Programming
Fail-Safe Systems
5-26 A5E00085588-03
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Safety Note – Startup Protection to handle short power failures in the F-I/O.
Following a power failure in the F-I/O that is shorter than the watchdog time set for
the F-I/O in HW Config (See Safety Engineering in SIMATIC S7 system
description), automatic reintegration can occur, as is the case when ACK NEC = 0,
regardless of your setting for ACK NEC. If automatic reintegration for the affected
process is not permitted for this case, you must program startup protection by
evaluating the variables QBAD or PASS_OUT (see Programming Startup
Protection).
When a power failure occurs in the F-I/O and lasts longer than the watchdog time
set for the F-I/O in HW Config, the F-system detects a communication error (see
Passivation and Reintegration of the F-I/O after Communication Errors).
Reintegration After User Acknowledgment
If the input ACK_NEC is set, the reintegration of the input or output channel does
not take place until after a user acknowledgment with a positive edge at the input
ACK_REI of the F channel drivers. At the output ACK_REQ of the F channel driver,
a value of 1 indicates that the error has gone and that a user acknowledgment of
the reintegration is possible.
You can implement the user acknowledgment of reintegration in the Safety
Program as follows:
• A manual input using OS/ES (see below) or
• A hardware switch connected to a fail-safe input module.
Note
In the event of a PROFIsafe communication error on the fail-safe input module with
the hardware switch, manual acknowledgment of the input ACK_REI is no longer
possible. This can lead to blocking, which can only be corrected by means of a
startup (cold or warm restart).
We therefore recommend that the acknowledgment is also always possible via
ES/OS.
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