Category: Combinational Logic

While it is OK to split an input signal into two wires it is problematic to just join two output signals on two wires without using a gate . Why is this the case? (for reference consider this diagram of the DFF gate in figure 3.1 in the textbook)

Question

Category: Combinational Logic

While it is OK to split an input signal into two wires it is problematic to just join two output signals on two wires without using a gate . Why is this the case? (for reference consider this diagram of the DFF gate in figure 3.1 in the textbook)

BlackTom AI · Accepted Answer

Topic: joining two output signals on the same wire without a gate in a combinational logic context.
Option 1: 'It is problematic because such diagrams are hard to read and code.' This rationale focuses on readability or implementation style, not on the electrical or logical behavior of signals. The core issue is not readability but what happens when two outputs are connected together, which can cause contention or indeterminate levels.
Option 2: 'It is problematic when the two outputs to be joined are different, then the signal produced is indeterminate.' This correctly points to the fundamental problem: if two outputs drive the same node and they disagree (one high, one low, or different strengths), the result is undefined or an intermediate voltage due to contention. In digital logic terms, this is a classic hazard: a direct short between conflicting outputs yields an indeterminate state or potential damage depending on the technology.
Option 3: 'It is problematic because it is physically impossible to join two output wires together.' This is incorrect in general: it is not physically impossible to connect two outputs, but doing so is undesirable because of contention. In real hardware, you can wire-join outputs, but you create a fight between drivers, which is the real problem described in the question. It's not a matter of impossibility, but of incorrect behavior and potential damage.
Summary: The only option that correctly captures the reason is that when the two outputs differ, the resulting signal becomes indeterminate due to contention. The other choices describe issues that are not the fundamental electrical/logical reason for avoiding direct fan-in of two outputs without a gate.

4520_COMP_SCI_X_0005 Practice Exam

Category: Combinational Logic While it is OK to split an input signal into two wires it is problematic to just join two output signals on two wires without using a gate . Why is this the case? (for reference consider this diagram of the DFF gate in figure 3.1 in the textbook)

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