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What is the difference in modbus communication between ATV312 and ATV320?

There is little difference on modbus RTU between ATV312 and ATV320.
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 maaping of these 8+2 varibales 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 suported. 
ATV312 supports codes: 3, 6, 16, 43
ATV320- supports 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 mulitple 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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