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Modbus communication
About protocol
11.4.1
Modbus is the communication protocol for controlling programmable logic controllers, which
were standardized for industrial electrical devices. That Modbus protocol allows connecting
devices to the supervisory device over the same network. The communication is carried out
with query and response messages, and the messages include an address, a function code.
The IEDs can communicate with the substation computer when the Modbus protocol is
implemented in the IEDs. The Modbus communication is facilitated by IED applications and
a communication application, which lie in the Application, Network, Data link and Physical
layers of the OSI.
Upper layer in IED
Lower layer in IED
Figure 11.4-1 Modbus layers in IED and corresponding OSI layers
†Note: Although the Modbus serial is fallen into the binary Modbus RTU and the
character Modbus ASCII, the IED—discussed here—supports only the Modbus
RTU. That is, we exclude discussion of Modbus ASCII mode in this chapter.
The Modbus addresses are assigned in the IED, and are unique numbers which can specify
IED memory area (see Figure 11.4-2 and Table 11.4-1). The substation computer (master) can
read or write the IED memories using a Function Code (FC), which the substation computer
(master) uses for the request command.
Modbus application layer (MBAP)
Serial line
Serial line
Master/Slave
Master/Slave
RS-485
RS-485
Modbus serial
Modbus ASCII
(Modbus RTU)
(Not supported)
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Application
Presentation
Session
Transport
Modbus on TCP
Network
IP
Data link
Ethernet
Physical
Modbus TCP
OSI layers
6F2T0207 (0.01)
GRE200 (5,6)
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