5.3 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
.
5.3.1 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
5.3.2 Exception and Error Codes
In the event of an error, the following exception codes may appear in the data field of a response message. For a full explanation of the structure of an
exception (i.e. error) response, please refer to
Function Field
in section
Modbus RTU Message Framing Structure
.
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 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 register 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 an incorrect implied length. 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 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
In case of an exception code 4 while accessing parameter values in the drive, detailed information about the latest exception can be read from the drives
Holding Register 0007. This register may contain one of the following, detailed error codes regarding the latest occurring MODBUS Exception.
VLT
®
Micro Drive FC 51 Operating Instructions
5 Modbus RTU
MG.02.A4.02 - VLT
®
is a registered Danfoss trademark
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