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Communication with SIMATIC S7-1200
11.2 Communication via Modbus
If the RS485 interface is used, the following character frame is valid:
Start bit
Number of data bits
Parity
Stop bit
The following baud rates can be set:
● 9 600 bit/s
● 19 200 bit/s (default setting)
● 38 400 bit/s
● 57 600 bit/s
● 115 000 bit/s
Functions which can be used by the master are listed below. The structure and contents of
the registers are shown in chapter "Scale parameters and functions (Page 57)".
Service
Read Holding Registers
Write Single Register
Write Multiple Registers
If a request of the master is answered by the SIWAREX module (slave), the SIWAREX
module sends a response frame with or without errors. In the case of a response without
error message, the response frame contains the received function code; in the case of
errors, the highest bit of the function code is set. This corresponds to the Modbus standard.
Afterwards, the master requests the data record DR 32 to find out which process-related
data or command errors exist.
11.2.3
Data record concept
The register assignment is an image of the data records. The chapter → Scale parameters
and functions (Page 57) describes the data records, variables and functions, including the
register addresses. The data records are always checked as complete data packets for
plausibility. For this reason, you must follow a specific procedure to change individual
parameters.
194
1
8
Even
1
Function code
03
06
16
Usage
Read one or more 16-bit parameter registers
Write a single parameter register
Write multiple registers
Manual, 12/2015, A5E37203357A
SIWAREX WP251
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