Up/down counters and direction control: The statement claims that once an up/down counter begins counting it cannot be reversed. Decide whether direction can be changed on-the-fly with proper control timing.

Introduction / Context:
Up/down counters include a control input that selects the counting direction. Practical systems often need to reverse direction—for example, in position encoders, elevator controllers, or digital tuning. This question challenges the idea that once counting starts, reversal is impossible.

Given Data / Assumptions:

• Counters have a direction or up/down input (often labeled U/D).
• Changing U/D synchronously with the clock prevents timing hazards.
• Asynchronous or synchronous clear/load inputs may also be present.

Concept / Approach:
Reversal is a core feature: the next state logic interprets the control line to add or subtract one. As long as setup/hold requirements for the U/D control relative to the active clock edge are met, the counter cleanly changes direction at the next count event without needing to stop or reset first.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets for standard up/down counters (for example, synchronous 4-bit devices) show truth tables where the state transition increments or decrements based on the U/D input each cycle.

Why Other Options Are Wrong:

Common Pitfalls:
Switching U/D near the active edge and violating setup/hold, causing metastability or an incorrect step.

Final Answer:
Incorrect