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The 74139 Decoder IC

The 74139 Decoder IC

Overview of the 74139 IC

The 74139 is a 1-of-4 binary decoder IC that converts 2 binary inputs into 4 active-low outputs. It contains 2 input pins labeled A and B, an active-low enable pin, and 4 active-low output pins labeled Y0 through Y3.

Key Features

  • 1-of-4 binary decoder
  • 2 binary inputs
  • 4 active-low outputs
  • Active-low enable pin
  • TTL logic compatible

How the 74139 Works

The 74139 works by decoding the 2 binary input pins A and B into 1 of 4 active-low outputs. The truth table below shows the function:

00Y0 = 0
01Y1 = 0
10Y2 = 0
11Y3 = 0

When the enable pin is high, the outputs are disabled and remain high. When enable is pulled low, the outputs are enabled and the decoding takes place according to the truth table.

The 74139 contains 4 2-input NAND gates followed by inverters to generate the 1-of-4 decoding logic.


  • Memory address decoding
  • Demultiplexing
  • Used in decoders/encoders
  • Switching circuitry

Some key advantages of using the 74139 decoder are:

  • Only requires 2 inputs for 4 outputs
  • Simplifies circuit design compared to discrete gates
  • Fast decoding speed
  • Wide logic level tolerance

Example Circuit

Here is a simple example of using the 74139 to switch on one of 4 LEDs based on a 2-bit binary input:

When input A = 0, B = 0, output Y0 goes low, lighting the first LED. The enable pin allows the outputs to be enabled or disabled externally.

Frequently Asked Questions

What is the difference between the 74139 and 74138?

The main difference is that the 74138 is a 1-of-8 decoder with 3 binary inputs, while the 74139 is a 1-of-4 decoder with 2 binary inputs. The 74138 can decode 3 inputs to 8 outputs, while the 74139 only handles 2 inputs and 4 outputs.

Can I cascade two 74139s to get more outputs?

Yes, you can cascade multiple 74139s by connecting the outputs of the first to the inputs of the second. This allows you to combine their decoding ability to handle more inputs and outputs. For example, cascading two 74139s would give you 4 inputs and 16 outputs.

How fast can the 74139 decoder operate?

The 74139 can typically operate at speeds up to 15-25 MHz, making it suitable for many medium-speed TTL logic applications. The propagation delay is around 13 ns.

What is the power supply voltage range for the 74139?

The 74139 is designed for a power supply voltage range of 4.75 – 5.25V, which is the standard TTL logic level supply voltage. Lower or higher voltages outside this range may not provide proper operation.

Can I directly interface the 74139 with a microcontroller?

Yes, the 74139 can be directly interfaced to a microcontroller if the I/O pins are 5V tolerant. Often it is convenient to use the microcontroller to generate the 2-bit binary input patterns to control the 74139 outputs.