Siemens SIMATIC S7-400H Manual page 94

Communications
Configuration
The availability of the system, including communications, is set during
configuration. Please refer to the STEP 7 documentation to find out how to
configure connections.
Only S7 communication is used for fault-tolerant S7 connections. To do this, select
in the "New Connection" dialog box the "S7 Connection Fault-Tolerant " as the
type.
The number of required redundant connections is determined by STEP 7 as a
function of the redundant nodes. If the network architecture allows them, up to four
redundant connections are generated. Higher redundancy cannot be achieved by
using more CPs.
In the "Properties - Connection" dialog box you can modify specific properties of a
fault-tolerant connection, should you require to do so. If you are using more than
one CP, you can also switch the connections in this dialog box. This may be
practical since, by default, all connections are routed initially through the first CP. If
all the connections have been assigned there, the other connections are routed
through the second CP etc.
Programming
Fault-tolerant communications can be used on the CPU 417-4H and are
accomplished via S7 communication.
This is possible solely within an S7 project.
The programming of fault-tolerant communications with STEP 7 is accomplished
by means of communication system function blocks. With these blocks, data can
be transmitted over subnets (Industrial Ethernet, PROFIBUS). The standard
communication SFBs integrated in the operating system offer you the option of
acknowledged data transmission. Not only data transmission is possible; other
communication functions for controlling and monitoring the communication peer
can also be used.
User programs written for standard communications can be run for fault-tolerant
communications as well, without being modified. Cable and connection redundancy
has no effect on the user program.
Note
You will find tips on programming communications in the S7 standard
documentation – for example, Programming with STEP 7 .
The communication functions START and STOP act on exactly one CPU or on all
CPUs of the fault-tolerant system (for more details refer to the Reference Manual
System software for S7-300/400, System and standard functions ).
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S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04