Description:
The 74LS139 works as a high-speed, dual 1-of-4 decoder/demultiplexer IC from the Low-Power Schottky (LS) family. Moreover, it includes two independent 2-line to 4-line decoders, and each one features an active-low enable input. Because of its flexibility, this IC serves as a versatile component for digital logic circuits. For example, engineers often use it in memory address decoding and data routing applications.
Key features:
- Dual functionality: The 74LS139 contains two completely independent 2-input, 4-output decoders, or demultiplexers, in a single 16-pin package.
-
Active-low outputs: The outputs work in an “active-low” configuration. When the decoder selects an output, it drives that line to a logic low (0). Meanwhile, it keeps the unselected outputs at a logic high (1).
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Active-low enable input (E): Each decoder includes an enable pin. For instance, when the enable input is low, the decoder becomes active. Conversely, if the enable input goes high, it drives all outputs high, no matter what the address inputs are.
- High-speed operation: The “LS” in the part number shows that the chip uses Low-Power Schottky technology. As a result, it delivers faster switching speeds compared to standard TTL logic, making it ideal for time-critical digital applications.
- TTL compatible: It is compatible with Transistor-Transistor Logic (TTL) circuits and operates on a typical 5V supply.
Functional description:
- The enable input, E, must be held low for the decoder to operate.
- By applying a 2-bit binary code to A0 (Least Significant Bit) and A1 (Most Significant Bit), you can select which of the four outputs (O0, O1, O2, and O3) goes low.
- This is useful for address decoding, where a memory bank or peripheral is selected by a specific address.
Truth table (for one decoder)
| Enable (E) | Select (A1) | Select (A0) | Output (O0) | Output (O1) | Output (O2) | Output (O3) |
|---|---|---|---|---|---|---|
| H | X | X | H | H | H | H |
| L | L | L | L | H | H | H |
| L | L | H | H | L | H | H |
| L | H | L | H | H | L | H |
| L | H | H | H | H | H | L |
As a 1-of-4 demultiplexer:
- In this application, the enable input, E, is used as the data input.
- The select inputs, A0 and A1, determine which output receives the data.
- The data signal (applied to E) is routed to the selected output. The selected output will be a low version of the data signal.
Pinout:
| Pin No. | Pin Name | Description |
|---|---|---|
| 1 | E_a |
Enable for decoder ‘a’ (active low) |
| 2 | A0_a |
Address input 0 for decoder ‘a’ |
| 3 | A1_a |
Address input 1 for decoder ‘a’ |
| 4 | O0_a |
Output 0 for decoder ‘a’ (active low) |
| 5 | O1_a |
Output 1 for decoder ‘a’ (active low) |
| 6 | O2_a |
Output 2 for decoder ‘a’ (active low) |
| 7 | O3_a |
Output 3 for decoder ‘a’ (active low) |
| 8 | GND |
Ground |
| 9 | E_b |
Enable for decoder ‘b’ (active low) |
| 10 | A0_b |
Address input 0 for decoder ‘b’ |
| 11 | A1_b |
Address input 1 for decoder ‘b’ |
| 12 | O0_b |
Output 0 for decoder ‘b’ (active low) |
| 13 | O1_b |
Output 1 for decoder ‘b’ (active low) |
| 14 | O2_b |
Output 2 for decoder ‘b’ (active low) |
| 15 | O3_b |
Output 3 for decoder ‘b’ (active low) |
| 16 | VCC |
Positive power supply (+5V) |
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