Technical data

Table Of Contents
Generating Block-Related Messages
System Software for S7-300/400 System and Standard Functions - Volume 2/2
A5E00739858-01
24-27
Message Storage
To avoid messages being lost when there is a lot of traffic on the communications
system, the SFCs 17, 18, 107 and 108 can both buffer two messages.
If, however, messages are lost, you are informed in RET_VAL. The logged on
display devices are informed of this the next time a message can be sent.
Message Acknowledgment with the SFCs 17 "ALARM_SQ" and 107
"ALARM_DQ"
If you have acknowledged an "entering event message" at a display device, this
acknowledgment information is first sent to the CPU where the message
originated. This then distributes the acknowledgment information to all stations
logged on for this purpose.
Disabling and Enabling Messages
Block-related messages generated with SFC 17 "ALARM_SQ", SFC 18
"ALARM_S", SFC 107 "ALARM_DQ" or SFC 108 "ALARM_D" cannot be disabled
and then enabled again.
Changes in Your Program that contains the SFC 17/SFC 18 calls
Note
When you download a block that is already on the CPU using SFC 17/SFC 18
calls, it is possible that the previous block has sent an entering state message but
that the new block does not send a corresponding leaving state message. This
means that the message remains in the internal message memory of the CPU.
This situation can also occur when you delete blocks with SFC 17/SFC 18.
You can remove such messages from the internal message memory of the CPU by
changing the CPU to STOP and then going through a warm or cold restart.
Changes in Your Program that contains the SFC 17/SFC 18 calls
Even though your program might contain SFC 107 and/or SFC 108 calls, the
described program modifications may cause the messages to become resident in
the internal message memory and thus permanently occupy system resources.
Contrary to system resources which were occupied by SFC 17/SFC 18 calls, you
can release system resources occupied by SFC 107/SFC 108 calls without having
to switch your CPU to STOP mode. This is carried out by using SFC 106 "DEL_SI",
see Reading Dynamic System Resources with SFC 106 "DEL_SI" . Before you
release dynamically occupied system resources by calling SFC 106 "DEL_SI", it
may be appropriate to fetch information on currently dynamically occupied system
resources of your CPU, with the help of SFC 105 "READ_SI", see Reading
Dynamic System Resources with SFC 105 "READ_SI".