{"schemaVersion":1,"slug":"iec-schmitt-trigger-buffer","name":"IEC Schmitt-trigger buffer (rectangular form)","kind":"symbol","abbreviation":"1","aliases":["IEC buffer with hysteresis","bi-threshold buffer","rectangular Schmitt buffer"],"category":"logic-signals","drawingTypes":["Schematic"],"referenceFamilies":["IEC","Manufacturer"],"summary":"Rectangular buffer carrying the hysteresis mark, meaning its input switches at two different thresholds.","description":["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."],"readingNotes":["Locate the hysteresis mark; it may be centred or next to an input.","Check signal levels against the device thresholds."],"drawingChecklist":["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."],"distinction":"A plain 1 buffer has a single threshold; only the hysteresis mark indicates Schmitt behaviour.","facts":[{"label":"Qualifier","value":"1 plus hysteresis mark"},{"label":"Typical location","value":"Field input conditioning"},{"label":"Artwork scope","value":"Original educational illustration; consult the applicable standard and the selected device documentation for the exact graphic"}],"references":[{"sourceId":"s28-ti-ieee91-logic-symbols","relevance":"Lists the Schmitt-trigger and bi-threshold input qualifiers."},{"sourceId":"pub-electronics-ti-lvc17","relevance":"Manufacturer Schmitt-trigger buffer documentation."}],"related":["schmitt-trigger-buffer","iec-hysteresis-input-qualifier","iec-buffer"],"artwork":"s28-iec-schmitt-trigger-buffer","publishedOn":"2026-10-28","updatedOn":"2026-10-28","provenance":"independent-public-research","symbolVariant":{"convention":"IEC","form":"1 element with the hysteresis qualifier inside the outline","reference":"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.","counterpart":"schmitt-trigger-buffer"},"plainEnglish":{"title":"What does IEC Schmitt-trigger buffer (rectangular form) mean?","introduction":"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.","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.","drawings":"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.","terms":[{"name":"Normally open / normally closed","meaning":"The contact state in the normal condition defined for the drawing."},{"name":"Signal","meaning":"Electrical information passed between devices, rather than necessarily the main power supply."},{"name":"Rating","meaning":"A value stating the conditions for which equipment is specified."}],"diagram":"flow","steps":["Identify the device and form","Read the connections and state","Check the equipment reference"],"caption":"A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.","sources":[{"label":"Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A)","url":"https://www.ti.com/lit/ml/sdyz001a/sdyz001a.pdf"},{"label":"SN74LVC1G17 — non-inverting Schmitt buffers","url":"https://www.ti.com/product/SN74LVC1G17"}],"detail":"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.","profileId":"reviewed-symbol-20261028"},"url":"https://eloragrid.ai/symbols/iec-schmitt-trigger-buffer","illustration":{"url":"https://eloragrid.ai/tools/electrical-library/illustrations/iec-schmitt-trigger-buffer.svg","scope":"Original educational concept; not an official standard graphic or product footprint","license":"https://creativecommons.org/licenses/by/4.0/"},"sources":[{"id":"s28-ti-ieee91-logic-symbols","title":"Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A)","publisher":"Texas Instruments","url":"https://www.ti.com/lit/ml/sdyz001a/sdyz001a.pdf","type":"manufacturer","scope":"Public explanation of rectangular logic symbols, general qualifiers, input and output qualifiers, dependency notation, bistable elements and coders, consistent with IEC 617-12 work.","checkedOn":"2026-10-28","reuse":"link-only","relevance":"Lists the Schmitt-trigger and bi-threshold input qualifiers."},{"id":"pub-electronics-ti-lvc17","title":"SN74LVC1G17 — non-inverting Schmitt buffers","publisher":"Texas Instruments","url":"https://www.ti.com/product/SN74LVC1G17","type":"manufacturer","scope":"Public device-function and terminal documentation used as an example of non-inverting schmitt buffers. Example part numbers are manufacturer identifiers, not IEC/IEEE symbol IDs. Device ratings, pin numbering and availability remain in the selected product documentation.","checkedOn":"2026-10-14","reuse":"link-only","relevance":"Manufacturer Schmitt-trigger buffer documentation."}]}