Guide to the 74LS138 3-to-8 Line Decoder/Demultiplexer
Introduction
The 74LS138 is a popular integrated circuit (IC) in digital electronics, designed as a 3-to-8 line decoder and demultiplexer. This IC takes a 3-bit binary input and activates one of eight active-low outputs, providing a straightforward solution for tasks like memory addressing, signal routing, and data distribution. Its compatibility with TTL logic, low power usage, and simple integration have made it a go-to component in classrooms, hobbyist projects, and professional systems alike. Whether you're decoding addresses for a microprocessor or directing signals in a custom circuit, the 74LS138 delivers reliable performance.
Key Features
- 3-to-8 Decoding: Transforms 3-bit binary inputs (A, B, C) into 8 distinct active-low outputs (Y0–Y7).
- Enable Inputs: Features three enable pins (G1, G2A, G2B) for precise control and the ability to chain multiple ICs.
- Active-Low Outputs: The selected output drops to LOW (0V), while unselected outputs stay HIGH (5V).
- TTL-Compatible: Runs on a 5V supply, pairing seamlessly with other TTL-based components.
- Low Power Dissipation: Draws roughly 2mW per gate, ideal for energy-conscious designs.
- 16-Pin DIP Package: Comes in a durable, breadboard-friendly dual in-line package.
- Fast Switching: Offers quick response times for real-time applications.
Pin Configuration
The 74LS138’s 16-pin layout is intuitive and functional:
| Pin No. | Name | Function |
|---|---|---|
| 1 | A | Least significant address input (LSB). |
| 2 | B | Middle address input. |
| 3 | C | Most significant address input (MSB). |
| 4 | G2A | Enable input, active LOW (must be 0V to activate). |
| 5 | G2B | Enable input, active LOW (must be 0V to activate). |
| 6 | G1 | Enable input, active HIGH (must be 5V to activate). |
| 7 | Y7 | Output 7 (active LOW). |
| 8 | GND | Ground connection (0V). |
| 9–15 | Y0–Y6 | Outputs 0 through 6 (active LOW). |
| 16 | VCC | Power supply (+5V). |
Functional Description
Truth Table
The 74LS138 activates only when its enable pins align: G1 = HIGH (5V), G2A = G2B = LOW (0V). If these conditions aren’t met, all outputs stay HIGH, effectively disabling the chip.
| G1 | G2A | G2B | C | B | A | Y0 | Y1 | Y2 | Y3 | Y4 | Y5 | Y6 | Y7 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 |
| 1 | 0 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 |
| 0 | X | X | X | X | X | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| X | 1 | X | X | X | X | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| X | X | 1 | X | X | X | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
Note: X = Don’t Care (can be 0 or 1).
Internal Logic
The 74LS138 relies on a network of AND and NAND gates to process inputs. The three address lines (A, B, C) determine which output goes LOW, while the enable pins act as a gatekeeper. For example, when A=0, B=0, C=0 and the chip is enabled, Y0 is driven LOW through a combination of inverted and non-inverted signals processed by the internal logic. This design ensures only one output is active at a time, preventing overlap.
Applications
Memory Addressing
- Purpose: Selects a specific memory chip in a multi-chip system.
- Setup: Connect higher-order address lines (e.g., A15–A13 from a CPU) to A, B, and C. Use the outputs (Y0–Y7) as Chip Select signals for up to eight memory devices.
- Benefit: Simplifies memory expansion in microprocessors like the 8085 or Z80.
Data Routing/Demultiplexing
- Purpose: Directs a single input signal to one of eight destinations.
- Setup: Feed the data signal into G2A or G2B, keep G1 HIGH, and use A, B, C to pick the output channel.
- Use Case: Distributing a serial data stream to multiple devices.
7-Segment Display Control
- Purpose: Manages multiplexed displays by activating one digit at a time.
- Setup: Tie Y0–Y7 to the enable pins of each digit’s driver circuit, cycling through inputs to display numbers sequentially.
- Tip: Pair with a 74LS47 BCD-to-7-segment decoder for full functionality.
Communication Systems
- Purpose: Routes data packets to specific ports or channels.
- Example: In a simple router, the 74LS138 can select output lines based on packet headers.
Cascading Multiple 74LS138s
- Purpose: Expands decoding beyond eight outputs (e.g., 4-to-16).
- Method: Use one 74LS138’s outputs to enable additional 74LS138s via their G1/G2 pins. Connect the same address bus to all ICs, adding an extra input line to differentiate between them.
Timing and Electrical Characteristics
- Supply Voltage: 4.75V to 5.25V (standard TTL range).
- Propagation Delay: Approximately 24ns from input to output, suitable for moderate-speed systems.
- Power Consumption: Typically 10–20mW, depending on switching frequency.
- Operating Temperature: 0°C to 70°C, reliable for most environments.
- Input High Voltage (VIH): Minimum 2V to register a HIGH.
- Input Low Voltage (VIL): Maximum 0.8V to register a LOW.
- Output Current: Can sink up to 8mA per output, enough to drive LEDs or small loads directly.
Example Circuit: Memory Address Decoder
- Components: 74LS138, 8 memory chips (e.g., SRAM), CPU address bus.
- Connections:
- CPU A0–A2 → 74LS138 A, B, C.
- CPU A15–A13 → G1, G2A, G2B (via logic to enable at a specific address range).
- Y0–Y7 → Chip Select pins of each memory chip.
- Operation: When the CPU outputs an address in the designated range (e.g., 0x8000–0xFFFF), the 74LS138 activates the corresponding memory chip.
- Design Tip: Add pull-up resistors (e.g., 10kΩ) to unused enable pins to prevent floating inputs.
Practical Design Tips
- Decoupling Capacitor: Place a 0.1µF capacitor between VCC and GND near the IC to filter noise.
- Unused Inputs: Tie unused enable pins to their inactive state (G1 to GND, G2A/G2B to VCC) to avoid erratic behavior.
- Fan-Out: The 74LS138 can drive up to 10 TTL loads per output—check your downstream components’ requirements.
- Heat Management: Though low-power, ensure good ventilation in dense circuits to stay within the 70°C limit.
Troubleshooting Common Issues
- All Outputs HIGH: Check enable pins—G1 must be HIGH, G2A and G2B must be LOW. Use a multimeter to verify voltages.
- Wrong Output Active: Confirm A, B, C inputs match the desired binary code. Test with a logic probe.
- No Response: Verify VCC is 5V and GND is connected. Inspect for shorts or broken pins.
- Glitches: Add a small delay (e.g., via an RC circuit) if inputs change too rapidly for the 24ns propagation time.
Comparison with Other Decoders
| IC | Type | Voltage | Power | Speed | Notes |
|---|---|---|---|---|---|
| 74LS138 | TTL | 5V | Moderate | 24ns | Standard choice for TTL systems. |
| 74HC138 | CMOS | 2–6V | Low | 15ns | Faster, wider voltage range. |
| 74LS139 | TTL | 5V | Low | 24ns | Dual 2-to-4 decoder alternative. |
| 74LS154 | TTL | 5V | High | 30ns | 4-to-16 decoder for larger systems. |
Conclusion
The 74LS138 stands out as a dependable, efficient decoder and demultiplexer for digital projects. Its active-low outputs, flexible enable controls, and TTL compatibility make it a practical choice for memory systems, signal routing, and beyond. With a solid grasp of its pinout, logic, and applications—plus a few design tricks—engineers and hobbyists can harness its capabilities to streamline circuits and boost performance across a wide range of electronics tasks.
Further Reading
Want to dive deeper into digital electronics or explore related topics? Check out these articles on our site:
- "Understanding TTL Logic: A Beginner’s Guide" – Learn the basics of TTL circuits and why the 74LS138 fits right in.
- "Decoders vs. Multiplexers: What’s the Difference?" – Clarify how the 74LS138 compares to other signal-routing ICs.
- "Building a Retro Microcomputer: Step-by-Step" – See the 74LS138 in action within a classic CPU design.
- "7-Segment Displays Made Easy" – Pair the 74LS138 with display drivers for your next project.
Frequently Asked Questions (FAQ)
Q: Can the 74LS138 work with CMOS circuits?
A: Yes, but with caution. The 74LS138 is TTL-based (5V), so ensure CMOS inputs tolerate TTL’s voltage levels (HIGH ≥ 2V, LOW ≤ 0.8V). Use level shifters if pairing with 3.3V CMOS systems.
Q: Why are the outputs active-low?
A: Active-low outputs simplify interfacing with many devices (e.g., memory chips) that use a LOW signal to activate. It’s a common convention in TTL design.
Q: How do I cascade two 74LS138s?
A: Use the first 74LS138’s outputs to control the enable pins of a second one. Add an extra address line to select between the two, expanding to 16 outputs (4-to-16 decoding).
Q: What happens if I leave enable pins floating?
A: Floating pins can cause unpredictable behavior. Tie G1 to GND and G2A/G2B to VCC if unused, or use pull-up/pull-down resistors.
Q: Can it drive an LED directly?
A: Yes, it can sink up to 8mA per output. Connect the LED’s anode to VCC via a current-limiting resistor (e.g., 330Ω) and the cathode to a Y output.
Q: Is the 74LS138 still relevant today?
A: Absolutely! While CMOS ICs like the 74HC138 are newer, the 74LS138 remains widely used in education, legacy systems, and TTL-based designs.
Resources
- Datasheet: Texas Instruments 74LS138 Datasheet – Official specs, timing diagrams, and electrical details.
- Tutorial: All About Circuits – Decoders – A beginner-friendly breakdown of decoder concepts.
- Simulation Tool: Tinkercad Circuits – Test 74LS138 circuits virtually before building.
- Purchase: DigiKey or Mouser Electronics – Reliable sources for buying the 74LS138 IC.
- Community: Electronics Stack Exchange – Ask questions and share projects with fellow enthusiasts.
- Book: Digital Electronics: Principles and Applications by Roger L. Tokheim – A solid reference for TTL and IC fundamentals.
