Example Of Address Mapping - Siemens SIMATIC S7 Manual
S7 open modbus / tcp
communication via the
integrated pn interface of the plc
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Also See for SIMATIC S7:
- System manual (866 pages) ,
- Functional safety manual (212 pages) ,
- User manual (134 pages)
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TI
WRITE_READ
6.3
Example of Address Mapping
Interpretation of
MODBUS Register
Addresses
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
The parameter TI (Transaction Identifier) is copied by the server from the
request telegram to the respond telegram according to the MODBUS
specification.
In the operating mode S7 is Client, this is an input parameter. The FB copies
this value to the request telegram and verifies it when receiving the respond
telegram.
In the operating mode S7 is Server, this is an output parameter. The FB
copies this value from the request telegram into the respond telegram.
The Transaction Identifier is used for the identification of telegrams and the
unambiguous allocation of the requests to the corresponding responds. The
FB MODBUSPN can only perform this function properly if the TI is changed
with each transaction. Only a change of the TI ensures a reliable operation of
the FB.
Therefore, we recommend increment the TI by 1 with any request.
This parameter defines whether a reading or writing function is to be carried
out. If the value of the input/output is FALSE, the reading mode is specified.
The value TRUE specifies the writing mode.
In the operating mode S7 is Client, this is an input parameter. In the
operating mode S7 is Server, this is an output parameter.
The MODBUS data model is based on a series of storage areas with distinct
properties. Some systems, e.g. MODICON PLCs, distinguish these areas by
the register or bit addresses. The Holding Register with offset 0, for example,
is called 40001 (memory type 4xxxx, reference 0001).
A potential source of confusion is the varying interpretation of the register
address in different manuals. Sometimes the register or bit address is defined
as the address of the application layer and in other manuals it is the actually
transferred address.
The FB MODBUSPN uses the register or bit address transferred for its
parameters start_x, end_x und START_ADDRESS. Therefore, it is possible to
use register or bit addresses from von 0000
Function Block MODBUSPN
to FFFF
for each function code.
H
H
6-12
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