Working With The Function Block - Siemens SIWAREX WP251 Manual

11.1.4

Working with the function block

Data records in SIWAREX weighing modules
All parameters in SIWAREX weighing modules are structured in data records. These data
records must be considered as connected packages and can only be respectively read into
the CPU or written to the SIWAREX as complete packages. Reading or writing of a single
parameter within a data record is not possible. You can find a description of all data records
and their parameters in chapter → Scale parameters and functions (Page 57).
Reading and writing of data records is carried out using special command codes which can
be sent with three command mailboxes handled according to priority within the instance DB:
Image 11-6
As shown in the graphics, a command mailbox always consists of a command code (Int) and
four bits (Bool). A command is set by entering the desired command code in the
"i_CMD_CODE" parameter and setting the respective command trigger
"bo_CMD_TRIGGER". The status bits "bo_CMD_InProgress" (command being processed),
"bo_CMD_FinishedOk" (command finished without errors) and "bo_CMD_FinishedError"
(command rejected or finished with error) can be evaluated in the user program.
In addition, the three command mailboxes are managed and processed according to priority.
CMD1 has the highest priority, CMD3 has the lowest priority. If all three command mailboxes
are triggered simultaneously by the user program, for example, the function block initially
processes CMD1, then CMD2, and finally CMD3. Cyclic triggering of command mailbox 3 is
also interrupted by intermediate sending of a command in mailbox 2 or 1 for processing of
the respective command.
Note
Cyclic triggering of the CMD1 command mailbox makes it impossible to send commands in
mailbox 2 or 3.
SIWAREX WP251
Manual, 12/2015, A5E37203357A
CMD command mailboxes
Communication with SIMATIC S7-1200
11.1 Integration in SIMATIC S7-1200
189