# IEC exclusive-NOR gate (rectangular form)

Canonical URL: https://eloragrid.ai/symbols/iec-xnor-gate
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: =1
Drawing use: Schematic
Reference families: IEC, Manufacturer

## Symbol convention

Convention: IEC
Form: =1 element with output negation
Qualifying-symbol and dependency conventions as explained in TI's public overview of IEEE Std 91-1984, which incorporates IEC 617-12 work. Independent educational drawing; not certified IEC geometry or an IEC database identifier. The counterpart is the matching functional concept; no equivalence of every detailed standard graphic is asserted.
Related representation: [Exclusive-NOR gate](https://eloragrid.ai/symbols/logic-xnor-gate)

IEC =1 box with a negated output, giving the two-input equivalence or exclusive-NOR function.

## In plain English

### What does IEC exclusive-NOR gate (rectangular form) mean?

An IEC exclusive-NOR is commonly drawn as the =1 element with a negation circle at its output. For two inputs, the output is active when both inputs are in the same state, which is why the function is also called equivalence. Some drawings instead use the = logic-identity additional drawing mark, which gives the same two-input result.

### A simple example

Equivalence checks appear where two redundant channels must agree, for example dual-channel position switches or duplicated sensor inputs. The drawing should make clear whether disagreement produces an alarm, a trip, or only a log entry, because the gate itself only produces the comparison.

### What the drawings help you do

The symbol does not tell you how long a disagreement may persist before action. That tolerance is a separate timing element or a setting in the implementing device.

### This particular reference

IEC =1 box with a negated output, giving the two-input equivalence or exclusive-NOR function. The = identity element extends to many inputs as all-the-same; a negated =1 with three inputs means not exactly one active.

### Terms

- Signal: Electrical information passed between devices, rather than necessarily the main power supply.
- Schematic: A drawing showing function and connections rather than physical appearance.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A)](https://www.ti.com/lit/ml/sdyz001a/sdyz001a.pdf)
- [SN74HC266 — XNOR gate logic](https://www.ti.com/lit/ds/symlink/sn74hc266.pdf)


## What it represents

An IEC exclusive-NOR is commonly drawn as the =1 element with a negation circle at its output. For two inputs, the output is active when both inputs are in the same state, which is why the function is also called equivalence. Some drawings instead use the = logic-identity qualifier, which gives the same two-input result.
Equivalence checks appear where two redundant channels must agree, for example dual-channel position switches or duplicated sensor inputs. The drawing should make clear whether disagreement produces an alarm, a trip, or only a log entry, because the gate itself only produces the comparison.
The symbol does not tell you how long a disagreement may persist before action. That tolerance is a separate timing element or a setting in the implementing device.

## How to read it

- Check whether the sheet uses =1 with negation or the = identity qualifier; both mean equivalence for two inputs.
- Trace the output to see what action follows a mismatch.

## Information to show or reference

- Choose one convention for equivalence and use it consistently.
- Limit to two inputs unless the identity function is intended.
- Show the mismatch timing element separately.

## Important distinction

The = identity element extends to many inputs as all-the-same; a negated =1 with three inputs means not exactly one active.

## Facts

- Qualifier: =1 with output negation
- Alternative: = (logic identity) for two inputs
- Artwork scope: Original educational illustration; consult the applicable standard and the selected device documentation for the exact graphic

## Public references

- [Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A)](https://www.ti.com/lit/ml/sdyz001a/sdyz001a.pdf) — Texas Instruments. Defines =1, = and output negation.
- [SN74HC266 — XNOR gate logic](https://www.ti.com/lit/ds/symlink/sn74hc266.pdf) — Texas Instruments. Manufacturer exclusive-NOR documentation.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.