Advanced Topics - Siemens Simatic S7-200 System Manual

S7-200 Programmable Controller System Manual

Advanced Topics

Optimizing the Network Performance
The following factors affect network performance (with baud rate and number of masters having the
greatest effect):
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Baud rate: Operating the network at the highest baud rate supported by all devices has the greatest
effect on the network.
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Number of masters 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; having fewer masters lessens the overhead.
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Selection of master and slave addresses: 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. You can set the slave addresses 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.
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Gap update factor (GUF): Used only when an S7-200 CPU is operating as a PPI master, the GUF
tells the S7-200 how often to check the address gap for other masters. You use STEP 7–Micro/WIN
to set the GUF in the CPU configuration for a CPU port. This configures the S7-200 to check
address gaps only on a periodic basis. For GUF=1, the S7-200 checks the address gap every time
it holds the token; for GUF=2, the S7-200 checks the address gap once every two times it holds the
token. If there are address gaps between masters, a higher GUF reduces the network overhead. 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, because the addresses are
checked less frequently. The default GUF setting is 10.
Highest station address (HSA): Used only when an S7-200 CPU is operating as a PPI master, the
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HSA defines the highest address at which a master should look for another master. You use
STEP 7–Micro/WIN to set the HSA in the CPU configuration for a CPU port. 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. As a general rule, set the
highest station address on all masters to the same value. This address should be greater than or
equal to the highest master address. The default value for the HSA is 31.
Calculating the Token Rotation Time for a Network
In a token-passing network, the only station that can initiate communications is the station that holds the
token. The token rotation time (the time required for the token to be circulated to each of the masters in the
logical ring) measures the performance of your network.
Figure 7-26 provides a sample network as an example for calculating the token rotation time for a
multiple-master network. In this example, the TD 200 (station 3) communicates with the CPU 222
(station 2), the TD 200 (station 5) communicates with the CPU 222 (station 4), and so on. The two
CPU 224 modules use the Network Read and Network Write instructions to gather data from the other
S7-200s: CPU 224 (station 6) sends messages to stations 2, 4, and 8, and the CPU 224 (station 8) sends
messages to stations 2, 4, and 6. In this network, there are six master stations (the four TD 200 units and
the two CPU 224 modules) and two slave stations (the two CPU 222 modules).
Refer to the Tips and Tricks on the documentation CD for a discussion about token rotation. See Tip 42.
Tips and Tricks
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