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Entry type: FAQ, Entry ID: 58496173, Entry date: 03/07/2012

What is the difference between the initialization and runtime parameters on the blocks for Modbus TCP?

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In this entry we describe the differences between the initialization and runtime parameters of the blocks for Modbus TCP. The Modbus blocks are called in the user program of the SIMATIC S7 for the communication.

The initialization parameters are written in lowercase characters. This makes it easier to distinguish them from the runtime parameters. The initialization parameters are only evaluated when called in OB100 and transferred to the instance data block of the Modbus block. Changing the initialization parameters during runtime has no effect, because only the stored value is processed. The initialization parameters specify what is possible. If you change these parameters, in Test mode, for example, the instance data block has to be re-initialized by a restart of the CPU (STOP > RUN).
In the initialization parameters, the data type of the Modbus register is specified (data_type_1), for example. Furthermore, the initialization parameters define whether the CPU is server or client (server_client).

The runtime parameters are written in uppercase characters. They can be changed in cyclic mode. The runtime parameters specify the current job.
In the "CPU/CP is client" mode, however, it is not useful to change the input parameters while a job is running. You should wait with the preparations for the next job and the associated change of parameters until the previous job has been terminated with DONE_NDR or ERROR.
In the "CPU/CP is server" mode you can only evaluate the output parameters if DONE_NDR is set.
The output parameters are dynamic displays and therefore apply only for 1 CPU cycle. For any necessary further processing or display in the variable table they must be stored in a different memory area.