7400 Series Guide: 74HC155/74LS155 (Dual 1-of-4 decoder)

The 74×155 (ex 74LS155) is a chip that features a dual 1-of-4 decoder/demultiplexer with complementary outputs.

In this guide, you’ll learn the essentials of this chip to use one-of-four decoders and demultiplexers effectively in your own projects.

What does the 74HC155 / 74LS155 do?

The 74×155 is a dual 1-of-4 decoder and demultiplexer.

Here’s how it works:

• It has two separate sections, each with its own set of inputs and outputs.
• For each section, you have two address inputs (usually labeled A and B) that select which one of the four outputs (Y0, Y1, Y2, or Y3) will be activated (LOW).
• The rest of the outputs will remain in the HIGH state.
• There’s also an enable input for each section that must be LOW for the outputs to be active.

The chip can be used to route a single input signal to one of several outputs based on the address inputs, making it useful in digital systems for functions like memory addressing and signal routing.

How To Use This Chip

The 74HC155 comes in a 16-pin package, and you need to connect it to power before you can use it. Most 7400 ICs support a VCC voltage of 5V. One difference between the HC and LS version of the chip is that the 74HC155 supports 2V to 6V, while the 74LS155 only supports 5V.

74HC chips can normally supply a maximum of 4 mA from an output pin. If you’re using the 74LS version, the maximum current you can pull out of one output pin is 0.4 mA when the pin is high (sourcing) or 8 mA when the pin is low (sinking).

But these values can differ between models, so check the datasheet of your model to verify.

Once you’ve connected it to power, you can use the 2-to-4 line decoders inside.

74×155 Pinout

The 74×155 has 16 pins and contains two 2-to-4 line decoder/demultiplexer with individual active-low outputs laid out as shown in the pinout diagram below:

Pin Name	Pin #	Type	Description
A1	1	Input	Select input for the first 2-to-4 line decoder.
B1	2	Input	Select input for the first 2-to-4 line decoder.
E1	3	Input	Enable input for the first 2-to-4 line decoder (active low).
Y3	4	Output	Output 3 from the first 2-to-4 line decoder.
Y2	5	Output	Output 2 from the first 2-to-4 line decoder.
Y1	6	Output	Output 1 from the first 2-to-4 line decoder.
Y0	7	Output	Output 0 from the first 2-to-4 line decoder.
GND	8	Power	Connect to ground (GND).
Y5	9	Output	Output 1 from the second 2-to-4 line decoder.
Y4	10	Output	Output 0 from the second 2-to-4 line decoder.
Y7	11	Output	Output 3 from the second 2-to-4 line decoder.
Y6	12	Output	Output 2 from the second 2-to-4 line decoder.
E2	13	Input	Enable input for the second 2-to-4 line decoder (active low).
B2	14	Input	Select input for the second 2-to-4 line decoder.
A2	15	Input	Select input for the second 2-to-4 line decoder.
VCC	16	Power	Positive power supply. Connect to +5V power.

Alternatives and Equivalents for 74HC155 / 74LS155

There are many versions of the 74×155 chip. They all have the same functionality, but with different specifications such as supported voltages and maximum current output.

Here’s a list of a few equivalents of this chip:

• 74HC155 (High-speed CMOS)
• 74HCT155 (High-speed CMOS, TTL compatible)
• 74LS155 (High-speed TTL)
• 74LVC155 (Low Voltage TTL)
• 74AC155 (Advanced CMOS)
• 74ALS155 (Advanced Low-Power Schottky TTL)
• 74F155 (Very High Speed)
• 74C155 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC155 and SN74LS155 by Texas Instruments.

Can’t find the 74×155 anywhere? Then try one of the following IC alternatives:

• 74×139 – Dual 2-to-4 line decoder/demultiplexer.
• 74×157 – Quad 2-to-1 line data selector/multiplexer.
• 74×253 – Quad 2-to-1 line data selector/multiplexer (with 3-state outputs).
• CD4512 – 8-channel data selector/multiplexer.
• CD4513 – BCD to 7-segment latch decoder.
• CD4514 – 4-to-16 line decoder/demultiplexer.
• CD4515 – 4-to-16 line decoder/demultiplexer.

If you can’t find the 74×155 IC in your local electronics store, don’t worry, you’ll most likely find it in one of the stores listed on this page of online stores where you’ll find components and tools for all your electronics projects.

Datasheet for the 74LS155 and 74HC155 chips

Download the PDF datasheet for your version of the 74×155 here:

• 74LS155 (Futurlec)
• SN74LS155 (Texas Instruments)
• 74LS155 (SYC)
• MM74HC155 (National Semiconductor)
• M74HC155 (STMicroelectronics)