Why many CMOS logic ICs can serve as both multiplexers and demultiplexers Explain the key device characteristic that allows certain CMOS logic ICs (such as transmission-gate based devices) to be used interchangeably as multiplexers and demultiplexers.

Introduction / Context:
Analog switch and transmission-gate CMOS ICs (e.g., CD4051/4052/4053 families) can route signals in either direction because their internal pass transistors act as bilateral switches. This enables the same device to function as a multiplexer (many-to-one) or as a demultiplexer (one-to-many).

Given Data / Assumptions:

• CMOS transmission gates are symmetrical bilateral devices.
• Signal levels remain within the device’s supply rails.
• Control inputs select which switch paths conduct.

Concept / Approach:
Because a transmission gate passes signals in both directions with similar impedance, the network does not impose directionality. If you feed the common node and select one branch, you demultiplex; if you feed multiple branches and select which one connects to the common node, you multiplex. The logic of selection does not change, only which side you call “input” or “output.”

Step-by-Step Solution:

Verification / Alternative check:
Datasheets for CD4051 (8-channel analog multiplexer/demultiplexer) explicitly specify bidirectional operation across channels.

Why Other Options Are Wrong:

• “Cannot be used as both” contradicts standard device behavior.
• Always tri-stated outputs or TTL compatibility are unrelated to bidirectionality.

Common Pitfalls:
Ignoring on-resistance and signal range limits; ensure the analog levels stay within VSS–VDD and bandwidth constraints.

Final Answer:
CMOS uses bidirectional switches.