SLo / STL Safe Torque Off (STO) Lock fault in delta drive

SLo / STL Safe Torque Off (STO) Lock fault in delta drive troubleshooting

Description

SLo (or STL on MS300) is not a defect but a safety status. It indicates that the hardware "Safe Torque Off" terminals (usually STO1 and STO2 connected to +24V) are open. In this state, the drive physically cannot generate a magnetic field to rotate the motor.

Causes

1. Emergency Stop: The hardwired safety circuit (E-Stop buttons, Door Gates, Light Curtains) is open.
2. Jumper Missing: On a new drive, the factory jumper bridging STO1-STO2-24V is missing or loose.
3. Wiring Fault: One of the two safety channels has a broken wire.

Solution

Verify safety circuit:
1. Check Safety Inputs: Look at the STO terminals. Both STO1 and STO2 must have +24V present to run.
2. Reset E-Stop: Pull out the Emergency Stop button.
3. Install Jumper: If you are not using the safety function (bench testing), ensure the factory metal link or wire jumper is installed between STO terminals and +24V/DCM.