7.8.9 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.
7.8.10 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
7.8.11 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 specifically 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 sig-
nificance 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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