S R
IEC SR latch (rectangular form)
Rectangular IEC bistable with S and R inputs that set or reset the stored state immediately.
IEC
IEC · Sm/Rm
IEC set and reset dependency (S, R)
Dependency labels Sm and Rm: the affected output behaves as if set or reset when the affecting input is active.
In plain English
What does IEC set and reset dependency (S, R) mean?
S and R dependencies specify the internal state of an output when both set and reset conditions could be active together. An Rm input at its internal 1 state makes the affected output react as it would to S=0, R=1; an Sm input does the opposite. At 0 they have no effect.
A simple example
These labels are used where a storage element must have a defined priority between set and reset. A reader can see from the symbol whether set or reset dominates, which matters in latching alarm and interlock logic.
What the drawings help you do
Plain S and R labels without numbers are the ordinary set and reset inputs of a bistable element. The dependency form only applies when a number follows the letter.
This particular reference
Dependency labels Sm and Rm: the affected output behaves as if set or reset when the affecting input is active. An ordinary R input resets the element directly; Rm dependency states the reaction of particular outputs.
- 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.
- 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 · Affecting inputs labelled Sm or Rm with affected outputs labelled m
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 · Sm/Rm
What it represents
S and R dependencies specify the internal state of an output when both set and reset conditions could be active together. An Rm input at its internal 1 state makes the affected output react as it would to S=0, R=1; an Sm input does the opposite. At 0 they have no effect.
These labels are used where a storage element must have a defined priority between set and reset. A reader can see from the symbol whether set or reset dominates, which matters in latching alarm and interlock logic.
Plain S and R labels without numbers are the ordinary set and reset inputs of a bistable element. The dependency form only applies when a number follows the letter.
How to read it
- S or R followed by a number is a dependency; without a number it is an ordinary input.
- Check which one dominates when both are active.
Information to show or reference
- Number dependency inputs explicitly.
- Label affected outputs with the matching number.
- State set or reset priority in the logic description.
A distinction that matters
An ordinary R input resets the element directly; Rm dependency states the reaction of particular outputs.
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)
S (set) and R (reset) dependencies and the summary table.
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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