Siemens SIMATIC PCS 7 Configuration Manual page 240

Configuring OS functions
10.3 Message system
Note
To prevent too many messages being generated, the SIMATIC PCS 7 message concept
employs cumulative message suppression.
Example: An I/O module with eight channels can signal a wire break for every channel. If the
I/O module is not connected to a power supply, just this one message is output and other
messages relating to wire breaks are suppressed.
Alarm, warning, and tolerance messages
Alarm, warning and tolerance messages indicate that a limit value has been exceeded. These
limit values have been defined in the user-configured CFC. The Interrupt message class is
usually used for emergency stop and other high-priority binary messages too.
The messages/signals are aimed at the operator.
AS process control messages
AS process control messages provide information about errors in I/O peripherals and in the
system, e.g. wire break, short circuit, channel error, module failure, access error, redundancy
failure, or failure of the auxiliary current. If a system-generated message or signal is displayed,
the operator must inform the service team without delay.
OS process control messages
OS process control messages provide information about errors in the OS system, e.g. errors
relating to database access, failure of OS server redundancy, errors relating to time
synchronization. If a system-generated message or a signal for field instruments is displayed,
the operator must inform the service team without delay.
Preventive maintenance
Preventive maintenance indicates that maintenance personnel will soon have to take a look
at devices/components (due to a dirty sensor, for example).
Process messages
Process messages are specified to a user.
Operational messages
Operational messages provide information about the plant status during conventional
operation (e.g. "Temperature setpoint reached").
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Compendium Part A - Configuration Guidelines (V8.2)
Configuration Manual, 08/2016, A5E38166869-AA