S5-115F Manual Rules Governing the Use of the S5-115F
10.8 I/O Module Types
There are different types of digital and analog I/Os which are suited to the type of sensors and
actuators and to the time characteristics of the input/output signals.
A distinction is made between safety-related and nonsafety-related types.
In the case of safety-related types, a further distinction is made between types for intermittent
signals and types for non-intermittent signals.
An intermittent digital signal must be subjected to sufficently frequent changes of state and these
must be detected by the CPU. For this purpose, the signal must assume the states ”0” and ”1” at
least once within the second error occurrence time and for a period in each case greater than the
PLC cycle time.
Thus, the EMERGENCY OFF input is
not
intermittent since it is activated extremely infrequently.
An analog signal is intermittent if the whole value range relevant to evaluation within the second
error occurrence time is run through, read in and coded at least once. In doing so, it is of special
importance that those values that lead to a safety response be reached.
For example, an analog input for measuring temperature cannot generally be configured as an
intermittent type since the critical temperatures leading to emergency shutdown are never
reached in normal operation.
Note
If an intermittent type is selected for a safety-related signal, this signal characteristic
must be proved to the licensing authority.
This characteristic is often impossible or difficult to prove. If this is the case, a non-
intermittent type should be selected for which no special proof is required.
I/O types differ according to
• Safety-related aspects
• Number of I/O channels
• Features and type of connection
• ”Intermittent” characteristic
• Feedback type
• Number of feedback channels.
EWA 4NEB 811 6148-02
10-15
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