Symbol of XNOR Gate: ANSI vs IEC Standards, Pinouts, and IC Selection

The symbol of XNOR gate depends entirely on the drafting standard your schematic follows. In the US and military applications (ANSI/IEEE Std 91), it is drawn as a D-shape with a curved back and an inversion bubble on the output. In European and international standards (IEC 60617), it is drawn as a rectangular box with an "=1" or "XNOR" qualifier inside. Electrically, the XNOR (Exclusive-NOR) acts as an equivalence detector: the output goes HIGH only when both inputs are at the same logic level.

XNOR Gate Symbol and Physical IC Reference Table

Before wiring a breadboard or routing a PCB, you must map the schematic symbol to the correct physical silicon. The table below bridges the gap between schematic drafting standards and the physical 14-pin DIP ICs you will actually buy.

Drafting Standard	Symbol Visual Description	Primary Region	Standard Push-Pull IC	Open-Drain / Special IC
ANSI/IEEE 91	Curved D-shape (like XOR) with a circle (bubble) on the output line.	USA, MIL-STD, Legacy	74HC7266 / 74LS7266	74HC266 / 74LS266
IEC 60617	Rectangular box with "=1" inside and a bubble on the output, or "XNOR" text.	EU, UK, Modern Global	74HC7266 / 74LS7266	74HC266 / 74LS266
DIN 40700 (Legacy)	Rectangular box with an ampersand (&) or specific function code, largely obsolete.	Older German/EU prints	CD4077 (CMOS)	N/A

Regional Standards and Schematic Ambiguities

When reading schematics, the region of the original engineer dictates the symbol you will see. If you are reading a US-based university textbook or a MIL-STD aerospace schematic, expect the ANSI/IEEE curved shapes. If you are reading a modern European industrial PLC manual or an IEC-compliant power system diagram, expect the IEC rectangular boxes. For a deeper dive into logic gate theory and symbol history, All About Circuits provides an excellent breakdown of digital logic symbols.

Safe Interpretation of Faded or Unmarked ICs

If you are scavenging parts or dealing with a 14-pin DIP where the laser marking has rubbed off, you can definitively identify an XNOR gate using a multimeter or logic probe.

1. Apply 5V to Pin 14 (VCC) and GND to Pin 7.
2. Tie both inputs of Gate A (Pins 1 and 2) to GND (Logic 0, 0).
3. Probe the output (Pin 3).
4. The Verdict: If Pin 3 reads HIGH (~5V), it is an XNOR gate (or a NOR gate, though quad NORs usually have different pinouts). If Pin 3 reads LOW, it is likely an XOR gate (like the 74HC86). An XNOR outputs HIGH when inputs are equal; an XOR outputs LOW when inputs are equal.

Physical IC Pinouts: Quad 2-Input XNOR (14-Pin DIP)

Whether you are using the 74HC7266 (push-pull), 74HC266 (open-drain), or the CD4077 (4000-series CMOS), the physical pinout for a standard 14-pin DIP package remains identical. This standardization allows you to swap between logic families without rewiring your breadboard, provided you account for voltage and output stage differences.

Pin Number	Function	Gate / Description
1	Input A1	Gate 1 Input A
2	Input B1	Gate 1 Input B
3	Output Y1	Gate 1 Output
4	Output Y2	Gate 2 Output
5	Input A2	Gate 2 Input A
6	Input B2	Gate 2 Input B
7	GND	Ground (0V)
8	Input A3	Gate 3 Input A
9	Input B3	Gate 3 Input B
10	Output Y3	Gate 3 Output
11	Output Y4	Gate 4 Output
12	Input A4	Gate 4 Input A
13	Input B4	Gate 4 Input B
14	VCC	Positive Supply (2V-6V for HC, 3V-15V for CD)

Decision Tree: Which XNOR IC Should You Buy?

Do not just order "an XNOR gate" from your distributor. Use this decision path to select the exact part number that matches your circuit's electrical requirements.

• IF you are building a standard 5V or 3.3V digital logic circuit, driving LEDs, microcontrollers, or standard CMOS inputs directly...
  THEN buy the 74HC7266 (Push-Pull). This is the default pick for 90% of hobbyist and prototyping applications.
• IF you need to wire-OR multiple gate outputs together on a shared bus, or you are interfacing with I2C-style open-collector buses...
  THEN buy the 74HC266 (Open-Drain). Remember to add 4.7kΩ pull-up resistors to VCC on every output pin. (Reference the NXP 74HC266 datasheet for bus capacitance limits).
• IF you are working with higher voltage systems (9V, 12V) like automotive logic, synthesizers, or legacy 4000-series CMOS designs...
  THEN buy the CD4077B. It accepts VCC from 3V to 15V, but expect propagation delays to be significantly slower (approx. 50ns to 100ns) compared to the HC series.
• IF you need 5V TTL compatibility but are running a 5V supply...
  THEN buy the 74HCT7266. The "T" denotes TTL-compatible input thresholds (Vih = 2.0V), making it ideal for interfacing older 5V microcontrollers with modern 3.3V logic on the output side.

Truth Table and Bench-Tested Propagation Delays

The XNOR gate is fundamentally an equality comparator. In parity-checker circuits and phase-detector loops, the propagation delay (the time it takes for the output to flip after the inputs change) dictates your maximum clock speed.

Input A	Input B	Output Y (Logic)	Practical Meaning
0 (LOW)	0 (LOW)	1 (HIGH)	Inputs match
0 (LOW)	1 (HIGH)	0 (LOW)	Inputs differ
1 (HIGH)	0 (LOW)	0 (LOW)	Inputs differ
1 (HIGH)	1 (HIGH)	1 (HIGH)	Inputs match

Real-World Timing Benchmarks

When designing high-speed digital systems or precise timing circuits, you must account for voltage-dependent propagation delays. Based on bench testing and manufacturer specifications (such as the Texas Instruments SN74HC7266 datasheet), here is what you can expect at room temperature (25°C):

• 74HC7266 at 5.0V VCC: Typical propagation delay (tpd) is 14 ns. Maximum guaranteed is 23 ns. Suitable for clock speeds up to ~20 MHz without timing margin issues.
• 74HC7266 at 3.3V VCC: Typical tpd increases to 20 ns. Maximum guaranteed is 32 ns. If you are running an ESP32 at 80 MHz and using external XNOR gates for address decoding, this delay will consume a significant portion of your setup/hold time window.
• CD4077B at 5.0V VCC: Typical tpd is 110 ns. Strictly limited to low-frequency applications (under 1 MHz), such as debounce logic, slow motor interlocks, or audio-frequency synthesizer logic.