RFC 4072S
28 / 272
PHOENIX CONTACT 108580_en_02
2.3 Calculating/determining the response time (Safety
Function Response Time, SFRT)
The procedure for determining the necessary times, which is explained in more detail below,
is recommended.
1. Determining the maximum permissible safety function response time (SFRT
max
)
depending on the relevant safety function to be implemented and
determining the resulting maximum monitoring/watchdog times
(F_WD_Time IN
max
/F_WD_Time OUT
max
) as an upper limit for each individual
safety function (see Section 2.3.1 on page 29).
2. Determining the minimum monitoring/watchdog times
(F_WD_Time IN
min
/F_WD_Time OUT
min
) required for optimum system availability
as a lower limit (see Section 2.3.2 on page 32).
3. Defining the monitoring/watchdog times (F_WD_Time IN/F_WD_Time OUT) to be
parameterized within the determined upper and lower limits and checking/validating
that each of the safety functions to be implemented may be implemented with the
defined monitoring/watchdog times (see Section 2.3.3 on page 38).
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