Siemens SIMATIC S7 Manual page 29

Connection
Handling
Multiple
Communication
Partners
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
Active connection establishment can be carried out by the Modbus client as
well as the Modbus server. It is recommended explicitly that the Modbus client
executes the active establishment. The relevant information is read from the
connection parameters of DB MODBUS_PARAM.
With a parameter of the connection parameter block (active_est), it is possible
to define whether the PLC performs active or passive connection
establishment.
During runtime the function TCON establishes the connection to the
communication partner for both types of connections, active and passive.
The point in time at which the connection is established is defined in DB
MODBUS_PARAM (parameter connect_at_startup).
The termination of the connection is defined with the parameter
DISCONNECT of FB MODBUSPN.
A PN PLC can establish multiple connections to various communication
partners. To ensure a proper operation and data transfer, a strict division of
the different connections to the corresponding communication partners is
inevitable.
Correspondingly, there are the following requirements for each connection:
•
one connection parameter block and the related Modbus parameter in
DB MODBUS_PARAM
•
call of FB MODBUSPN in OB100
•
call of FB MODBUSPN in OB121
•
call of FB MODBUSPN in OB1 or cyclic interrupt OB
In this case, the calls of the FB MODBUSPN in the OB100, OB121 and in the
OB1 receive the same instance DB for one connection. A different instance
DB is required for each additional connection.
Function Block MODBUSPN
6-3