ABB FSPS-21 - PROFIsafe Response Time and Watchdog

PROFIsafe error: Cyclic PROFIsafe error was detected (watchdog time or communication
error [PROFIsafe status bits "WD_Timeout" or "CE_CRC"] will result in "FV_activated" to
be active also, as according to PROFIsafe standard).
No PROFIsafe errors: Cyclic PROFIsafe communication is error-free.
Safe state, see States (page 30).
￭ 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 ΔT_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 ΔT_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 FSPS-21 module
is listed in the table below.
Device WDWCDTDevice
10 ms10 msFSPS-21
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 (3ADR025091M0202 [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 ΔT_WD
where
• Device_WD1 is an internal input device watchdog time
• F_WD_Time1 is the watchdog time for receipt of the new valid telegram (from the
input device to the safety PLC)
• F_Host_WD is the watchdog time of the safety PLC
• F_WD_Time2 is the watchdog time for receipt of the new valid telegram (from the
safety PLC to the output)
• Device_WD2 is an internal output device watchdog time.
Instead of WCDT values, the calculation uses watchdog times. See AC500-S Safety
User Manual (3ADR025091M0202 [English]) for details.
For example, when using the ABB AI581-S as the input device, the SM560-S safety
PLC and the FSPS module as the output device, SFRT can be calculated as follows:
122 PROFIsafe