6.8.9 How to Control the Frequency Converter
This section describes codes which can be used in the function and data fields of a Modbus RTU message. For a complete description of all the message fields
please refer to the section
Modbus RTU Message Framing Structure.
6.8.10 Function Codes Supported by Modbus RTU
Modbus RTU supports use of the following function codes in the function field
of a message:
Function
Function Code
Read coils
1 hex
Read holding registers
3 hex
Write single coil
5 hex
Write single register
6 hex
Write multiple coils
F hex
Write multiple registers
10 hex
Get comm. event counter
B hex
Report slave ID
11 hex
Function
Function Code
Sub-function code
Sub-function
Diagnostics
8
1
Restart communication
2
Return diagnostic register
10
Clear counters and diagnostic register
11
Return bus message count
12
Return bus communication error count
13
Return bus exception error count
14
Return slave message count
6.8.11 Modbus Exception Codes
For a full explanation of the structure of an exception code response, please refer to the section
Modbus RTU Message Framing Structure, Function Field.
Modbus Exception Codes
Code
Name
Meaning
1
Illegal function
The function code received in the query is not an allowable action for the server (or slave). This may be
because the function code is only applicable to newer devices, and was not implemented in the unit selected.
It could also indicate that the server (or slave) is in the wrong state to process a request of this type, for
example because it is not configured and is being asked to return register values.
2
Illegal data address
The data address received in the query is not an allowable address for the server (or slave). More specifically,
the combination of reference number and transfer length is invalid. For a controller with 100 registers, a
request with offset 96 and length 4 would succeed, a request with offset 96 and length 5 will generate
exception 02.
3
Illegal data value
A value contained in the query data field is not an allowable value for server (or slave). This indicates a fault
in the structure of the remainder of a complex request, such as that the implied length is incorrect. It spe-
cifically does NOT mean that a data item submitted for storage in a register has a value outside the
expectation of the application program, since the Modbus protocol is unaware of the significance of any
particular value of any particular register.
4
Slave device failure
An unrecoverable error occurred while the server (or slave) was attempting to perform the requested action.
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