SET & RESET Command for Latching or Unlatching in Siemens TIA Portal V16 with PLC Sim

 SET & RESET Command for Latching or Unlatching in Siemens TIA Portal V16 with PLC Sim

Behaviour of Set and Reset

Here’s how outputs behave under different input conditions:

1. Both Set and Reset inputs are OFF → Output holds its last state (ON or OFF).
2. Set is ON, Reset is OFF → Output is turned ON.
3. Set is OFF, Reset is ON → Output is turned OFF.
4. Both Set and Reset are ON → Output is OFF (Reset has higher priority than Set).

This priority behavior ensures that if both instructions are triggered at the same time, the output is forced to a safe OFF condition.

 Using PLC SIM to provide user inputs

Setting Up PLC SIM

PLC SIM SIM Table

 

Imagine you want to control two indicator lights (Q0.0 and Q0.1) using two push buttons:

• Button A (I0.0) → Latches both outputs ON.

• Button B (I0.1) → Unlatches (resets) both outputs OFF.

Ladder Logic with Set/Reset

• Input I0.0 (A) → Set Q0.0 and Q0.1
• Input I0.1 (B) → Reset Q0.0 and Q0.1

How it works:

• Pressing A once will turn ON both lights, and they will stay ON even after releasing the button.

• Pressing B will reset them back to OFF.

This is a common and clean way to implement a Start/Stop lamp logic with simple push buttons.

Alternate Approach Without Set/Reset: The same functionality can be built using normal and inverted contacts (NO/NC).

Example logic:

• Use a self-holding contact of the output in parallel with the push-button input (creating a latch).
• Use a second input (NC contact) to break the latch when needed.

Functionally, the result is the same:

• A pulse on A latches the outputs ON.

• Pressing A once will turn ON both lights, and they will stay ON even after releasing the button.

• A pulse on B breaks the latch, turning the outputs OFF.

• Pulse on both A and B breaks the latch, turning the outputs OFF.

This approach offers flexibility and sometimes makes debugging easier for those used to traditional coil/contact logic.

Set/Reset in FBD (Function Block Diagram)

The same principle applies in FBD programming. Siemens provides dedicated blocks:

• S (Set block)
• R (Reset block)

The logic is identical to ladder, just represented visually with blocks and signal lines:

• An input signal to S sets the output ON.
• An input signal to R resets the output OFF.
• If both are active, Reset dominate