74LS266 (74266) Logic Gate - XNOR, Pinout, and Applications

The 74LS266 (also known as 74266) is an integrated circuit containing four two-input XNOR (Exclusive-NOR) logic gates. Its most distinctive feature is that it has open-collector outputs. It is powered with +5V, and each gate performs the Boolean operation Y = (A ⊕ B)'. The output is "1" (HIGH) only when its inputs are equal (both "0" or both "1"). If the inputs are different, the output is "0" (LOW).

This component is ideal for equality comparator circuits and "wired-AND" logic thanks to its open-collector outputs.

What is the 74LS266?

The 74LS266 is a quad XNOR gate with open-collector outputs. Its main features are:

• Four independent two-input XNOR gates.
• Open-collector outputs, which require an external pull-up resistor.
• Functions as an "equality detector."
• Compatible with standard TTL logic levels (5V).

Basic Operation and Truth Table

Each XNOR gate in the 74LS266 implements the "Exclusive-NOR" function, which is the negation of the XOR. The output is activated only if the inputs are identical.

Truth table for each XNOR gate:

Open-Collector Outputs

Unlike most TTL gates, the outputs of the 74266 are "open-collector." This means the output can pull the level down to 0V, but it cannot pull it up to 5V on its own. For the output to reach a HIGH level, an external pull-up resistor must be connected between the output and VCC (+5V).

This feature allows for:

• Connecting multiple outputs together: A "wired-AND" logic can be created by connecting several outputs to a single pull-up resistor.
• Driving loads at different voltages: The pull-up resistor can be connected to a different voltage than the chip's VCC (as long as it does not exceed the IC's maximum rating).
• Driving higher currents: It allows for more direct control of loads like LEDs or relays.

Pinout and Connections

The 74LS266 uses the standard DIP-14 package with the same pinout as the 7486 (XOR):

• Pin 7: GND (Ground)
• Pin 14: VCC (+5V)
• Gates organized as follows:
  • Gate 1: Inputs (Pin 1, 2), Output (Pin 3)
  • Gate 2: Inputs (Pin 4, 5), Output (Pin 6)
  • Gate 3: Inputs (Pin 9, 10), Output (Pin 8)
  • Gate 4: Inputs (Pin 12, 13), Output (Pin 11)

Don't forget the pull-up resistor on each output you use! A typical value is between 1kΩ and 10kΩ.

Detailed Technical Specifications

Practical Applications

• Equality comparator: Its primary use. The output is "1" if the input bits are equal.
• "Wired-AND" logic: Allows for implementing complex AND functions without needing additional gates.
• Data bus control: Open-collector outputs are common on buses where multiple devices may need to take control.

Practical Resources

• Datasheet SN74LS266 (Texas Instruments): [Quad 2-Input XNOR Gates with Open-Collector Outputs]
• Simulations: Simulators like Proteus or Falstad allow for experimenting with open-collector outputs and pull-up resistors.

Frequently Asked Questions (FAQ)

What gate is the 74266? The 74266 (or 74LS266) is an integrated circuit with four two-input XNOR logic gates and open-collector outputs.

What is the truth table for the 74266? The output is 1 if the inputs are equal (0-0 or 1-1). The output is 0 if they are different (0-1 or 1-0).

What does the 74266 integrated circuit do? It functions as an equality detector. Its special feature is the open-collector outputs, which require a pull-up resistor to work correctly.

What is the difference between the 7486 (XOR) and the 74266 (XNOR)? The 74266 is the logical negation of the 7486. Additionally, the 74266 has open-collector outputs, while the 7486 has standard TTL (totem-pole) outputs.

What voltage does the 74LS266 operate at? It operates with a VCC of 5V (range 4.75–5.25V).

References: Information is based on official datasheets from Texas Instruments and digital electronics technical guides.