RS (Reset-Set Flip-Flop, Reset Dominant)
Reset-dominant flip-flop: reset wins if both set and reset are active simultaneously.
The RS (Reset-Set) bistable function block is a reset-dominant flip-flop. It operates identically to the SR flip-flop except that when both Set (S) and Reset (R1) inputs are TRUE simultaneously, the reset takes priority and Q1 goes FALSE. RS bistables are preferred in safety-critical applications where the reset (stop/safe) condition must always override the set condition, ensuring the system defaults to a safe state when conflicting signals are present.
Parameters
Inputs
| Name | Type | Description |
|---|---|---|
| S | BOOL | Set input - sets Q1 to TRUE |
| R1 | BOOL | Reset input (dominant) - resets Q1 to FALSE |
Outputs
| Name | Type | Description |
|---|---|---|
| Q1 | BOOL | Bistable output |
Ladder Logic Example
// Ladder Logic - RS Flip-Flop (Safety Latch) // // |----[ Start_PB ]----------( S )-- Motor_Latch ---| Set // |----[ Stop_PB ]----+------( R )-- Motor_Latch ---| Reset (dominant) // |----[ E_Stop ]----+ | // |----[ Motor_Latch ]------( Motor_Contactor )-----|
Structured Text Example
VAR
Motor_Latch : RS;
Start_PB : BOOL;
Stop_PB : BOOL;
Motor_Run : BOOL;
END_VAR
Motor_Latch(S := Start_PB, R1 := Stop_PB);
Motor_Run := Motor_Latch.Q1;20 Production-Ready Ladder Logic Templates
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Platform-Specific Implementation
Siemens (TIA Portal)
Use RS instruction in TIA Portal. Reset-dominant for safety applications.
Allen-Bradley (Studio 5000)
Implement with OTL/OTU where OTU rung is below OTL for reset-dominant behavior.
CODESYS
Standard RS in Standard library. Reset input is R1 (dominant).
Common Applications
- Safety-critical motor control
- E-stop circuits
- Valve control with safety priority
- Process shutdown latches
Common Mistakes to Avoid
- Using SR when RS is needed for safety applications
- Not understanding the difference between SR and RS priority
Frequently Asked Questions
What is RS (Reset-Set Flip-Flop, Reset Dominant) in PLC programming?
Reset-dominant flip-flop: reset wins if both set and reset are active simultaneously. The RS (Reset-Set) bistable function block is a reset-dominant flip-flop. It operates identically to the SR flip-flop except that when both Set (S) and Reset (R1) inputs are TRUE simultaneously, the reset takes priority and Q1 goes FALSE.
What are common applications of RS (Reset-Set Flip-Flop, Reset Dominant)?
RS (Reset-Set Flip-Flop, Reset Dominant) is commonly used for: Safety-critical motor control, E-stop circuits, Valve control with safety priority, Process shutdown latches.
How do I use RS (Reset-Set Flip-Flop, Reset Dominant) in different PLC platforms?
Siemens: Use RS instruction in TIA Portal. Reset-dominant for safety applications. Allen-Bradley: Implement with OTL/OTU where OTU rung is below OTL for reset-dominant behavior. CODESYS: Standard RS in Standard library. Reset input is R1 (dominant).
Related Function Blocks
SR (Set-Reset Flip-Flop, Set Dominant)
Set-dominant flip-flop: output latches TRUE on set, cleared by reset. Set wins if both active.
View Reference →R_TRIG (Rising Edge Detection)
Detects FALSE-to-TRUE transition, outputs single-scan pulse on rising edge.
View Reference →F_TRIG (Falling Edge Detection)
Detects TRUE-to-FALSE transition, outputs single-scan pulse on falling edge.
View Reference →Stop Writing Ladder Logic From Scratch
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