74138 3-to-8 Line Decoder/Demultiplexer IC (DIP)

Overview of the 74138 3-to-8 Line Decoder/Demultiplexer

The 74138 IC is a TTL-based 3-to-8 line decoder and demultiplexer in a standard 16-pin DIP package. It decodes a 3-bit binary input into one of eight active-low outputs, making it an essential component for memory addressing, data routing, and many digital logic designs.

Why choose the 74138

• TTL compatibility for direct interfacing with common digital logic families
• Active-low outputs for simple integration into TTL and CMOS systems
• Three enable inputs for selective device enabling and easy cascading to build larger decoders

Key Features

• 3-to-8 decoding and demultiplexing functionality
• Eight active-low outputs (Y0 to Y7)
• Three enable inputs – two active-low (G1, G2A) and one active-high (G2B)
• TTL logic family with fast switching and low power consumption
• Standard 16-pin DIP packaging for prototyping and PCB use

Technical Details and Typical Operation

Inputs A, B, and C represent the 3-bit binary select lines. The selected output goes low while all other outputs remain high. The enable inputs must be set correctly to allow decoding:

• G1 (active-high) must be high to enable the device
• G2A and G2B (active-low) both must be low to enable the device

Pin Summary

• 16-pin DIP package
• Select inputs: A, B, C
• Outputs: Y0 to Y7 (active-low)
• Enables: G1 (active-high), G2A and G2B (active-low)
• Vcc and GND for TTL power

Cascading and System Integration

The three enable inputs make the 74138 easy to cascade for larger decoding networks. Use the enable pins to gate multiple 74138 devices and create a larger N-to-2^N decoder tree. Typical system uses include memory page selection and I/O port enabling.

Applications

• Memory address decoding and chip select generation
• I/O port selection in microprocessor systems
• Data routing and demultiplexing
• Digital logic design, educational projects, and prototyping

Packaging and Prototyping

The 74138 comes in a standard 16-pin DIP package which is ideal for breadboards, educational workbenches, and through-hole PCBs. Its TTL-based design gives predictable levels when interfacing with legacy TTL or TTL-compatible CMOS devices.

Ordering and Notes

When ordering, verify supply voltage and logic family compatibility for your design. Images are for illustration purposes only. For datasheet-level timing and electrical characteristics, consult the manufacturer’s datasheet.