Cycle Time Calculation; Factors That Influence The Cycle Time; Operating System Scan Time At Scan Cycle Checkpoint - Siemens SIMATIC S7-400 Reference Manual

Cycle and Reaction Times of the S7-400
3.2

Cycle Time Calculation

Increasing the Cycle Time
Basically, you should note that the cycle time of a user program is increased by the
following:
• Time-driven interrupt processing
• Hardware interrupt processing (see also Section 3.8)
• Diagnostics and error handling (see also Section 3.9)
• Communications via the MPI and CPs connected via the communication bus
(for example, Ethernet, Profibus, DP); contained in the communication load
• Special functions such as control and monitoring of variables or block status
• Transfer and clearance of blocks, compression of the user program memory

Factors that Influence the Cycle Time

The following table indicates the factors that influence the cycle time.
Table 3-2 Factors that Influence the Cycle Time
Factors
Transfer time for the
process-image output table
(PIQ) and the process-image
input table (PII)
User program processing
time
Operating system scan time
at scan cycle checkpoint
Increase in the cycle time
from communications
Impact of interrupts on the
cycle time
Note
With CPUs produced prior to October 1998, updating of the process image of the
outputs takes place before the scan cycle checkpoint.
3-4
Remark
... see Table 3-5
... is calculated from the execution times of the different
instructions (see S7-400 Instruction List). For a special
characteristic of the CPU 417-4H refer to Table 3-5.
... see Table 3-6
You set the maximum permissible cycle load expected for
communication in % in STEP 7 (manual Programming with
STEP 7 Version 5.0). See Section 3.4.
Interrupt can interrupt the user program at any time.
... see Table 3-7
Automation System S7-400 CPU Specifications
A5E00165965-01