standard cannot provide the real-time capability. C/OS-II is an
source-open and real-time operating system with high reliability.
If it is possible to design a real-time Modbus/TCP based on
C/OS-II, we can not only accelerate the application of industrial
Ethernet to factory automation system, but also reduce the cost
greatly. Zhang et al. [21] transplanted Modbus/TCP into C/OS-II,
but they did little to improve the real-time capability. Xu et al.
[20] tested the real-time capability of Modbus/TCP between the
Ethernet communication module of PLC (Programmable Logic Controller)
made in Schneider electric company and the monitoring
station, the response time is between 15 ms and 25 ms, which is
not able to be applied in the industrial automation field.
In this paper, we proposed and verified a basic token-ring realtime
response algorithm of Modbus/TCP, and the average delay is
about 1.117 ms. This reaches the level of EPL (Ethernet Powerlink)
working in open mode [22–24]. So, the designed algorithm can be
applied in the industrial field. Finally, we proposed two variations
of the basic algorithm, which can meet the advanced application in
the future.
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