PLC SET/RESET Logic: Why Stop Priority Matters

A clean Run state starts by defining every condition that must turn it OFF.

The core problem

A latched SET bit remains ON after the set condition disappears. That makes the reset conditions just as important as the start conditions.

SET M0 : M10 AND X5 AND (X3 OR M20)
RST M0 : /M10 OR /X5 OR M21

In the training logic, the reset rung is placed below the set rung so a simultaneous SET and RST resolves to the reset state in the same scan.

Separate the run permissive

M10 = X4 AND X10 AND X11 AND X12 AND X13 AND /X14

A reusable permissive bit keeps safety/ready conditions separate from operator start logic and makes troubleshooting easier.

Why rung order matters

The program executes top to bottom. If both SET and RST execute in the same scan, the later instruction can determine the final state.

Field checklist

• Run drops when the common permissive drops
• Auto-to-Manual transfer does not leave Auto Run latched
• HMI Stop and physical Stop do not conflict
• Fault reset is not confused with Run reset
• No unintended automatic restart after power recovery