224 PROFIsafe
PROFIsafe response time
The safety function response time (SFRT) is the time within which the safety system
must react after an error has occurred in the system.
SFRT is also the maximum time within which the safety system must respond to a
change in the input signals.
According to PROFIsafe System Description, Version November 2010, SFRT for
PROFIsafe devices can be defined as:
SFRT = TWCDT + Longest ∆T_WD
where
• TWCDT (total worst-case delay time) is the maximum time for input signal transfer
in the safety system until the output reaction under worst-case conditions (all
components require the maximum time)
• Longest ∆T_WD is the longest time difference between the watchdog time for a
given entity and the worst-case delay time.
In safety systems, to define SFRT you must take into account a potential single fault
in one of the components during the signal transfer. It is enough to consider a single
fault only (see PROFIsafe System Description, Version November 2010).
The worst-case delay time (WCDT) and watchdog (WD) values for the FSO and
FENA modules are listed in the table below.
The documentation of the safety PLC defines how you can calculate the processing
time and transmission time of the PROFIsafe connection.
For example, AC500-S Safety User Manual (3ADR025091M0207 [English]) proposes
that SFRT is calculated using the following formula:
SFRT = Device_WD1 + 0.5 x F_WD_Time1 + F_Host_WD + 0.5 x F_WD_Time2
+ Device_WD2 + Longest ∆T_WD
Device WCDT Device WD
FSO 50 ms 50 ms
FPNO-21 3 ms -
FENA-21 3 ms -
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