11. Modbus TCP server
11. 1. Modbus TCP frames
Modbus TCP frames consist of a header and a Modbus request.
Header format:
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 | |
5 | low order | 256 bytes, the value of the significant byte therefore equals 0. | ||
| ||||
6 | Destination identifier (Unit ID) |
|
| |
7 | Modbus request function code |
|
|
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:
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. |
31
