XNOR
Exclusive-NOR gate
For two positive-logic inputs, produces a true output when the input states are equal.
Schematic
IEC · =1
IEC exclusive-NOR gate (rectangular form)
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.
- Signal
- Electrical information passed between devices, rather than necessarily the main power supply.
- Schematic
- A drawing showing function and connections rather than physical appearance.
Read the source: Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A) · SN74HC266 — XNOR gate logic
IEC · =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.
Shown as an independently authored educational illustration. Use the cited publication and drawing legend to verify the required drafting form.
Schematic · =1
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.
A distinction that matters
The = identity element extends to many inputs as all-the-same; a negated =1 with three inputs means not exactly one active.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A)
Defines =1, = and output negation.
SN74HC266 — XNOR gate logic
Manufacturer exclusive-NOR documentation.
About this graphic
This illustration was authored independently by Elora Grid to explain the concept. It is not reproduced from a customer drawing, an official standard graphic or a manufacturer CAD file. Its correspondence to an exact IEC or IEEE symbol identifier has not been certified.
SVG and DXF downloads contain this original artwork. DXF uses unscaled drawing units; a GA envelope supplies no product dimensions, clearances or fabrication information. Original artwork is available under CC BY 4.0. Third-party references retain their own terms.
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