# IEC Schmitt-trigger buffer (rectangular form)

Canonical URL: https://eloragrid.ai/symbols/iec-schmitt-trigger-buffer
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 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.
Related representation: [Schmitt-trigger buffer](https://eloragrid.ai/symbols/schmitt-trigger-buffer)

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.

### Terms

- 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.

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)
- [SN74LVC1G17 — non-inverting Schmitt buffers](https://www.ti.com/product/SN74LVC1G17)


## 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.

## Important distinction

A plain 1 buffer has a single threshold; only the hysteresis mark indicates Schmitt behaviour.

## Facts

- Qualifier: 1 plus hysteresis mark
- Typical location: Field input conditioning
- 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. Lists the Schmitt-trigger and bi-threshold input qualifiers.
- [SN74LVC1G17 — non-inverting Schmitt buffers](https://www.ti.com/product/SN74LVC1G17) — Texas Instruments. Manufacturer Schmitt-trigger buffer 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.