There is little difference between Altivar ATV312 and ATV320 on modbus RTU
ATV320 offers i/o scanning table where you can prepare (map) variables you want to exchange with PLC. See picture in attachment with 8 variables for reading, 2 for writing. The mapping of these 8+2 variables must be done in ATV320
The addresses of scan table can be found in ATV320 communication vriables.xls
ATV312 does not have scanner table. Therefore registers had to be read using other type of modbus request or using more requests.

Look at ATV312 modbus manual especially which function codes that are supported.
ATV312 supports modbus function codes: 3, 6, 16, 43
ATV320- supports modbus funciton codes 3,6,16,23,43, 8
Therefore on ATV320 is is easy to access scanner table with function code 23 (=16#17) read/write multiple registers.
For function codes 3, 6 or 16 there is no difference, both ATV312 and ATV320 uses the same modbus telegram.

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