Siemens SIMATIC S7-400H System Manual page 178

Communication
11.8 General issues in communication
SIMATIC OPs, SIMATIC MPs
Do not install more than 4 OPs or 4 MPs in a fault-tolerant system. If you do need more
OPs/MPs, your automation task may have to be revised. Contact your SIMATIC sales
partner for support.
Do not select a screen refresh cycle time of less than 1s, and increase it to 2 s as required.
Verify that all screen tags are requested within the same cycle time, in order to form an
optimized group for read requests.
OPC servers
When OPC was used to connect several HMI devices for your visualization tasks to a fault-
tolerant system, you should keep the number of OPC servers accessing the fault-tolerant
system as low as possible. OPC clients should always address a shared OPC server, which
then fetches the data from the fault-tolerant system.
You can tune data exchange by using WinCC and its client/server concept.
Various HMI devices of third-party vendors support the S7communication protocol. You
should utilize this option.
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System Manual, 09/2007, A5E00267695-03
S7-400H