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IEC Schmitt-trigger buffer (rectangular form)
Rectangular buffer carrying the hysteresis mark, meaning its input switches at two different thresholds.
IEC
IEC · HYST
IEC hysteresis (bi-threshold) qualifier
Small hysteresis loop mark showing a Schmitt-trigger function or a bi-threshold input.
In plain English
What does IEC hysteresis (bi-threshold) qualifier mean?
The hysteresis mark, a small loop drawn with two offset steps, appears either as a general additional drawing mark for a Schmitt-trigger element or next to a single input to show that input has two thresholds. TI's public overview lists both uses.
A simple example
When the mark is next to one input only, the other inputs of the element have normal single thresholds. That matters on devices such as gates with one Schmitt input used to accept a slow external signal.
What the drawings help you do
The mark does not quantify hysteresis. Threshold and hysteresis values must be read from device data and compared with the signal source.
This particular reference
Small hysteresis loop mark showing a Schmitt-trigger function or a bi-threshold input. A drawn square-wave inside a G element indicates an astable, not hysteresis.
- 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)
IEC · Hysteresis loop mark inside the outline or at an input
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.
Shown as an independently authored educational illustration. Use the cited publication and drawing legend to verify the required drafting form.
Schematic · HYST
What it represents
The hysteresis mark, a small loop drawn with two offset steps, appears either as a general qualifier for a Schmitt-trigger element or next to a single input to show that input has two thresholds. TI's public overview lists both uses.
When the mark is next to one input only, the other inputs of the element have normal single thresholds. That matters on devices such as gates with one Schmitt input used to accept a slow external signal.
The mark does not quantify hysteresis. Threshold and hysteresis values must be read from device data and compared with the signal source.
How to read it
- Check whether the mark applies to the whole element or one input.
- Compare thresholds with the signal source levels.
Information to show or reference
- Draw the mark clearly, not as a waveform sketch.
- Place it at the input it applies to where only some inputs have hysteresis.
- Reference device thresholds.
A distinction that matters
A drawn square-wave inside a G element indicates an astable, not hysteresis.
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)
Lists the Schmitt-trigger qualifier and the bi-threshold input symbol.
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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