Siemens S7-300 appendix Calculating the cycle time, Process image update

Cycle and reaction times 5.2 Cycle time

5.2.2Calculating the cycle time

Introduction

The cycle time is derived from the sum of the following influencing factors.

Process image update

The table below shows the time a CPU requires to update the process image (process image transfer time). The times specified might be prolonged as a result of interrupts or CPU communication. The process image transfer time is calculated as follows:

Table 5-2 Formula for calculating the process image (PI) transfer time

The transfer time of the process image is calculated as follows:

Base load K

+number of bytes in PI in module rack 0 x (A)

+number of bytes in PO in module rack 1 to 3 x (B)

+number of words in PO via DP x (D)

+number of words in PO via PROFINET x (P)

=Transfer time for the process image

Table 5-3 CPU 31xC: Data for calculating the process image (PI) transfer time

Const.	Portions	CPU	CPU	CPU	CPU	CPU	CPU
		312C	313C	313C-2	313C-2	314C-2	314C-2
				DP	PtP	DP	PtP
K	Base load	150 μs	100 μs	100 μs		100 μs
A	per byte in module rack 0	37 μs	35 μs	37 μs		37 μs
B	per byte in module	-	43 μs	47 μs		47 μs
	racks 1 to 3 *
D	per WORD in the DP area	-	-	1 μs	-	1 μs	-
(DP only)	for the integrated DP
	interface

CPU 31xC and CPU 31x, Technical data	5-5
Manual, Edition 08/2004, A5E00105475-05