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11. Modbus TCP server
11. 1. Modbus TCP frames
Modbus TCP frames consist of a header and a Modbus request.
Header format:
The frame header returned by the Altivar 71 server is identical to that of the frame sent by the client.
11. 2. Drive Modbus servers
The destination identifier (Unit ID) is used to access 4 drive Modbus TCP servers:
Byte Description Comments
0
Transaction identifier
high order
1 low order
2
Protocol identifier
high order
This identifier always equals 0.
3 low order
4
Length of data
high order
Number of bytes in the Modbus request +1. The frame length is always less than
256 bytes, the value of the significant byte therefore equals 0.
5 low order
6 Destination identifier (Unit ID)
7 Modbus request function code
Unit ID Modbus TCP server Accessible parameters
0 Variable speed drive See the Altivar 71 Communication parameters Manual.
251, AMOC Ethernet card See the “Ethernet card parameters” section.
252, AMOA Controller Inside card 2048 words (%MW0 to %MW2047).
255 IO Scanner See the “IO Scanner” section.