An Overview of This Versatile Logic Chip
The 74ls139 is a popular TTL logic chip that can function as a decoder or demultiplexer. As a decoder, it takes a binary input code and activates one of 8 output lines corresponding to the input. As a demultiplexer, it routes a single input to one of 8 outputs according to a 3-bit selection code.
Some key features of the 74ls139:
- Decodes 3 binary address inputs into 8 separate outputs
- Active low enable input
- Outputs source up to 20 mA
- High noise immunity
With its ability to expand a small binary input into a wider range of outputs, the 74ls139 finds use in many digital logic applications.
74ls139 Pinout
The 16-pin DIP package of the 74ls139 has the following pin assignments:
| Pin | Name | Description |
|---|---|---|
| 1 | G2a | Output 2 input A |
| 2 | G2b | Output 2 input B |
| 3 | G1a | Output 1 input A |
| 4 | G1b | Output 1 input B |
| 5 | C3 | Input C3 |
| 6 | C2 | Input C2 |
| 7 | C1 | Input C1 |
| 8 | C0 | Input C0 |
| 9 | GND | Ground |
| 10 | EI | Enable Input (Active Low) |
| 11 | A7 | Output A7 |
| 12 | A6 | Output A6 |
| 13 | A5 | Output A5 |
| 14 | A4 | Output A4 |
| 15 | A3 | Output A3 |
| 16 | VCC | Power Supply |
How the 74ls139 Decoder Works
The 74ls139 takes the 3 address inputs C0, C1, and C2 and activates one of the 8 outputs based on the binary address. For example, if C2C1C0 = 010, output A6 will go low while the other outputs remain high.
The truth table shows the decoding logic:
| C2 | C1 | C0 | Output |
|---|---|---|---|
| 0 | 0 | 0 | A0 |
| 0 | 0 | 1 | A1 |
| 0 | 1 | 0 | A2 |
| 0 | 1 | 1 | A3 |
| 1 | 0 | 0 | A4 |
| 1 | 0 | 1 | A5 |
| 1 | 1 | 0 | A6 |
| 1 | 1 | 1 | A7 |
The enable input (EI) acts as an override that forces all outputs high when it is high, regardless of the address inputs. This allows easy tristating of the outputs.
Example Circuits Using the 74ls139
The 74ls139 can be used in many digital logic circuits:
- Memory address decoding – The 8 outputs can be used to select 1 of 8 memory chips based on a 3-bit address.
- 7-segment display decoder – With open collector outputs, the 74ls139 can sink current to drive common cathode 7-segment LED displays.
- Input data routing – As a demultiplexer, input data on the EI pin can be routed to one of 8 destinations.
Here is an example circuit using the 74ls139 to decode a 3-bit address and drive an 8-bit bus:
Frequently Asked Questions
What is the difference between the 74ls139 and 74hc139?
The main difference is the logic family. The 74ls139 is TTL logic, while the 74hc139 is CMOS. The hc version has higher input impedance and lower power consumption.
How do I enable or disable the outputs of the 74ls139?
The active low EI pin serves as an output enable. Tie EI low to enable the outputs based on the address pins. Tie high to disable all outputs.
Can I connect the outputs of the 74ls139 directly to LEDs?
No, the 74ls139 cannot source enough current to drive LEDs directly. Use transistors or buffers to drive higher current loads.
What speed grade is the 74ls139?
The 74ls139 is usually offered in LS (Low-power Schottky) speed grade, which has propagation delays of 15-30 ns. Faster speed grades are often available too.
Are the outputs inverted logic?
No, the address decoding logic makes the outputs active low, but they are not inverted outputs. The selected output goes low when enabled.
