Converteam 700.002 735, Руководство по эксплуатации

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4 Modbus/TCP
Ethernet Option Board /
700.002 735 AE01
4-1
4 Modbus/TCP
MODBUS/TCP is a variant of the MODBUS family. It is a manufacturer-independent protocol for monitoring
and controlling automatic devices.
MODBUS/TCP is a client server protocol. The client makes queries to the server by sending “request”
messages to the server's TCP port 502. The server answers client queries with “response” messages.
The term 'client' can refer to a master device that runs queries. Correspondingly, the term 'server' refers to a
slave device that serves the master device by answering its queries.
Both the request and response messages are composed as follows:
Byte 0: Transaction ID
Byte 1: Transaction ID
Byte 2: Protocol ID
Byte 3: Protocol ID
Byte 4: Length field, upper byte
Byte 5: Length field, lower byte
Byte 6: Unit identifier
Byte 7: Modbus function code
Byte 8: Data (of variable length)
Figure 4-1: Modbus Transaction