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Difference Between 74×138 and 74×238 Decoders
The 74×138 and 74×238 are popular 3-to-8 line decoders/demultiplexers used in digital electronics. Both ICs take 3 binary inputs and generate 8 outputs, but they differ in output type and enable configuration, which makes them suitable for slightly different applications.
Let’s dive into their basic features, pinout comparison, and differences.
✅ 74×138: 3-to-8 Decoder with Active-Low Outputs
The 74×138 is a 3-to-8 decoder/demultiplexer with active-low outputs.
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Inputs: 3 (A0, A1, A2)
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Enable Pins: 3 (G1 active high, G2A and G2B active low)
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Outputs: 8 (Y0–Y7), all active-low
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Function: Only one output goes LOW based on the input combination, while others remain HIGH.
Common Uses:
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Memory address decoding
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Chip select in microprocessor systems
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Logic minimization circuits
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✅ 74×238: 3-to-8 Decoder with Active-High Outputs
The 74×238 is also a 3-to-8 decoder/demultiplexer, but it has active-high outputs.
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Inputs: 3 (A0, A1, A2)
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Enable Pins: 3 (G1 active high, G2A and G2B active low)
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Outputs: 8 (Y0–Y7), all active-high
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Function: Only one output goes HIGH based on the input combination, while others remain LOW.
Common Uses:
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Driving LEDs
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Selecting one of many logic devices
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Data routing and demultiplexing
Pinout Similarities
Both ICs (74×138 and 74×238) have the same pin configuration:
- Inputs: A0, A1, A2
- Enable: G1, G2A, G2B
- Outputs: Y0–Y7
- Power: Vcc and GND
⚡ Key Differences Between 74×138 and 74×238
Feature 74×138 74×238
Output Type Active-Low (only one goes LOW) Active-High (only one goes HIGH)
Default State All outputs HIGH All outputs LOW
Best for Memory decoding, chip select LED driving, logic control
Pin Count 16 pins (same as 74×238) 16 pins (same as 74×138)
Enable Pins 3 (same logic configuration) 3 (same logic configuration)
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Summary
- The 74×138 and 74×238 are both 3-to-8 decoders/demultiplexers.
- 74×138 outputs are active-low, while 74×238 outputs are active-high.
- Functionally, they are identical except for output polarity, which determines their usage.
- 74×138 is widely used for memory decoding and chip select logic, while 74×238 is more suitable for driving indicators and active-high logic circuits.
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