RS and SR Latches

1. RS and SR Latches

RS and SR are memory blocks. One signal turns the output on; another turns it off:

Set   → store TRUE
Reset → clear to FALSE
Q     → current stored state

A latch is useful when the output must retain its state after a brief input pulse.

2. Why a Latch Is Needed

Without memory, an output often follows the input: press a button and it turns on; release the button and it turns off.

A latch remembers the command:

Press Set   → Q = TRUE
Release Set → Q stays TRUE
Press Reset → Q = FALSE

3. Inputs and Outputs

Programming environments may use Set, Reset, S, or R; the principle is the same.

4. Example: Motor Start and Stop

[Start_Button] ──> S ┌────┐
                    │ RS │ Q ──> [Motor_Run]
[Stop_Button] ───> R └────┘

Start_Button sets Q; Stop_Button resets it.

5. State Table

The simultaneous TRUE case is the important difference between the two blocks.

6. RS versus SR

For mechanism control, reset priority is often preferable so a stop or fault-reset condition overrides a start command.

7. Reset-Priority RS

S = TRUE, R = FALSE  → Q = TRUE
R = TRUE            → Q = FALSE
S = FALSE, R = FALSE → Q retained

Even when Start and Stop are active together, Reset clears the output.

8. Set-Priority SR

SR gives priority to Set. When S and R are both TRUE, Q is TRUE. This is useful for some logic tasks, but requires care when controlling equipment starts.

9. Where Latches Are Used

• retaining a start command;
• holding an alarm until operator reset;
• remembering a selected mode;
• retaining a sequence step;
• storing a cycle-running state;
• recording a sensor event.

10. Simulator Practice

Open the RS/SR lesson example and start the simulation.

In the supplied diagram, Input 1 resets both blocks, Input 2 sets SR, and Input 3 sets RS. Output 1 shows SR; Output 2 shows RS.

• 1. Leave Input 1 FALSE and activate Inputs 2 and 3 briefly.
• 2. Return Inputs 2 and 3 to FALSE; both outputs remain TRUE.
• 3. Activate Input 1; both outputs become FALSE.
• 4. With Input 1 still TRUE, activate Inputs 2 and 3. Output 1 becomes TRUE because SR has Set priority; Output 2 remains FALSE because RS has Reset priority.

The exercise shows how the blocks retain state between scans and apply different priorities.

11. Key Points

RS and SR store Boolean state: S sets Q, R resets Q, and Q carries the retained result. Always account for priority when both commands are active, especially for start and stop logic.