Advanced Topics - Siemens SIMATIC S7-200 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):
Baud rate: Operating the network at the highest baud rate supported by all devices has the
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greatest effect on the network.
Number of masters on the network: Minimizing the number of masters on a network also
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increases the performance of the network. Each master on the network increases the
overhead requirements of the network; having fewer masters lessens the overhead.
Selection of master and slave addresses: The addresses of the master devices should be
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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.
Gap update factor (GUF): Used only when an S7-200 CPU is operating as a PPI master,
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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
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master, the 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 7-31 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).
Communicating over a Network Chapter 7
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