7400 Series Guide: 74HC240/74LS240 (Octal 3-state buffer)

The 74×240 (ex 74HC240) is a chip that contains eight inverting buffer/drivers with tri-state outputs. This configuration is particularly useful for driving data buses and providing temporary output isolation.

In this guide, you’ll learn the things you need to know about this chip in order to effectively use buffer/drivers in your own projects.

What does the 74HC240 / 74LS240 do?

The 74×240 is an octal buffer/line driver, meaning it has eight (octal) buffer gates. It is used to send signals from one part of a circuit to another without weakening them.

This chip has three-state outputs, so it can drive high (send a strong signal), drive low (send no signal), or be in a high-impedance state (like it’s disconnected). The high-impedance state lets multiple devices share the same output lines without interfering with each other.

The 74×240 has two sets of four buffer gates, and each set is controlled by an enable pin. When the enable pin is low, the buffers are active. The chip also inverts, or flips, the input signal. This means a high input becomes low, and a low input becomes high.

This makes the 74×240 convenient for controlling data flow in circuits. It is often used in computers, communication devices, and more!

How To Use This Chip

The 74HC240 comes in a 20-pin package. Most 7400 ICs support a VCC voltage of 5V. But one difference between the HC and LS version of the chip is that the 74HC240 supports 2V to 6V, while the 74LS240 only supports 5V.

The 74HC240 can supply up to 25 mA of current from an output pin. The 74LS240 can sink 24 mA and source 15 mA of current from an output pin.

(These values can differ between models, so check the datasheet of your model to verify.)

74×240 Pinout

The 74×240 has 20 pins and contains two 4-bit 3-state inverting buffer/line driver laid out as shown in the pinout diagram below:

Pin Name	Pin #	Type	Description
1OE	1	Input	Output Enable for the first set of four buffers (active low).
1A1	2	Input	First input to the first set of four buffers.
1Y1	3	Output	Inverted output from the first buffer.
1A2	4	Input	Second input to the first set of four buffers.
1Y2	5	Output	Inverted output from the second buffer.
1A3	6	Input	Third input to the first set of four buffers.
1Y3	7	Output	Inverted output from the third buffer.
1A4	8	Input	Fourth input to the first set of four buffers.
1Y4	9	Output	Inverted output from the fourth buffer.
GND	10	Power	Connect to ground (GND).
2A1	11	Input	First input to the second set of four buffers.
2Y1	12	Output	Inverted output from the fifth buffer.
2A2	13	Input	Second input to the second set of four buffers.
2Y2	14	Output	Inverted output from the sixth buffer.
2A3	15	Input	Third input to the second set of four buffers.
2Y3	16	Output	Inverted output from the seventh buffer.
2A4	17	Input	Fourth input to the second set of four buffers.
2Y4	18	Output	Inverted output from the eighth buffer.
2OE	19	Input	Output Enable for the second set of four buffers (active low).
VCC	20	Power	Positive power supply. Connect to +5V power.

Alternatives and Equivalents for 74HC240 / 74LS240

There are many versions of the 74×240 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:

• 74HC240 (High-speed CMOS)
• 74HCT240 (High-speed CMOS, TTL compatible)
• 74LS240 (High-speed TTL)
• 74LVC240 (Low Voltage TTL)
• 74AC240 (Advanced CMOS)
• 74ALS240 (Advanced Low-Power Schottky TTL)
• 74F240 (Very High Speed)
• 74C240 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC240 and SN74LS240 by Texas Instruments.

Can’t find the 74×240 anywhere? Then try one of the following IC alternatives:

• 74×241 – Octal buffer/line driver with 3-state outputs.
• 74×244 – Octal buffer/line driver with 3-state outputs.
• 74×245 – Octal bus transceiver with 3-state outputs.
• 74×541 – Octal buffer/line driver with 3-state outputs.
• 74×640 – Octal bus transceiver with 3-state outputs.
• CD4503 – Hex non-inverting buffer with 3-state outputs.

If you can’t find the 74×240 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 74LS240 and 74HC240 chips

Download the PDF datasheet for your version of the 74×240 here:

• SN74HC240 (Texas Instruments)
• SN74LS240 (Texas Instruments)
• 74HC240 (Futurlec)
• 74LS240 (Futurlec)
• 74HC240 (Nexperia)