4. Configuration
175
N = 19.6
That is, in this configuration the MODBUS Server answers, on average, 19 requisitions per second.
In case the obtained value is multiplied by the number of bytes in each requisition, we will obtain a
transfer rate of n bytes per second.
Remote Interfaces
The performance of a device MODBUS Server in one remote Ethernet interface is similar to the
performance in the CPU’s local interfaces.
However, due to time of the communication between the CPU and the modules, the maximum
performance is limited. For only one active connection, the number of answers is limited in the
maximum of 18 answers per second. With more active connections, the number of answers will
increase linearly, exactly like the local interfaces, however being limited at the maximum of 90
answers per second. So, for a remote Ethernet interface, we will have the following forms to
calculate his performance:
For T ≤ 55 ms is used:
N = C x (18,18 – (18,18 / (0,055 x 1000)))
And for T ≥ 55 ms is used:
N = C x (Z – (Z / (T x 1000)))
Where N is the medium number of answers per second, C is the number of active connections and T
is equal to the cycle time of the MODBUS task (in seconds).
The user must pay attention to the fact that the maximum performance of a device MODBUS Server
in one remote Ethernet interface is 90 answers of requisitions per second.
OPC Server
Communication performance with OPC Server was tested by creating POUs with 1,000 INT
variables each. All scenarios were tested with Single profile and MainTask Interval at 100 ms. The
communication was enabled by the attribute {attribute 'symbol' := 'readwrite'}, to make the data
available to the OPC Server. The measurements were made while MasterTool was disconnected from
the CPU, and MainTask duration was made to last 5%, 50% and 80% of the configured Interval, as
seen in Table 4-112
At the OPC Client’s side, a SCADA system driver was used. Configured update time was 50 ms.
Performance results in these conditions are described in Table 4-112.
Total quantity of
variables in the
PLC’s project
Variable update time at OPC Client
Table 4-112. Communication Rate of an OPC Server
Server IEC60870-5-104
The Server IEC 60870-5-104 driver is runned by the CPU in the same way as anothers
communication server drivers, that is, in second instance, with priority under the user application and
cyclically. This driver’s task specifically runs at each 50ms, and one cycle is enough to process and
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General