74138 IC 3-to-8 Line Decoder/Demultiplexer
The IC 74138 is a 3-to-8 line decoder/demultiplexer that converts a 3-bit binary input into one of eight active-low outputs. The chip includes three enable inputs (two active-low and one active-high), giving users flexible control over when the decoding occurs.
When enabled, the 74138 sets one of its eight outputs (Y0 to Y7) LOW based on the binary value of the three input lines (A, B, and C). All other outputs remain HIGH. If the enable conditions are not met, all outputs stay HIGH, regardless of the inputs.
This behavior makes it ideal for chip select logic, memory decoding, function selection, and demultiplexing data lines. Its fast switching and full TTL compatibility ensure reliable performance in digital systems.
The IC supports cascading for larger decoding needs, and it integrates easily with microcontrollers, microprocessors, and programmable logic circuits.
Key Features
Converts 3 binary inputs to 1 of 8 active-low outputs
Fast decoding with low propagation delay
Three enable inputs for flexible control
Outputs drive TTL and CMOS loads
TTL-compatible logic levels
Easy cascading for wide decoders
DIP-16 or SOIC-16 standard packages
Applications
Select one output line from eight using 3-bit address logic
Implement address decoding in memory or I/O maps
Use as a chip select decoder in microprocessor systems
Build function-select units in control logic
Simplify complex decision trees in embedded devices
Create demultiplexers for data routing or display driving
Develop priority logic systems in automation controllers
Build educational logic trainers for digital circuits
Expand peripheral interfaces in embedded applications
Organize command decoding in communication systems
Specifications Table
| Parameter | Value |
|---|---|
| Logic Family | TTL |
| Function | 3-to-8 Decoder/Demux |
| Input Lines | 3 (A, B, C) |
| Output Lines | 8 (Y0 to Y7, active LOW) |
| Enable Inputs | 3 (G1 active-HIGH, G2A/G2B active-LOW) |
| Output Type | Active LOW |
| Output Current (HIGH/LOW) | -0.4 mA / 8 mA |
| Input HIGH Voltage | ≥ 2.0V |
| Input LOW Voltage | ≤ 0.8V |
| Output HIGH Voltage | ≥ 2.4V |
| Output LOW Voltage | ≤ 0.4V ‘ |
| Propagation Delay | ~25 ns |
| Supply Voltage (Vcc) | 4.75V to 5.25V |
| Operating Temperature | 0°C to 70°C |
| Package Type | DIP-16 / SOIC-16 |
| Number of Pins | 16 |
