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7400 Series Guide: 74HC125/74LS125 (3-state buffers)

The 74×125 (ex 74HC125) is a chip that contains four independent tri-state buffer gates with three-state outputs. This chip allows you to control the flow of data and isolate different circuit sections when needed.

In this guide, you’ll learn the essential information about this chip to effectively use tri-state buffers in your own projects.

74HC125 / 74LS125 Package And Pinout

What does the 74HC125 / 74LS125 do?

The 74×125 contains four independent buffer gates. Each gate has a 3-state output, which means it can be in one of three modes: HIGH, LOW, or high-impedance (effectively disconnected).

The high-impedance state is useful for connecting multiple outputs to a single wire or bus without interfering with each other. Each buffer gate has an input, an output, and an enable pin. When the enable pin is LOW, the output is disconnected (high-impedance state). When the enable pin is HIGH, the output will follow the input, either HIGH or LOW.

Buffers like this are often used in digital circuits to strengthen signals or to control which devices are active on a shared line, like in a data bus system.

How To Use This Chip

The 74HC125 comes in a 14-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 74HC125 supports 2V to 6V, while the 74LS125 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 any of the tri-state buffers inside.

7400 series IC connected to 5V on a breadboard
The 74HC125/74LS125 IC connected to power

74×125 Pinout

The 74×125 has 14 pins and contains four 3-state noninverting buffer with active low enables laid out as shown in the pinout diagram below:

74125 Pinout
Pin NamePin #TypeDescription
1OE1InputOutput Enable for the first tri-state buffer (active low).
1A2InputData input for the first tri-state buffer.
1Y3OutputData output from the first tri-state buffer.
2OE4InputOutput Enable for the second tri-state buffer (active low).
2A5InputData input for the second tri-state buffer.
2Y6OutputData output from the second tri-state buffer.
GND7PowerConnect to ground (GND).
3Y8OutputData output from the third tri-state buffer.
3A9InputData input for the third tri-state buffer.
3OE10InputOutput Enable for the third tri-state buffer (active low).
4Y11OutputData output from the fourth tri-state buffer.
4A12InputData input for the fourth tri-state buffer.
4OE13InputOutput Enable for the fourth tri-state buffer (active low).
VCC14PowerPositive power supply. Connect to +5V power.
Pin overview for the 74×125 IC

Alternatives and Equivalents for 74HC125 / 74LS125

There are many versions of the 74×125 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:

  • 74HC125 (High-speed CMOS)
  • 74HCT125 (High-speed CMOS, TTL compatible)
  • 74LS125 (High-speed TTL)
  • 74LVC125 (Low Voltage TTL)
  • 74AC125 (Advanced CMOS)
  • 74ALS125 (Advanced Low-Power Schottky TTL)
  • 74F125 (Very High Speed)
  • 74C125 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC125 and SN74LS125 by Texas Instruments.

Can’t find the 74×125 anywhere? Then try one of the following IC alternatives:

  • 74×126 – Quad 3-state buffer (similar to 74×125 but with different enable configuration).
  • CD4503 – Hex non-inverting 3-state buffer.
  • CD4504 – Hex inverting 3-state buffer.

If you can’t find the 74×125 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 74LS125 and 74HC125 chips

Download the PDF datasheet for your version of the 74×125 here:

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