Safe Torque Off
6-3
890CS Common Bus Supply - Frames B & D; 890CD Common Bus Drive and 890SD Standalone Drive - Frames E & F
STO Functional Description
STO is a means of preventing an 890 drive from delivering power to its connected electric motor. Please refer to
EN61800-5-2:2007 para 4.2.2.2 for the formal definition.
To ensure a high degree of safety, two independent STO control channels are implemented in hardware. The
STO circuit in the 890 is designed such that a fault in one control channel will not affect the other channel’s
ability to prevent the drive from starting, i.e. the STO function of the 890 drive is tolerant to any single fault. It
may not be tolerant to an accumulation of faults. This is in keeping with its declared safety ratings.
STO always overrides any attempt to start the drive. If one or both STO control inputs is requesting the STO
function, the drive will not start, even if for example, the drive’s software malfunctions and tries to cause the
motor to turn.
The STO function is implemented in hardware; it overrides all software activities. The only software
involvement is to report STO status to the user via an MMI, serial communications link or user terminal on the
890 control board as defined by the drive configuration.
WARNING
THE DECLARED SIL/PL CAPABILITY OF THIS STO PRODUCT CAN BE ACHIEVED ONLY
WHEN THE TWO STO USER INPUTS ARE DRIVEN INDEPENDENTLY. THEY MUST NOT BOTH BE
DRIVEN FROM A COMMON SOURCE; OTHERWISE THE SINGLE FAULT DETECTION WILL BE
COMPLETELY INOPERATIVE.
USE OF THE PRODUCT IN THIS “COMMON SOURCE” CONDITION INVALIDATES THE STO
PRODUCT SPECIFICATION AND IS ENTIRELY AT THE USER’S OWN RISK.
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