Siemens SIMATIC S7-1500 System Manual page 231

Parallel processing of asynchronous instruction jobs
A CPU can process several asynchronous instruction jobs in parallel. The CPU processes the
jobs in parallel under the following conditions:
• Jobs for an asynchronous instruction are started while other jobs for that instruction are
still running.
• The maximum number of simultaneously running jobs for the instruction is not exceeded.
The figure below shows the parallel processing of two jobs of the WRREC instruction. The two
instructions are executed simultaneously for a certain duration.
Figure 9-6 Parallel processing of the asynchronous instruction WRREC
NOTE
Dependencies between asynchronous statements
The call sequence in the user program may differ from the processing sequence of the
asynchronous instructions. This can lead to problems with dependencies between
asynchronous instructions.
Solution: To ensure correct timing of processing, use the status outputs of the asynchronous
statements in a sequencer. If an asynchronous instruction is finished and this is
acknowledged via the parameter DONE, then only the next asynchronous instruction should
start.
Example: For the RecipeImport and RecipeExport recipe functions, you need a CSV file for the
recipe data. If you use the same CSV file for import and export, then the two asynchronous
statements are dependent on each other. In a sequencer, link the status of the DONE
parameter of the RecipeImport instruction in the transition to the next step where the
RecipeExport is executed. The link thus ensures correct processing.
Assigning calls of an instruction to a job
To execute an instruction over multiple calls, the CPU must be able to uniquely relate a
subsequent call to a running job of the instruction.
To relate a call to a job, the CPU uses one of the following two mechanisms, depending on
the type of the instruction:
• Using the instance data block of the instruction (for "SFB" type)
• Using the input parameters of the instruction that identify the job. These input parameters
must match in each call during processing of the asynchronous instruction.
Example: The instruction "RD_DPARA" is identified by LADDR and RECNUM.
S7-1500R/H redundant system
System Manual, 11/2022, A5E41814787-AD
Basics of program execution
9.5 Asynchronous instructions
231