Back to FBD referenceTypical FBD symbol for RS
Memory
RS
Reset-dominant latch
The RS block stores Q between scans. Input S sets the state, while input R resets it with priority.
Typical FBD symbol
R
S
RS
Q
Inputs
| Name | Type | Required | Description |
|---|---|---|---|
| R | BOOL | Yes | Reset for Q. This input has priority over set. |
| S | BOOL | Yes | Set for Q when reset is not active. |
Outputs
| Name | Type | Required | Description |
|---|---|---|---|
| Q | BOOL | Yes | The retained boolean state. |
Stateful
Yes, the block keeps internal state between scans.
Behavior rule
Q = (previousQ OR S) AND NOT R
Practical use
- start seal-in logic with reset or stop priority;
What to watch
- If R and S are TRUE at the same time, reset has priority, so Q = FALSE.
- Do not confuse it with SR: SR gives priority to set when S and R are both TRUE.