Configuring Hardware and Communication Connections STEP 7.pdf

Configuring Connections
12.6 Using Connection Resources with Fault-Tolerant S7
Connections
For H-systems, there are a number of possible configurations that differ with
respect to the number of H-CPUs, number of CPs and number of subnets.
Depending on the configuration, two or four partial connections are possible for
each fault-tolerant S7 connection. These connections ensure communication is
maintained even if a component should fail.
The following sections describe the most common configurations and how they use
connection resources for a fault-tolerant connection.
Basic Information
At each H-CPU, a connection resource is occupied for each of the endpoints of a
fault-tolerant S7 connection (that is, for redundant connections on both H-CPUs
concerned).
For each fault-tolerant S7 connection, STEP 7 establishes two partial connections
as alternative communication paths. To ensure the availability of both paths, each
path must have resources reserved for it. If both partial connections run through
the same intermediate CP, then two connection resources are also reserved on
this CP.
The following sections explain this situation in detail.
Notes on the Illustrations
For redundant H-stations, the two CPUs are labeled "H-CPU 0" and "H-CPU 1".
The CPs are numbered sequentially (CP 0, CP1, ...).
Configuring Hardware and Communication Connections with STEP 7
A5E00706939-01 12-13