Demultiplexers (DEMUX)

A demultiplexer (DEMUX) is the inverse of a multiplexer. It takes a single input and routes it to one of several outputs based on select lines. All non-selected outputs are held at 0. Demultiplexers are closely related to decoders - in fact, a decoder with an enable input IS a demultiplexer.

Objectives

• Understand the function of 1:2 and 1:4 demultiplexers
• Design DEMUX circuits from AND gates and inverters
• Recognize the relationship between demultiplexers and decoders
• Use DEMUX for data distribution and serial-to-parallel conversion
• Identify DEMUX ICs (74HC138, 74HC139)

Key Takeaways

• DEMUX routes one input to one of 2ⁿ outputs using n select lines
• Non-selected outputs are held at 0
• DEMUX is the inverse operation of MUX
• A decoder with enable = DEMUX (same circuit, different perspective)
• Common ICs: 74HC138 (1:8), 74HC139 (dual 1:4)
• Used for data distribution, address decoding, and serial-to-parallel conversion

Applications

• Data Distribution: Sending serial data to one of several destinations.
• Address Decoding: Selecting memory or I/O devices based on address bits.
• LED Displays: Driving multiple displays by rapidly switching between them.
• Communication: Time-division multiplexing at the receiving end.

Practice Problems

Problem 1: Write the expressions for all outputs of a 1:4 DEMUX.

Problem 2: If a 1:8 DEMUX has select lines S2S1S0 = 101 and data input D = 1, which output is HIGH?

Problem 3: How can you use a 74HC138 (3-to-8 decoder) as a demultiplexer?

Problem 4: Design a 1:16 DEMUX from two 1:8 DEMUX chips.