7.2 Supported function codes
The Modbus definitions for writing and reading of data are not directly compatible with parameter
access by a frequency inverter (irrespective of the manufacturer of the frequency inverter). Modbus is
designed for reading bits and captures data in a different way. Data access is limited to a bit width of
16.
In order to meet the requirements of Modbus, data access is defined in the frequency inverters by the
following function codes.
16-bit values:
• Function code 3, read ONE data width of 16 bits (reading of hold register)
• Function code 6, write ONE data width of 16 bits (writing of single register)
• Function code 16, read ONE data width of 16 bits (writing of multiple registers)
32-bit values:
For access to 32-bit data, frequency inverters use the following adapted function codes:
• Function code 3, read TWO data widths of 16 bits (=32 bits) (reading of hold register)
• Function code 16, write TWO data widths of 16 bits (=32 bits) (writing of multiple registers)
• Function code 100, read ONE bit width 32
• Function code 101, write ONE bit width 32
The Modbus specification does not describe handling of 32
-
bit values. The implemented
handlings and function codes are quite common and frequently used. These functions
enable data access to 32
-bit “Long” variables in the frequency inverter.
In all data fields containing more than one byte, the highest
-value byte will be tran
-Endian, Motorola Format).
10/13
ACU
Modbus/TCP 27
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