Optimizing Network Performance - Siemens SIMATIC S7-200 System Manual

Setting Up Communications Hardware and Network Communications
7.8 Network Performance

Optimizing Network Performance

The two factors which have the greatest effect on network performance are the
baud rate and the number of masters. Operating the network at the highest baud
rate supported by all devices has the greatest effect on the network. Minimizing the
number of masters on a network also increases the performance of the network.
Each master on the network increases the overhead requirements of the network.
Fewer masters lessen the overhead.
The following factors also affect the performance of the network:
Selection of master and slave addresses
Gap update factor
The addresses of the master devices should be set so that all of the masters are at
sequential addresses with no gaps between addresses. Whenever there is an
address gap between masters, the masters continually check the addresses in the
gap to see if there is another master wanting to come online. This checking
requires time and increases the overhead of the network. If there is no address
gap between masters, no checking is done and so the overhead is minimized.
Slave addresses may be set to any value without affecting network performance as
long as the slaves are not between masters. Slaves between masters increase the
network overhead in the same way as having address gaps between masters.
The S7-200 CPUs can be configured to check address gaps only on a periodic
basis. This checking is accomplished by setting the gap update factor (GUF) in the
CPU configuration for a CPU port with STEP 7-Micro/WIN 32. The GUF tells the
CPU how often to check the address gap for other masters. A GUF of one tells the
CPU to check the address gap every time it holds the token. A GUF of two tells the
CPU to check the address gap once every two times it holds the token. Setting a
higher GUF reduces the network overhead if there are address gaps between
masters. If there are no address gaps between masters, the GUF has no effect on
performance. Setting a large number for the GUF causes long delays in bringing
masters online since addresses are checked less frequently. The GUF is only used
when a CPU is operating as a PPI master.
The highest station address (HSA) defines the highest address at which a master
should look for another master. Setting an HSA limits the address gap which must
be checked by the last master (highest address) in the network. Limiting the size of
the address gap minimizes the time required to find and bring online another
master. The highest station address has no effect on slave addresses. Masters
can still communicate with slaves which have addresses greater than the HSA.
The HSA is only used when a CPU is operating as a PPI master. The HSA can be
set in the CPU configuration for a CPU port with STEP 7-Micro/WIN 32.
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S7-200 Programmable Controller System Manual
A5E00066097-02