3.6.6 Signal Filtering
If a sensor with 2NC or 1NC/NO is selected, the safety option checks the signals of the safe digital input for consistency. Consistent
signals at both inputs always assume the same signal state (high or low). If 1NC/1NO is selected, it checks the right state of each input.
With electromechanical sensors (for example, emergency stop buttons or door switches), the 2 sensor switches never switch at the
same time (discrepancy). A long-term discrepancy points towards a fault in the wiring of a safe input, for example, a wire break. An
adjustable filter in the safety option prevents faults caused by temporary or short-term discrepancy. Within the filter tolerance time
(parameter 42-22 Discrepancy Time), the safety option suppresses the discrepancy monitoring of the safe inputs.
Illustration 15: Discrepancy Time
Parameterize the discrepancy time of the switching elements connected to the digital inputs. The default value is 10 ms.
NOTICE
The discrepancy time does not extend the safety option response time. The safety option activates its safety function when 1 of
the 2 DI signals changes from high to low.
3.6.7 Stable Signal Time from Safe Outputs
The safety option normally responds immediately to signal changes at its safe input DI1 or DI2. This response is not required in the
following cases:
• When interconnecting the safe input of the option with an electromechanical sensor, contact bounce may result in signal changes
occurring, to which the option could respond.
• Several control modules test their safe outputs using test pulse pattern (on/off tests), to identify faults due to either short or cross
circuiting. When interconnecting the safe input of the option with a safe output of a control module, the option could respond to
these test signals.
A signal change during a test pulse pattern usually lasts 1 ms.
Functions and Systems Overview
Installation Guide | VLT® Safety Option MCB 150/151
AQ279747441421en-000101 / 130R0292 | 33
Danfoss A/S © 2018.09
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