SCHMITT
Schmitt-trigger buffer
A non-inverting digital element with separate rising and falling input thresholds.
Schematic
IEC · 1
IEC Schmitt-trigger buffer (rectangular form)
Rectangular buffer carrying the hysteresis mark, meaning its input switches at two different thresholds.
In plain English
What does IEC Schmitt-trigger buffer (rectangular form) mean?
A Schmitt-trigger buffer is an IEC 1 element with the hysteresis additional drawing mark drawn inside the box. The additional drawing mark states that the input has two thresholds, one for a rising signal and one for a falling signal, so the output does not chatter when the input passes slowly through the switching region.
A simple example
In control panels this element usually sits at the boundary between field wiring and logic, where a slow or noisy contact or sensor signal needs squaring up. Drawings sometimes place the hysteresis mark at the input line rather than at the centre; both indicate the same bi-threshold behaviour.
What the drawings help you do
The symbol does not give threshold voltages or hysteresis width. Those values belong to the device datasheet and should be checked against the field signal levels.
This particular reference
Rectangular buffer carrying the hysteresis mark, meaning its input switches at two different thresholds. A plain 1 buffer has a single threshold; only the hysteresis mark indicates Schmitt behaviour.
- Normally open / normally closed
- The contact state in the normal condition defined for the drawing.
- Signal
- Electrical information passed between devices, rather than necessarily the main power supply.
- Rating
- A value stating the conditions for which equipment is specified.
Read the source: Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A) · SN74LVC1G17 — non-inverting Schmitt buffers
IEC · 1 element with the hysteresis qualifier inside the outline
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
A Schmitt-trigger buffer is an IEC 1 element with the hysteresis qualifier drawn inside the box. The qualifier states that the input has two thresholds, one for a rising signal and one for a falling signal, so the output does not chatter when the input passes slowly through the switching region.
In control panels this element usually sits at the boundary between field wiring and logic, where a slow or noisy contact or sensor signal needs squaring up. Drawings sometimes place the hysteresis mark at the input line rather than at the centre; both indicate the same bi-threshold behaviour.
The symbol does not give threshold voltages or hysteresis width. Those values belong to the device datasheet and should be checked against the field signal levels.
How to read it
- Locate the hysteresis mark; it may be centred or next to an input.
- Check signal levels against the device thresholds.
Information to show or reference
- Draw the hysteresis mark clearly so it is not mistaken for a waveform note.
- Reference the device for threshold values.
- Keep the input filter components on the same sheet.
A distinction that matters
A plain 1 buffer has a single threshold; only the hysteresis mark indicates Schmitt behaviour.
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 and bi-threshold input qualifiers.
SN74LVC1G17 — non-inverting Schmitt buffers
Manufacturer Schmitt-trigger buffer 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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