Communication Performance - Siemens SIMATIC S7-400H System Manual

11.7

Communication performance

Compared to a fault-tolerant CPU in stand-alone mode or to a standard CPU, the
communication performance (reaction time or data throughput) of a fault-tolerant system
operating in redundant mode is significantly lower.
The aim of this description is to provide you with criteria which allow you to assess the
effects of the various communication mechanisms on communication performance.
Definition of communication load
Communication load is the sum of requests per second issued to the CPU by the
communication mechanisms, plus the requests and messages issued by the CPU.
Higher communication load increases the reaction time of the CPU, meaning the CPU takes
more time to react to a request (such as a read request) or output requests and messages.
Operating range
In every automation system there is a linear operating range in which an increase in
communication load will also lead to an increase in data throughput. This then results in
reasonable reaction times which are acceptable for the automation task faced.
A further increase in communication load will push data throughput into the saturation range.
Under certain conditions, the automation system may as a result be no longer be capable of
processing the request volume within the response time demanded. Data throughput
reaches its maximum, and the reaction time rises exponentially; see the figures below.
Data throughput may also be reduced by a certain amount due to additional internal loads
inside the device.
Figure 11-14 Communication load as a variable of data throughput (basic profile)
S7-400H
System Manual, 09/2007, A5E00267695-03
Communication
11.7 Communication performance
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