Communication
4.3 S7 connections
S7-400 Automation System, CPU Specifications
4-20 Manual, 10/2006, 6ES7498-8AA04-8BA0
Time sequence for allocation of S7 connection resources
When you configure your project in STEP 7, the system generates parameter assignment
blocks which are read by the modules during startup. This allows the module's operating
system to reserve or allocate the relevant S7 connection resources. That is, for instance,
OPs cannot access a reserved S7 connection resource for PG communication. The CPU's
S7 connection resources which were not reserved can be used freely. These S7 connection
resources are allocated in the order they are requested.
Example:
If there is only one free S7 connection left on the CPU, you can still connect a PG to the bus.
The PG can then communicate with the CPU. The S7 connection is only used, however,
when the PG is communicating with the CPU. If you connect an OP to the bus while the PG
is not communicating, the OP can establish a connection to the CPU. Since an OP maintains
its communication link at all times, in contrast to the PG, you cannot subsequently establish
another connection via the PG.
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