Ensure safe, instant shutdown of a servo drive in the event of an emergency

Used In:

  • Any servo drive

Activation of the Safe Torque Off (STO) function causes the drive to stop providing power, which controls the motor movement. The STO may be used to prevent unexpected motor rotation while the drive remains connected to a power supply.

The motor can only be electrically activated, when both STO inputs are active, i.e., current flows through the diode of the Optically Isolated STO inputs (see following figure). Whenever one of the inputs is no longer active, power that can generate motion is no longer provided to the motor by the drive. The motor in this case will be “free running” and will come to a rest under its own inertia and frictional forces.

Elmo’s STO input can be configured in the following two options:

  • TTL connection
  • PLC Source connection (default option)


Our Gold line servo drives support safe torque off (STO) according to the following industry standards:

Design Features

    • Ensure high safety operation while working on servo system
    • Prevents motor operation while the drive remains connected to the power supply
    • Support IEC and EN STO safety standard
    • Two STO configuration options for your convenience