available as single IC packages.” This statement is ________.

Accepted Answer

Introduction / Context: Classic small- and medium-scale integration (SSI/MSI) catalogs include standard building blocks for common digital functions, including parity generation and checking. Recognizing the availability of these functions as off-the-shelf ICs is useful in both legacy maintenance and educational labs. Given Data / Assumptions: Parity generator: computes parity bit from data. Parity checker: detects parity mismatches on received data. Commercial logic families (TTL, CMOS) provide such functions as fixed packages. Concept / Approach: Manufacturers have long offered parity generator/checker chips. While part numbers vary by family, their presence in logic catalogs demonstrates that both functions exist as single ICs. Some chips even combine generation and checking in the same package for convenience. Step-by-Step Solution: Identify the function needed (generation, checking, or both).Consult standard logic catalogs for corresponding MSI devices.Note that these devices interface easily with data buses and UARTs.Use them where simple, fast parity functions are required without programmable logic. Verification / Alternative check: Datasheets and textbooks list parity generator/checker ICs in common TTL/CMOS lines, confirming availability as single, dedicated parts. In modern designs, equivalent functionality can also be done in programmable logic, but the discrete ICs still exist for specific needs. Why Other Options Are Wrong: Incorrect/logic-family limitations: Availability does not hinge on a particular high-voltage family or exotic logic like ECL; standard TTL/CMOS offer such parts. Common Pitfalls: Believing parity must require a microcontroller or CPLD; overlooking simple MSI solutions; conflating parity with more complex error-correcting codes which indeed require more logic. Final Answer: Correct

Question 17

Digital communications and noise immunity: Consider the statement: “Electrical noise does not affect the transmission of binary information.” Evaluate the statement based on noise margins, bit errors, and practical digital links.

Accepted Answer

Introduction / Context: Digital systems use two discrete logic levels to represent bits, often labeled 0 (LOW) and 1 (HIGH). While this representation provides tolerance against minor disturbances, electrical noise can still perturb signals, cause timing violations, and create bit errors. The statement claims noise does not affect binary transmission; this is an oversimplification and is therefore incorrect. Given Data / Assumptions: Binary signaling relies on defined thresholds (VIL/VIH) and noise margins. Real interconnects and channels introduce noise, crosstalk, attenuation, and jitter. Error detection (parity, checksum, CRC) and sometimes error correction are used because errors can occur. Concept / Approach: Noise can shift instantaneous voltage levels or timing such that a receiver samples the wrong level. Even with generous noise margins, sufficiently large or frequent disturbances lead to bit flips. Engineering mitigates this with impedance control, shielding, filtering, proper rise/fall times, and coding with redundancy, but the residual error probability is never exactly zero. Step-by-Step Solution: Identify the claim: “Noise does not affect binary transmission.”Recall that noise can push a signal past VIH/VIL thresholds or distort edges.Conclude that errors are possible (non-zero bit error rate), hence the claim is false.Therefore the correct evaluation is: Incorrect. Verification / Alternative check: High-speed buses and serial links specify bit error rates and include integrity features (pre-emphasis, equalization, CRC). The very existence of these features confirms that noise affects transmission outcomes and must be managed, not ignored. Why Other Options Are Wrong: Correct: Ignores well-known error mechanisms.Depends on noise margins only: Margins help but do not guarantee zero errors.Not enough information: Sufficient general knowledge shows the claim is false. Common Pitfalls: Assuming “digital is immune to noise” because it works reliably most of the time; forgetting that thresholds and timing create finite tolerance but not absolute immunity. Final Answer: Incorrect

Question 18

Exclusive-OR behavior (2-input XOR): Assess the statement: “An exclusive-OR provides a LOW output if one input or the other input is HIGH.”

Accepted Answer

Introduction / Context: The exclusive-OR (XOR) gate is fundamental in digital electronics for parity, addition (sum bit), and data comparison. Its defining property is that the output is HIGH when the inputs are different and LOW when they are the same. The given statement claims the output is LOW when exactly one input is HIGH, which contradicts the XOR truth table. Given Data / Assumptions: XOR has two inputs A, B and output X. Definition: X = 1 when A ≠ B; X = 0 when A = B. Standard positive-logic symbols are assumed. Concept / Approach: Check the XOR truth table for the two cases where exactly one input is HIGH: (A, B) = (1, 0) and (0, 1). In both cases, inputs differ, so XOR must output HIGH. Therefore, the statement that it outputs LOW is incorrect. Step-by-Step Solution: Case 1: A = 1, B = 0 → A ≠ B → XOR = 1 (HIGH).Case 2: A = 0, B = 1 → A ≠ B → XOR = 1 (HIGH).Same-input cases (0,0) or (1,1) give LOW. The statement is the opposite of correct behavior. Verification / Alternative check: Use the Boolean form: X = ĀB + AB̄ (read as (NOT A)*B + A*(NOT B)). When exactly one input is 1, one of the two product terms equals 1, so X = 1. Why Other Options Are Wrong: Correct: Contradicts the XOR definition.Only true for active-LOW XOR symbols: Symbol polarity does not invert XOR function, it changes annotation for active levels.Not enough information: Standard XOR behavior is fully defined. Common Pitfalls: Confusing XOR with XNOR or with OR; forgetting XOR outputs HIGH when inputs differ. Final Answer: Incorrect

Question 19

Parity generator vs. parity checker (4-bit system): Evaluate the statement: “A parity checker is constructed like a parity generator, except a 4-bit system needs five inputs (four data + parity), and the single output is used as an error indicator.”

Accepted Answer

Introduction / Context: Parity circuits are standard for low-cost error detection. A parity generator produces a parity bit from data, whereas a parity checker compares received data with the associated parity bit and flags mismatches. For 4-bit data words, a checker must consider all four data inputs plus the received parity input. Given Data / Assumptions: Data width: 4 bits. Parity scheme: even or odd (either works similarly for structure). Parity generator: 4 inputs → 1 parity output. Parity checker: 5 inputs (4 data + 1 parity) → 1 error flag output. Concept / Approach: The same XOR building blocks used to generate parity are used to check parity. A checker XORs all five inputs. For even parity, a correct word yields 0; for odd parity, conventions vary (you may invert the result), but the single output still serves as the error indicator. Thus, the structural similarity and the five-input requirement are accurate. Step-by-Step Solution: Form the XOR of D3, D2, D1, D0 to generate parity in a generator.In a checker, XOR D3, D2, D1, D0, and the received parity P.If the result equals 0 for even parity (or 1 for odd, depending on convention), no error; otherwise, assert error. Verification / Alternative check: Test a valid even parity case: data 1011 has three ones; parity P = 1 to make total 4. XOR(1,0,1,1,1) = 0 → no error. Flip one bit and observe nonzero result → error. Why Other Options Are Wrong: Incorrect: Ignores the five-input requirement for checking.Only true for odd parity: Structure does not depend on parity type; only interpretation changes.Not enough information: The definition is standard and sufficient. Common Pitfalls: Confusing generator and checker roles; misinterpreting checker output polarity; forgetting that the checker consumes both data and parity. Final Answer: Correct

Question 20

XOR notation in Boolean algebra: Assess the statement: “The exclusive-OR operation is denoted by a plus sign with a circle around it in Boolean equations.”

Accepted Answer

Introduction / Context: In symbolic logic and digital design, standard operators use consistent notation across textbooks and datasheets. The exclusive-OR function (XOR) has a distinctive symbol to distinguish it from ordinary OR. Recognizing the correct symbol avoids ambiguity in equations and schematics. Given Data / Assumptions: Standard Boolean operators: + for OR, · for AND, overbar or prime for NOT. XOR has a special symbol to avoid confusion with inclusive OR. We assume positive logic conventions. Concept / Approach: The common notation for XOR is a plus inside a small circle, read as “circle plus,” often written as ⊕. In ASCII text, authors may write “XOR” or use the caret ^ symbol, but the circle-plus is the canonical printed symbol in Boolean algebra. Step-by-Step Solution: Identify the operator in question: exclusive-OR.Recall canonical symbol: ⊕ (circle-plus).Conclude that the statement is accurate. Verification / Alternative check: Check a typical identity: A ⊕ B = ĀB + AB̄. Here ⊕ is the circle-plus notation, confirming the standard usage. Why Other Options Are Wrong: Incorrect: Conflicts with well-established notation.Only valid in arithmetic SOP: The symbol is general for logic, not limited to any form.Not enough information: Symbol conventions are widely standardized. Common Pitfalls: Confusing XOR with OR (+) or with plus in arithmetic; misreading symbols in low-resolution prints. Final Answer: Correct

Ex-OR and Ex-NOR Gates Questions