Siemens SIMATIC S7 System Manual page 152

Glossary
Sequence Number
Time-monitoring of the message-frame update in the PROFIsafe protocol is performed
through the transfer of a sequence number from the -> F-CPU to the -> F-I/O. A valid,
current -> safety message frame with a valid sequence number must be received by the F-
CPU and the F-I/O within an assignable monitoring time. If a valid sequence number is not
detected within the monitoring time, the F-I/O is passivated.
Single-channel I/O
Single-channel I/O is a configuration variant of S7 Distributed Safety/S7 F Systems in ->
safety mode. The -> F-CPU and -> F-I/O are not redundant. In the event of a fault, the F-I/O
is no longer available.
Single-channel Switched I/O
Single-channel switched I/O are a configuration variant of S7 FH Systems in -> safety mode
to increase availability. The -> F-CPU is redundant and the -> F-I/O is not redundant; in the
event of a fault, the system switches over to the other F-CPU. In the event of a fault, the F-
I/O is no longer available.
Standard Communication
Standard communication is communication used to exchange non-safety-related data.
Standard Mode
In standard mode of -> F-I/O, -> safety-related communication using -> safety message
frames is not possible; only -> standard communication is possible in this operating mode.
-> S7-300 F-SMs can be used in standard mode or -> safety mode. -> Fail-safe modules
ET 200S, ET 200pro und ET 200eco are designed for safety mode only.
Standard User Program
The standard user program is a non-safety-related user program.
Test Signals
For -> F-I/O with outputs, the required -> safety class is achieved by injecting test signals (->
light period, -> dark period).
User Safety Function
The -> safety function for the process can be provided through a user safety function or a
fault reaction function. The user only programs the user safety function. If the -> F-system
can no longer execute its actual user safety function, it executes the fault reaction function;
for example, the associated outputs are deactivated, and the -> F-CPU switches to STOP
mode, if necessary.
Glossary-14
Safety Engineering in SIMATIC S7
System Manual, 04/2006, A5E00109529-05