Parts And Composition Of The Cycle Time; Cyclic Program Scanning - Siemens SIMATIC S7-400 Reference Manual

The Cyclic Program Scanning Process
The following table and figure illustrate the phases of cyclic program scanning.
Table 3-1
Step
1
2
3
4
5
6
Parts of the Cycle Time
PIQ:
PII:
SCC:
OS:
Time slice (1 ms each)
Time slice (1 ms)
Figure 3-1

Parts and Composition of the Cycle Time

Automation System S7-400 CPU Specifications
A5E00165965-01

Cyclic Program Scanning

The operating system starts the scan cycle monitoring time.
The CPU writes the values from the process-image output table in the
output modules.
The CPU reads out the status of the inputs at the input modules and
updates the process-image input table.
The CPU processes the user program in time slices and performs the
operations specified in the program.
At the end of a cycle, the operating system executes pending tasks,
such asthe loading and clearing of blocks.
The CPU then goes back to the beginning of the cycle after the
configured minimum cycle time, as necessary, and starts cycle time
monitoring again.
Process-image output table
Process-image input table
scan cycle checkpoint
Operating system
Operating system
User program
Communication Functions
Cycle and Reaction Times of the S7-400
Process
PIQ
PII
User Program
SCC (OS)
3-3