Specifications

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Guidance for the Employment of CPUs with 2 PU Interfaces Section 1-7
1.7 Guidance for the Employment of CPUs with 2 PU
Interfaces
CPUs with 2 PU interfaces (e.g. 928B with PU module) are used during system
commissioning thus keeping the PU interface available for program debugging.
Please note that the interfaces are interacting (see Siemens CPU manual). For
example, if a PLC program is analysed by means of the STATUS-function on the
first interface of the CPU, this will cause the speed of protocol handling on the
second interface to decrease significantly. With the STATUS-function, the PLC
program can, so to speak, be operated in the single-step-mode, i.e. the result of
every program line is displayed on the screen.
In order to maintain the connection, the timeout period must be increased to 5
seconds. Using the regular timeout period results in the terminal generating a com-
munication error message.
In this case, each transmission requires 3-4 seconds, in contrast to a regular com-
munication cycle which requires approximately 60 ms. This means a slow-down of
the interface by a factor of 50!