1. Opposite Enable Logic Levels (inferred from general logic device naming conventions): - The "125" in the part number typically indicates an active-low Output Enable (OE#), while "126" denotes an active-high enable. - Under the same enable signal control, the output states of the two devices will be completely opposite: when one is in an output-enabled (active) state, the other will be in a high-impedance (disabled) state. 2. Practical Impact of the Difference: - Direct substitution may prevent the system from properly controlling the data bus, potentially leading to bus contention, signal latch failure, or communication errors. - If substitution is required, the driving logic of the enable signal must be modified accordingly (e.g., by adding an inverter); otherwise, the circuit will not function correctly. Note: Both devices are identical in supply voltage, drive capability, package dimensions, and other parameters. The only difference lies in the logic level requirement of the enable control.