Technical data

Table Of Contents
Generating Block-Related Messages
System Software for S7-300/400 System and Standard Functions - Volume 2/2
A5E00739858-01
24-29
24.13 Generating Acknowledgeable Block-Related Messages
with SFC 17 "ALARM_SQ" and Permanently
Acknowledged Block-Related Messages with SFC 18
"ALARM_S"
Note
Newly created programs should only use SFCs 107 and 108 (if your CPU supports
SFCs 107 and 108) since they provide improved options for managing system
resources.
Description
Each time they are called, SFC 17 "ALARM_SQ" and SFC 18 "ALARM_S"
generate a message to which you can add associated values. The message is sent
to all stations that have logged on for the message. SFC 17 and SFC 18 provide
you with a simple mechanism for sending messages. You must make sure that you
only call SFC 17 or SFC 18 when the value of the triggering signal SIG is inverted
compared with the last call. If this is not the case, this is indicated in RET_VAL and
no message is sent. The very first time that SFC 17 or SFC 18 is called, you must
make sure that the SIG input has the value 1. Otherwise, RET_VAL contains error
information and no message will be sent.
Note
Call SFC 17 and SFC 18 in an FB to which you have previously assigned suitable
system attributes! For more detailed information about assigning system attributes
to blocks, refer to /234/.
Use of system resources
When generating messages with the SFCs 17 "ALARM_SQ" and 18 "ALARM_S",
the operating system uses one system resource for the duration of the signal cycle.
For SFC 18 "ALARM_S", the signal cycle lasts from the SFC call SIG=1 until
another call with SIG=0. For SFC 17 "ALARM_SQ", this time period also includes
the time until the incoming signal is acknowledged by one of the reported display
devices, if necessary.
If, during the signal cycle, the message-generating block is overloaded or deleted,
the associated system resource remains occupied until the next restart (warm
restart).