SR-LATCH
Set-reset latch
Retains a digital state that is changed by the defined set or reset input conditions.
Schematic
IEC · S R
IEC SR latch (rectangular form)
Rectangular IEC bistable with S and R inputs that set or reset the stored state immediately.
In plain English
What does IEC SR latch (rectangular form) mean?
The simplest IEC bistable is a rectangle with inputs labelled S and R and outputs Q and complemented Q. An active S sets the element to 1 and an active R resets it to 0, with no clock involved. Negation or polarity marks show active-low inputs, as on devices built from cross-coupled NAND gates.
A simple example
SR latches appear in alarm-latching and seal-in logic on control drawings. The key question for a reviewer is what happens when both S and R are active; dependency notation or a note should state whether set or reset dominates.
What the drawings help you do
The symbol does not specify the power-up state. Where it matters, add the I=0 or I=1 additional drawing mark or a note. A separate reset or initialisation circuit is needed when the system requires a defined starting condition.
This particular reference
Rectangular IEC bistable with S and R inputs that set or reset the stored state immediately. A clocked SR flip-flop has 1S and 1R inputs under C1 control; the plain latch has none.
- 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) · SN74LS279A — set-reset latches
IEC · Rectangular bistable with S and R inputs
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 · S R
What it represents
The simplest IEC bistable is a rectangle with inputs labelled S and R and outputs Q and complemented Q. An active S sets the element to 1 and an active R resets it to 0, with no clock involved. Negation or polarity marks show active-low inputs, as on devices built from cross-coupled NAND gates.
SR latches appear in alarm-latching and seal-in logic on control drawings. The key question for a reviewer is what happens when both S and R are active; dependency notation or a note should state whether set or reset dominates.
The symbol does not specify the power-up state. Where it matters, add the I=0 or I=1 qualifier or a note. A separate reset or initialisation circuit is needed when the system requires a defined starting condition.
How to read it
- Check for active-low marks on S and R.
- Look for a stated set or reset priority.
Information to show or reference
- Label S and R inside the outline.
- State the priority when both are active.
- Specify the power-up state where relevant.
A distinction that matters
A clocked SR flip-flop has 1S and 1R inputs under C1 control; the plain latch has none.
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)
Usual meanings of S and R inputs and bistable symbols.
SN74LS279A — set-reset latches
SR latch device 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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