Q: Equality testing in MSI — definition of a comparator: A logic circuit that determines whether one binary input is equal to another (optionally also indicating greater-than or less-than) is called a comparator. Judge the correctness of this definitio…

Introduction / Context:Magnitude comparators are standard MSI devices (e.g., 74xx85) that evaluate the relationship between two binary words. They report whether A=B, A>B, or A<B, and often allow cascading to larger widths. Recognizing this function is essential when specifying digital decision hardware and control logic.Given Data / Assumptions:Two input words A and B of the same bit-width.Outputs include equality and possibly inequality flags.No arithmetic (sum/difference) is required to decide equality.Concept / Approach:An equality comparator can be implemented by XNORing each bit of A and B, then ANDing all XNOR results to produce an A=B signal. MSI comparators add dedicated outputs for A>B and A B, A=B, A B,” “A=B,” and “A<B,” verifying the definition. HDL synthesis of equality comparators follows the same logic with XNOR-and-AND structures.Why Other Options Are Wrong:“Incorrect” contradicts standard device naming. Restricting correctness to adders or mismatched widths is wrong; comparators are independent blocks, and equal-width inputs are assumed by specification.Common Pitfalls:Feeding unequal widths without zero/ sign extension; misunderstanding that subtraction is unnecessary for equality detection.Final Answer:Correct

Question 1

Function of a demultiplexer – does a demultiplexer convert a binary or other code into a recognizable number or character for display, or is that the role of a decoder/driver?

Accepted Answer

Introduction / Context: Digital subsystems often include blocks with similar names but different roles. Demultiplexers and decoders are frequently confused. This question distinguishes a demultiplexer (DEMUX) from a decoder/driver that generates segment patterns for displays or translates codes into human-recognizable output.Given Data / Assumptions:A demultiplexer routes a single input to one of many outputs based on select lines.A decoder maps binary codes to one-hot outputs or to display segments.Display drivers (for example, BCD-to-7-segment) convert numeric codes to segment patterns.Concept / Approach: A DEMUX is a data-routing device: 1 input → N outputs, with only one output active at a time. It does not “interpret” the input data as a code. A decoder accepts an n-bit code and asserts exactly one of 2^n outputs, or generates a pattern (as in 7447/7448). Therefore, converting a code into a recognizable number or character is the job of a decoder/driver, not a demultiplexer.Step-by-Step Solution:1) Define DEMUX: controlled routing of a single input signal to one of many outputs.2) Define decoder: code-to-one-hot or code-to-pattern translation.3) For display generation, use dedicated decoder/driver ICs.4) Conclude DEMUX does not perform code conversion to display characters.Verification / Alternative check: 74HC238 (decoder) and 74HC154 (decoder) translate binary inputs to one-of-N outputs; 74HC4051 (analog MUX/DEMUX) simply routes signals and does not “recognize” codes beyond selection.Why Other Options Are Wrong: Marking “Correct” confuses roles; references to BCD or 7447 drivers are about decoders, not demultiplexers; common-anode vs common-cathode pertains to 7-seg wiring, not DEMUX function.Common Pitfalls: Assuming any device with select lines does decoding of data content; DEMUX only uses select lines to choose a path.Final Answer: Incorrect

Question 2

Demultiplexer behavior – does a demultiplexer take a single input signal and “turn it into a binary number,” or does it distribute that signal to one of many outputs based on select lines?

Accepted Answer

Introduction / Context: The names “encoder,” “decoder,” “multiplexer,” and “demultiplexer” can be confusing. This question clarifies the specific function of a demultiplexer (DEMUX) and whether it performs numeric code generation from an input signal.Given Data / Assumptions:DEMUX has one data input, multiple outputs, and select inputs.An encoder generates a binary number from one-of-N active inputs.A decoder translates a binary code into one-of-N outputs or display segments.Concept / Approach: A DEMUX routes a single input to one selected output among many, as determined by the select lines. It does not analyze the content of the input to produce a binary number; that function belongs to an encoder. Therefore, the claim that a DEMUX “turns an input into a binary number” is incorrect. Instead, a DEMUX is essentially the inverse of a multiplexer with respect to routing, not coding.Step-by-Step Solution:1) Identify DEMUX connections: one input D, outputs Y0..YN-1, selects S.2) Operation: Yi = D when S selects i; other outputs inactive.3) No code generation occurs; the signal is merely distributed.4) For generating a binary number from lines, use an encoder IC.Verification / Alternative check: Look at truth tables: 1-of-N DEMUX truth tables show output selection, while encoder truth tables map one-hot inputs to binary codes—two distinct behaviors.Why Other Options Are Wrong: “Correct” confuses DEMUX with encoder; references to “active-low” or specific device counts do not change the fundamental routing function.Common Pitfalls: Misapplying DEMUX in applications that require code generation or decoding; ensure you select the right primitive for the job.Final Answer: Incorrect

Question 3

BCD-to-7-segment 7447 outputs – if segment outputs a, b, c, d, and g are asserted (active), which digit appears on the display, and is it 5 as claimed?

Accepted Answer

Introduction / Context: Seven-segment displays use segments labeled a through g to form numerals. The 7447 is a BCD-to-7-segment decoder/driver whose outputs correspond to these segments (with active-low drive for common-anode displays). This question checks whether lighting segments a, b, c, d, and g produces the numeral 5 or another digit.Given Data / Assumptions:Segments are labeled a (top), b (upper-right), c (lower-right), d (bottom), e (lower-left), f (upper-left), g (middle).“Outputs active” means those segments are illuminated (active-low driver like 7447 pulls them on).Set considered: a, b, c, d, g are on; e and f are off.Concept / Approach: Draw or visualize the 7-segment layout. With a, b, c, d, and g illuminated, the display shows the top bar, both right-side bars, the bottom bar, and the middle bar. That pattern corresponds to the numeral 3. In contrast, the digit 5 requires a, f, g, c, and d—note that f (upper-left) is on and b is off for 5. Therefore, the statement claiming the displayed number is 5 is incorrect; it is actually 3.Step-by-Step Solution:1) Enumerate segments for 3: a, b, c, d, g (e = off, f = off).2) Enumerate segments for 5: a, f, g, c, d (b = off, e = off).3) Compare to given set: matches digit 3 exactly.4) Conclude the claim “displays 5” is false.Verification / Alternative check: Reference segment maps in data sheets or standard charts; active-low nature of 7447 does not alter which segments must be lit—only the logic level required to light them.Why Other Options Are Wrong: 0 lacks g; 7 lacks g and d; 9 requires f (upper-left) which is not active here; “5 as stated” conflicts with the actual segment pattern.Common Pitfalls: Forgetting which side segments b and f occupy; mixing common-cathode vs common-anode drive with the visual pattern.Final Answer: The digit is 3, not 5

Question 4

Hardware description languages – does AHDL (Altera Hardware Description Language) lack a multiplication operator, or does it support “*” for arithmetic in synthesizable expressions?

Accepted Answer

Introduction / Context: AHDL (often mistyped as ADHL) is a hardware description language historically used with Altera (now Intel) programmable logic devices. This question addresses whether the language supports a multiplication operator, which is relevant when describing arithmetic circuits or parameter calculations.Given Data / Assumptions:We refer to AHDL language constructs intended for synthesis onto PLDs/FPGAs.Arithmetic operators are used within combinational or registered logic descriptions.Tool support maps high-level operations to hardware resources (LUTs, DSP blocks, etc.).Concept / Approach: AHDL provides standard arithmetic operators, including addition (+), subtraction (−), and multiplication (*). Synthesis tools translate “*” into appropriate hardware: LUT-based multipliers for small widths, or dedicated DSP/multiplier blocks when available on the target device. While designers may sometimes prefer shift–add techniques for area or timing reasons, the language itself does not forbid multiplication; it offers a direct operator to describe the intended behavior.Step-by-Step Solution:1) Identify the required arithmetic: x * y.2) Express it in AHDL using the “*” operator within a combinational or clocked assignment.3) Constrain synthesis (if needed) to map to DSP blocks for efficiency.4) Verify timing/resource usage in the fitter/map reports.Verification / Alternative check: Vendor documentation and example designs show multiply operations and parameterizable multipliers described directly in AHDL and inferred by tools.Why Other Options Are Wrong: Claiming no multiplication operator contradicts the language feature set; limiting to simulation only is incorrect—synthesis supports it. Division-only or mandatory LUT tables are misconceptions about implementation choices, not language capability.Common Pitfalls: Assuming that because older CPLDs lacked dedicated multipliers, the language banned “*”; synthesis may still implement small multiplies using logic resources if DSP blocks are absent.Final Answer: AHDL supports “*” (multiplication) in arithmetic expressions

Question 5

Definition check – is a multiplexer (MUX) the device that converts a code into a recognizable number or character for display, or is that the specific function of a decoder/driver?

Accepted Answer

Introduction / Context: Multiplexers and decoders are foundational MSI logic blocks but serve different roles. The question tests whether a multiplexer is the element that performs code-to-symbol translation for human-readable outputs such as seven-segment displays, or whether that is the decoder’s job.Given Data / Assumptions:Multiplexer: selects one of many data inputs based on select lines; data routing element.Decoder/driver: translates coded inputs (for example, BCD) into a specific pattern (for example, seven-segment).Recognizable symbols require mapping logic, not mere routing.Concept / Approach: A MUX does not interpret codes; it forwards the selected input to the output. In contrast, a BCD-to-7-segment decoder (such as 7447/7448) contains logic that asserts specific segment lines to draw digits 0–9. Therefore, stating that a MUX converts a code to a number or character is incorrect; that function belongs to decoders and display drivers.Step-by-Step Solution:1) Identify need: map input code to a segment/character pattern → requires decoding logic.2) Recognize MUX role: choose among inputs; no code translation.3) Use 7447-like devices or FPGA logic for display encoding.4) Conclude the statement about MUX as code converter is false.Verification / Alternative check: Examine truth tables: MUX tables specify selection behavior; decoder tables map binary inputs to one-of-N or to segment outputs—explicit code conversion.Why Other Options Are Wrong: Marking “Correct” confuses device categories; BCD specifics, enable pins, and display polarity (common-anode/cathode) do not convert a MUX into a decoder.Common Pitfalls: Using a MUX to multiplex display digits over time (time-multiplexing) and mistaking that mechanism for code translation; time-multiplexing is about sharing a driver, not generating segment patterns.Final Answer: Incorrect

Question 6

MSI logic concept — multiplexer behavior: In medium-scale integration (MSI) digital design, a multiplexer (MUX) is a data selector that routes exactly one of several input lines to a single output line under the control of select signals. Evaluate t…

Accepted Answer

Introduction / Context:A multiplexer (often abbreviated as MUX) is one of the most widely used MSI building blocks. It appears in datapaths, bus interconnects, arithmetic logic units, and test access paths. The defining purpose of a MUX is selection: many inputs, one output, with select lines choosing which input is forwarded. This question checks your understanding of that core definition and distinguishes it from related components such as demultiplexers, encoders, and bus transceivers.Given Data / Assumptions:A device has multiple data inputs (e.g., D0…D7).It provides a single data output (Y).Control (select) inputs (e.g., S2 S1 S0) choose which Di connects to Y.Enable pin, if present, gates the function on or off.Concept / Approach:By definition, a multiplexer performs one-of-N selection. For a 2^n-to-1 MUX, there are n select lines. Internally, logic gates steer exactly one input to the output. The function can be viewed as a truth table where Y equals Di when S encodes i. This is distinct from a demultiplexer (1-to-N), which routes one input to exactly one of several outputs, and from encoders/decoders, which transform between one-hot and binary codes.Step-by-Step Solution:Identify the direction: many inputs → one output, controlled by selects = multiplexer.Confirm that only one input is active at the output for any select combination.Note that enabling disables or permits the selection without changing the definition.Therefore, the statement matches the formal definition of a MUX.Verification / Alternative check:Datasheets for 74xx151 (8-to-1) or 74xx157 (quad 2-to-1) describe “data selector/multiplexer,” confirming this exact behavior. Textbooks likewise define MUX as one-of-N selection.Why Other Options Are Wrong:“Incorrect” denies the standard definition. The options invoking demultiplexers or tri-state bus drivers describe different components: DEMUX routes one input to many outputs; bus drivers buffer or isolate lines but do not inherently select among many inputs.Common Pitfalls:Confusing MUX with serializer hardware; serialization requires timing and switching over time, whereas a basic MUX is an instantaneous selector governed by static select inputs.Final Answer:Correct

Question 7

Computer engineering terminology — meaning of a “variable”: In algorithm design and hardware description, a VARIABLE can be viewed as a named storage placeholder used to record numeric values (or symbols) for later use in expressions, decisions, or …

Accepted Answer

Introduction / Context:Whether in software (C, Python) or in hardware description languages (VHDL, Verilog/SystemVerilog), a “variable” represents storage associated with an identifier. The mental model in the prompt compares a variable to a sheet of paper where you jot a number for use later. This question checks whether that metaphor is broadly accurate and useful for beginners.Given Data / Assumptions:A variable has a name (identifier) and an associated value.The value may change over time via assignment.Scope, lifetime, and update semantics depend on the language (procedural vs HDL).The analogy is qualitative, aiming to build intuition.Concept / Approach:The analogy holds: a variable is writable storage that can be read later. In SW, a variable occupies memory; in HDLs, a variable/signal represents a modeled storage element or a driver value that changes with events or clock edges. The “scrap paper” captures two essentials: (1) you can write (assign) a value; (2) you can read and reuse it later in computation or control logic.Step-by-Step Solution:Identify the role: remember a value for future reference.Recognize mutability: variables can be reassigned; constants cannot.Map to implementation: RAM/registers/flip-flops in hardware; memory locations in software.Conclude the analogy is pedagogically sound, if simplified.Verification / Alternative check:Intro texts define variables as named memory locations; HDL references define variables/signals with types, drivers, and update rules, which still fulfill the “write then read later” concept.Why Other Options Are Wrong:Claiming “incorrect” ignores the core storage-and-retrieval nature. Limiting to assembly language or constants is inaccurate: variables are fundamental across languages and are specifically non-constant.Common Pitfalls:Equating HDL variables with wires trivially; in HDLs, assignment timing (blocking vs nonblocking) matters and affects synthesized hardware.Final Answer:Correct

Question 8

Identify the MSI function — decoder vs comparator: A circuit that translates an n-bit binary code word into exactly one active output line (one-hot) corresponding to its numeric value is called a ________. The statement labels it a “comparator”; eva…

Accepted Answer

Introduction / Context:MSI parts include decoders, encoders, multiplexers, demultiplexers, and comparators. Each serves a distinct role. The device described—one active output out of many corresponding to the numeric value of a binary input—is a decoder. This question ensures you can differentiate decoders from comparators, which instead judge the relationship between two values.Given Data / Assumptions:Input: an n-bit binary number.Output: a single asserted line among 2^n possibilities (one-hot code).No second value is provided for comparison.Concept / Approach:A binary-to-2^n decoder activates exactly one output line Yk when the input equals k. Examples include 74xx138 (3-to-8 decoder) and 74xx154 (4-to-16). In contrast, a comparator (e.g., 74xx85) accepts two binary inputs A and B and asserts relationship outputs such as A>B, A=B, A<B. Thus, calling the one-hot generator a “comparator” is misclassification.Step-by-Step Solution:Match behavior: one-of-many output → decoder.Check for comparison outputs; none are present.Conclude that “comparator” is incorrect; the accurate term is “decoder.”Therefore the original statement is false.Verification / Alternative check:Consult truth tables: a 3-to-8 decoder shows Y0..Y7 with single active output for each input code; a comparator shows three relationship outputs, not one-hot decoding lines.Why Other Options Are Wrong:“Correct” confuses functions. Priority encoders perform the inverse mapping (one-hot to binary) with priority, not what was described. “Magnitude comparator with enable” still compares two inputs, not decode one.Common Pitfalls:Mixing encoder/decoder directions; assuming any device producing a single output is a comparator—selection criteria differ.Final Answer:Incorrect (it is a decoder, not a comparator)

Question 9

Equality testing in MSI — definition of a comparator: A logic circuit that determines whether one binary input is equal to another (optionally also indicating greater-than or less-than) is called a comparator. Judge the correctness of this definitio…

Accepted Answer

Introduction / Context:Magnitude comparators are standard MSI devices (e.g., 74xx85) that evaluate the relationship between two binary words. They report whether A=B, A>B, or AB and AB, A=B, AB,” “A=B,” and “A

Question 10

Terminology — multiplexer select inputs: In data selectors, the select inputs are often referred to as address lines because they choose which input line is routed to the output. Evaluate the statement.

Accepted Answer

Introduction / Context:Multiplexers (MUXes) map a binary code on select inputs to the choice of one input among many. Because those select bits effectively “address” one of the inputs, the term address lines is commonly used, especially when MUXes connect to memory or register banks.Given Data / Assumptions:A 2^n-to-1 MUX uses n select inputs.Only one input is connected to the output for any select code.The naming reflects function, not necessarily memory addressing.Concept / Approach:Addressing means picking one resource from many using a binary code. In a MUX, the select pins perform exactly this role for input lines. Thus, calling them address lines is reasonable and widely seen in schematics and datasheets, although “select” remains the formal pin name.Step-by-Step Solution:Map select code to input index (e.g., S=101 selects D5).Recognize the selection is analogous to addressing.Conclude the terminology is acceptable and accurate.Therefore the statement is correct.Verification / Alternative check:Parts such as 74xx151 (8-to-1 MUX) label pins S2..S0; application notes often describe them as address inputs when used in memory/register selection roles.Why Other Options Are Wrong:Limiting correctness to 2:1 devices or memory maps confuses a general naming convention with a specific context.Common Pitfalls:Assuming address lines always refer to external memory buses; the concept applies anywhere one-of-N selection is required.Final Answer:Correct

Question 11

Decoder behavior — number of active outputs: For standard binary-to-one-of-N decoders, only one output line should be active for any valid input code (one-hot). Assess the claim: “It is normal for more than one decoder output to be active at the sam…

Accepted Answer

Introduction / Context:Decoders map an n-bit input into 2^n mutually exclusive outputs. Proper operation requires exactly one active output at a time for valid codes, enabling clean chip selects, address decoding, and resource enabling in digital systems. This question checks your understanding of the “one-hot” nature of decoders.Given Data / Assumptions:Binary input is valid and stable within setup/hold windows.Outputs are active-low or active-high but mutually exclusive.No enable pin or glitching is considered during transitions.Concept / Approach:For a device like the 74xx138 (3-to-8 decoder), each input combination selects one and only one output (with enable asserted). Multiple active outputs would break address decoding, causing bus contention and undefined system behavior. While hazards or transition glitches can momentarily assert multiple outputs if inputs change asynchronously, this is not “normal” design intent; careful timing or synchronized inputs prevent that.Step-by-Step Solution:Examine truth table: for each input code, exactly one output is asserted.Enable disabled → all outputs inactive (still not multiple active).Glitch-free design ensures exclusivity under synchronous changes.Therefore, the statement is false for correct decoder use.Verification / Alternative check:Datasheets specify “one-of-eight” or “one-of-sixteen” behavior; timing diagrams emphasize hazard-free changes when inputs are synchronized.Why Other Options Are Wrong:“Correct” contradicts the definition. Gray code and priority encoders are unrelated: Gray code minimizes bit changes; priority encoders perform the inverse function and still produce one code, not multiple active decoder outputs.Common Pitfalls:Ignoring enables; changing multiple inputs asynchronously leading to temporary glitches; misinterpreting active-low outputs as multiple actives.Final Answer:Incorrect

Question 12

HDL fundamentals — meaning of a “process”: In hardware description languages (e.g., VHDL, SystemVerilog), a process (or always block) models behavior that executes on events or clock edges, not a mere physical wire. Evaluate the statement: “In HDL, …

Accepted Answer

Introduction / Context:HDLs provide two complementary views: structural (nets/wires, instances) and behavioral (processes/always blocks). A “process” captures behavior driven by sensitivity lists or clocks, specifying how signals change over time. It is not synonymous with a wire, which represents a physical connection for signal propagation.Given Data / Assumptions:Process/always blocks execute when signals change (combinational) or on clock edges (sequential).Wires/nets represent interconnect.Synthesis maps behavior to gates, flops, and routing.Concept / Approach:Think of a process as an encapsulated piece of logic: combinational processes describe functions Y=f(X); sequential processes describe state updates at clock edges. The wire merely carries signals between such logic elements. Conflating the two obscures timing, concurrency, and state—central to digital design.Step-by-Step Solution:Identify: a process has a sensitivity list or clock and contains assignments.Relate: synthesis converts the described behavior to LUTs, flip-flops, and routing.Contrast: a wire/net has no behavior; it transmits values driven by sources.Conclude: the statement equating a process to a wire is false.Verification / Alternative check:VHDL defines process as a concurrent statement containing sequential statements; SystemVerilog uses always_comb/always_ff/always_latch to model distinct behaviors. None are defined as “wires.”Why Other Options Are Wrong:Claims about “combinational nets” or “routing” misinterpret implementation: even when a combinational process synthesizes to gates connected by wires, the process itself is not the wire.Common Pitfalls:Using blocking/nonblocking incorrectly; missing signals in sensitivity lists; mistaking signals vs variables and their assignment semantics.Final Answer:Incorrect

Question 13

Priority encoder truth — 74148 I1 active case: For the 74148 8-to-3 priority encoder (active-low inputs and active-low outputs), when data input I1 is the only active input (asserted), what is the output code? The statement claims A0 = 0, A1 = 0, A2…

Accepted Answer

Introduction / Context:The 74148 is a classic MSI priority encoder with active-low inputs (I7 highest priority … I0 lowest) and active-low outputs (A2, A1, A0). Understanding its active-level conventions avoids frequent mistakes when interpreting output codes. This item checks your ability to translate between true binary values and active-low outputs for a specific asserted input.Given Data / Assumptions:Device: 74148, 8-to-3 priority encoder.Inputs I0–I7 are active-low; lower numerical index implies lower priority.Outputs A2, A1, A0 are active-low representations of the binary index of the highest-priority active input.Only I1 is asserted (I1 = 0, others = 1), and enable is appropriately asserted.Concept / Approach:First, determine the true binary code for input 1: that is 001 (A2_true=0, A1_true=0, A0_true=1). Because the 74148 outputs are active-low, the pins A2, A1, A0 present inverted logic: A2 = NOT(A2_true), A1 = NOT(A1_true), A0 = NOT(A0_true). Thus, the pins read A2=1, A1=1, A0=0 for input 1 active (and no higher-priority input asserted).Step-by-Step Solution:Find true code: index 1 → 001.Apply inversion: A2 = 1, A1 = 1, A0 = 0.Check priority: no I2–I7 asserted, so index 1 is valid.Compare with claim: claimed A2=1, A1=0, A0=0 is different → statement is false.Verification / Alternative check:Consult a 74148 truth table: with I1 low, outputs are A2=A1=1, A0=0, and GS/EI pins reflect valid encoding/enable conditions.Why Other Options Are Wrong:“Correct” lists A2=1, A1=0, A0=0, which corresponds to true code 010 (index 2), not input 1. Enable polarity alone does not change the encoded pattern. Non-priority encoders differ in conflict handling but do not flip the active-low output convention of the 74148.Common Pitfalls:Forgetting outputs are active-low; neglecting priority (I7 dominates); misreading the bit order A2..A0 vs A0..A2.Final Answer:Incorrect (actual outputs are A2 = 1, A1 = 1, A0 = 0)

Question 14

Does a multiplexer “serialize” data by definition? A multiplexer provides one-of-N selection at its output. Consider the statement: “Basically, a multiplexer changes parallel data inputs to a serial output.” Decide if this characterization is accura…

Accepted Answer

Introduction / Context:It is tempting to equate “many inputs to one output” with serialization. However, a bare multiplexer merely selects one input at a time based on static select lines. Serialization, in contrast, implies time-division transmission of multiple channels in sequence, which requires dynamic control (a clocked selector) and framing—not just a MUX gate.Given Data / Assumptions:Multiplexer: one-of-N selector governed by select inputs.Serializer: cycles through channels, outputting them in time sequence.No clocking behavior is implied in the MUX definition itself.Concept / Approach:A MUX can be used as a building block in a serializer by driving its select lines with a counter or finite-state machine. But the MUX alone does not imply sequencing; with fixed selects, it simply forwards a single chosen input continuously. Therefore, saying a MUX “basically changes parallel to serial” is misleading without the crucial timing/control context.Step-by-Step Solution:Define MUX: Y = D[S], static data selection.Define serializer: Y(t) = D0, D1, … in a timed sequence governed by a clock.Note dependency: serialization = MUX + time-varying selects + protocol.Conclude the original statement is false as a general definition.Verification / Alternative check:Reference block diagrams: UART/SerDes include shift registers, clocks, encoding—far beyond a single MUX block.Why Other Options Are Wrong:“Correct” overstates the MUX role. Conditional options that restrict to “1-of-2” or rely on cycling omit that serialization is a system behavior, not the definition of a MUX.Common Pitfalls:Confusing data selection with time-division multiplexing; ignoring buffering/clock domains needed for true serial links.Final Answer:Incorrect

Question 15

Using a 7485 four-bit magnitude comparator — specific case check: Given A = 0110 (decimal 6) and B = 1010 (decimal 10), evaluate the following claimed outputs for the 7485 comparator: A < B = 1, A = B = 0, A > B = 1. Determine whether this set is va…

Accepted Answer

Introduction / Context:The 7485 magnitude comparator outputs three mutually exclusive flags: AB. Only one of these should be asserted for any well-defined comparison. This problem tests your ability to evaluate a specific input pair and verify consistent outputs.Given Data / Assumptions:A = 0110 (6 unsigned).B = 1010 (10 unsigned).Comparator is a standard 4-bit device without cascade inputs affecting the result.Concept / Approach:Compare A and B numerically as unsigned binary. Since 6 is less than 10, the correct flags are AB = 0. The claim that both AB are 1 simultaneously violates mutual exclusivity and is logically inconsistent.Step-by-Step Solution:Convert to decimal or compare bitwise from MSB: A(0110) vs B(1010).MSB comparison: A3=0, B3=1 → immediately implies AB = 0.Compare with claim: claim has A>B = 1 as well → inconsistent → false.Verification / Alternative check:7485 truth tables show that when the highest unequal bit indicates A

Question 16

Computer organization — Evaluate the statement: “Modern computers transfer data between different devices over a common set of connecting lines called a data bus.”

Accepted Answer

Introduction / Context: In digital systems, processors, memories, and peripherals exchange information over shared pathways. The term “data bus” describes a common group of conductors used to move data words among components. This question checks foundational literacy in computer organization and I/O architecture.Given Data / Assumptions:“Modern computers” include microcontrollers, microprocessors, and SoCs.A bus may be parallel (multiple lines) or serialized (logical bus over few wires).We consider on-chip and board-level interconnects conceptually.Concept / Approach: A bus is a shared communication medium comprising signals for data, address, and control. The “data bus” specifically carries payload bits between endpoints such as CPU registers, memory, and I/O devices. Even when the physical implementation uses point-to-point serial links, the logical abstraction remains a bus protocol coordinating access and transfer semantics.Step-by-Step Solution:Recognize that a bus allows multiple devices to communicate via defined rules (arbitration, timing, signaling).Identify examples: memory buses (DDR), on-chip buses (AHB/AXI), peripheral buses (PCIe, USB, I2C, SPI).Conclude that the shared “data bus” description is accurate at the conceptual level.Verification / Alternative check:Block diagrams in datasheets show a CPU core connected to memory and peripherals through a central bus or interconnect fabric.Why Other Options Are Wrong:Incorrect: Ignores standard bus-based designs.Applies only to serial links: Buses can be parallel (e.g., internal data buses) or serial (logical buses like I2C/PCIe).True only for microcontrollers, not CPUs: General-purpose CPUs also rely on buses and interconnects to access memory and devices.Common Pitfalls:Assuming “bus” must be wide parallel lines; modern buses are often serialized but still shared protocols.Confusing the data bus with address and control buses; they are distinct signal groups.Final Answer:Correct

Question 17

VHDL semantics — Evaluate the statement: “A very important attribute of the conditional signal assignment statement is its sequential operation.”

Accepted Answer

Introduction / Context: VHDL distinguishes between concurrent statements (evaluate in parallel) and sequential statements (execute in order inside a process or subprogram). The conditional signal assignment (using “when … else …”) is a canonical concurrent construct. This question checks whether learners can separate VHDL’s two semantic domains.Given Data / Assumptions:“Conditional signal assignment” refers to the concurrent form: S Sequential signal assignments occur inside processes, if/elsif/case statements.Signals (not variables) are being discussed.Concept / Approach: Concurrent statements describe hardware that exists and operates simultaneously. A conditional signal assignment synthesizes a combinational network (often a multiplexer) evaluated continuously. Sequential statements live inside a process; their order matters and they simulate as a time-ordered sequence. The prompt attributes sequential behavior to a concurrent construct, which is incorrect.Step-by-Step Solution:Identify the construct: conditional signal assignment is a concurrent statement.Map meaning to hardware: synthesizes mux logic driven by conditions.Conclude: not sequential; sequential equivalents would use “if … then … else …” inside a process.Verification / Alternative check:Any number of concurrent conditional assignments are evaluated in parallel; changing a condition updates outputs without stepwise execution order.Why Other Options Are Wrong:Correct: Would misclassify a concurrent construct as sequential.Valid only inside processes: The concurrent conditional form is outside processes; inside a process you use sequential if/case.True for variables, not signals: Variables relate to sequential context; this statement remains incorrect for the discussed construct.Common Pitfalls:Confusing similar syntax across concurrent and sequential domains.Assuming simulation waveforms imply sequential execution for all constructs.Final Answer:Incorrect

Question 18

74×151 8-to-1 multiplexer — To select data input I3, what binary levels must be applied to the select inputs (S2, S1, S0)?

Accepted Answer

Introduction / Context: The 74151 is an 8-to-1 multiplexer with three select inputs choosing one of eight data inputs I0–I7. Understanding how the select lines encode the chosen channel is fundamental to using multiplexers in datapaths and control logic.Given Data / Assumptions:Three select inputs labeled S2 (MSB), S1, S0 (LSB).Input lines I0–I7 are selected by the binary value on S2 S1 S0.Active-low enable must be asserted for the device to function; assume it is enabled.Concept / Approach: With binary encoding, the select code corresponds to the index of the input: code 000 selects I0, 001 selects I1, …, 011 selects I3, up to 111 selecting I7. Therefore, to route I3 to the output, the select inputs must represent binary 3, namely 0 1 1 on S2 S1 S0 respectively.Step-by-Step Solution:Map desired input: I3 → binary index 3.Write code in S2 S1 S0 order: 0 1 1.Verify alternate choices correspond to I4 (100), I1 (001), I6 (110), not I3.Verification / Alternative check:Consult truth tables: S2 S1 S0 = 011 is documented to select I3.Why Other Options Are Wrong:S2 = 1, S1 = 0, S0 = 0 → selects I4.S2 = 0, S1 = 0, S0 = 1 → selects I1.S2 = 1, S1 = 1, S0 = 0 → selects I6.Common Pitfalls:Reversing bit order and treating S0 as MSB.Forgetting to enable the device, which tri-states or forces the outputs to known inactive states.Final Answer:S2 = 0, S1 = 1, S0 = 1

Question 19

74LS138 decoder — Evaluate: “Output 6 is selected when A0 = 1, A1 = 1, A2 = 0 (with the device enabled).”

Accepted Answer

Introduction / Context: The 74LS138 is a 3-to-8 decoder with active-LOW outputs Y0–Y7. It asserts exactly one output LOW based on the binary code on inputs A (LSB), B, and C (MSB) when properly enabled. The prompt tests correct output indexing for a specific input pattern.Given Data / Assumptions:Inputs: A0 ≡ A (LSB), A1 ≡ B, A2 ≡ C (MSB).Enable conditions satisfied: G1 = 1, G2A = 0, G2B = 0.Outputs are active LOW; “selected” means driven LOW.Concept / Approach: The output index equals the binary value ABC (with C as MSB, A as LSB). For A2 A1 A0 = 0 1 1, the binary value is 3, so Y3 goes LOW, not Y6. Therefore, saying “Output 6 is selected” for 0 1 1 is incorrect.Step-by-Step Solution:Write inputs in MSB→LSB order: C B A = 0 1 1.Compute index: 04 + 12 + 1*1 = 3.Map to output: Y3 is asserted LOW (active); Y6 remains HIGH (inactive).Verification / Alternative check:Truth tables in datasheets confirm A=1, B=1, C=0 → Y3 LOW.Why Other Options Are Wrong:Correct: Would wrongly claim Y6 is selected.True only if outputs are active HIGH: 74LS138 outputs are active LOW by design.Depends on enable levels only: Enables must be correct, but the output index still follows ABC coding.Common Pitfalls:Swapping bit significance (treating A as MSB).Forgetting that “selected” means LOW on an active-LOW decoder.Final Answer:Incorrect

Question 20

Comparator definition — Evaluate: “A device that determines which of two numbers is greater is a comparator.”

Accepted Answer

Introduction / Context: Comparators are used to determine ordering between two inputs. In digital logic, magnitude comparators output signals indicating A>B, A=B, or AGiven Data / Assumptions:“Numbers” may refer to digital binary words or analog magnitudes represented as voltages/currents.We focus on the core function—establishing greater/less/equal relationships.Implementation details (bit width, technology) do not change the definition.Concept / Approach: A comparator’s essential behavior is to compare two inputs and assert outputs representing relative magnitude. Digital magnitude comparators (e.g., 74×85) produce A>B, A=B, AStep-by-Step Solution:Identify inputs A and B.Evaluate their magnitudes; hardware sets outputs accordingly.Use the outputs to steer subsequent logic (select max, trigger thresholds, etc.).Verification / Alternative check:Datasheets and textbooks define comparators by their ability to indicate relative size or threshold crossing.Why Other Options Are Wrong:Incorrect: Conflicts with standard definitions in both digital and analog domains.Only true for analog signals: Digital comparators exist; the definition is broader.Only true for 8-bit data: Bit width is irrelevant; comparators scale to various widths.Common Pitfalls:Confusing a comparator with a subtractor; subtractors compute differences, comparators report ordering.Assuming a comparator outputs the larger value; that requires a multiplexer controlled by the comparator.Final Answer:Correct

MSI Logic Circuits Questions

Questions

Practice smarter