Communication Load - Siemens SIMATIC S7-400 Manual

Cycle and Response Times of the S7-400

9.4 Communication Load

9.4 Communication Load
Overview
The CPU operating system continually makes available to communications the percentage
you configured for the overall CPU processing performance (time sharing). Processing
performance not required for communication is made available to other processes.
In the hardware configuration, you can set the load due to communications between 5% and
50%. By default, the value is set to 20%.
This percentage should be regarded as an average value, in other words, the
communications component can be considerably greater than 20% in a time slice. On the
other hand, the communications component in the next time slice is only a few or zero
percent.
This fact is also expressed by the following equation:
Actual
cycle time
Round up the result to the next whole number !
Figure 9-4
Data Consistency
The user program is interrupted for communications processing. The interrupt can be
executed after any instruction. These communication jobs can modify the program data. This
means that the data consistency cannot be guaranteed for the duration of several accesses.
The section
when there is more than one command.
Figure 9-5
Of the part remaining, the operating system of the S7-400 requires only a negligibly small
amount for internal tasks.
9-8
Cycle time
= x
100 - "configured communication load in %"
Equation: Influence of Communication Load
Consistent Data
provides more information about how to ensure consistency
Time slice (1 ms)
Operating system
User program
Communication
Breakdown of a time slice
100
Interrupt of the
user program
Component configurable
between 5 and 50%
S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0