8 STO (safe torque off)
8.5.3 Control inputs STOA, GNDA/STOB, GNDB [X3]
The control inputs STOA and STOB are used for requesting the STO safety function
("Safe Torque Off") via two channels. They enable the direct connection of safe
semiconductor outputs (electronic safety relays, active safety sensors, e.g. light grid with
OSSD signals) and of switching contacts (safety relays with relay outputs, passive safety
sensors, e.g. positively driven position switches). See, for example, section 13.12
Connector: STO [X3] on page 257.
To request the STO safety function ("Safe Torque Off"), the 24 V control voltage is
switched off at both control inputs STOA and STOB (0 V). If both control inputs are
switched off simultaneously or within a predefined discrepancy time, the STO function
becomes active.
The table in section 12.11.1 Electrical data of the STO function on page 230 includes the
technical data of the control inputs for the specified operating range of logic voltages.
Certain tolerance ranges have been defined for the input voltage range of the control inputs
STOA and STOB. The amount of energy that is stored in the components of the STO
circuit (e.g. capacitors) depends on this input voltage. During switching processes, this
energy must be charged or discharged. This results in specific values for the switch-off
time for the transition to the safe state (STO) and for the tolerance time with regard to the
OSSD signals (buffer time). These values depend on the input voltage.
The time response is described in section 12.11.2 Time response on page 231.
8.5.4 Discrepancy time
The transition between the safe and unsafe state is initiated by level changes at the
control inputs STOA and STOB of the servo drive. In accordance with the specification of
the safety function, both levels must be identical. Otherwise, an error message will be
generated. The finite state machine in the servo drive monitors the driver supply voltage
as a result of the actuation of the control inputs. Usually, these level changes do not occur
exactly at the same time, e.g. due to component tolerances or bouncing outputs of safety
control systems. The firmware tolerates this as long as the second input follows within a
predefined time, the so-called discrepancy time. If it is exceeded, the servo drive will
generate an error message.
A discrepancy time of 100 ms is preset. Recommendation: Ensure to always switch
STOA and STOB simultaneously.
In addition, to the integrated STO circuit, the finite state machine in the servo drive has its
own status. Due to the evaluation of the discrepancy time, this finite state machine may
reach the "safe state" only with a considerable delay. Consequently, this state can only be
signalled via the digital outputs or a fieldbus with a considerable delay. The power output
stage has already been "switched off safely" when this happens. The status of the finite
state machine is processed every 10 ms.
Product manual BL 4000-C Page 198 of 298
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