Siemens SIMATIC S5-90U System Manual page 138

Addressing and Access to I/Os
Note
Note the following points if you wish to reduce the scan time and the response time:
• DB1 enables you to parameterize the number of analog inputs to be read into the PII
cyclically from the onboard I/Os of the S5-95U (see Chapter 12). It is
recommendable to keep the number of analog inputs read cyclically to a minimum.
• DB1 enables you to parameterize the number of internal timers to be processed by
the operating system (see Section 6.5).
In the default setting, all timers available (128) are executed by the operating
system. Parameterize only those timers you are actually using in your program.
Please note that the scan time and the response time are extended
• Through the use of time-controlled program execution (OB13)/interrupt processing
(OB3). In this case, you have to take into account the corresponding runtimes when
calculating the scan time and the response time.
• Through the use of the programmer interface. In this case, the scan time and the
response time mainly depend on the functions initiated via the programmer interface.
1. Process Image Transfer Time
The process image transfer time is the time required for reading the input signals into the PII and
writing the output states into the PIQ.
The process image transfer time can be divided into the
• Process image transfer time for onboard I/Os and the
• Process image transfer time for external I/Os.
Process image transfer time for onboard I/Os:
The process image transfer time for the digital onboard I/Os is fixed and already included in the
operating system runtime.
There is a linear relationship between the process image transfer time for the analog onboard I/Os
(S5-95U) and the number of analog channels to be read into the PII. The transfer time can thus be
calculated as follows:
PI transfer time for the
analog onboard I/Os
A certain period for writing the analog output of the onboard I/Os is always required and therefore
included in the operating system runtime of the S5-95U.
Process image transfer time for external I/Os:
The time required for this function is identical for the S5-90U and the S5-95U. There is a linear
dependency between the time and the number and type of modules used. It can be calculated as
follows:
PI transfer time for the
external I/Os
6-18
= Number of analog channels of the onboard I/Os
read into the PII
= Bits of the PII/PIQ used by
all modules
S5-90U/S5-95U
·80 µs
·34 µs
EWA 4NEB 812 6115-02b