15
5
MODBUS FUNCTIONS
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5.10
User Defined – Function Code 65
5.10.1
User Defined Query
The user defined function makes available to the user a range of non-standard Modbus query responses. The information field
comprises of a control byte which defines control type, an internal function byte designating the action to be taken, a qualifier byte
the usage of which is dependent on the user defined task and a byte specifying the number of bytes to follow in the information
data string.
Example
– a user defined read instrument version (internal function 24) of slave 01 is shown below.
The control byte has no effect, valid qualifier and byte counts are zero only and other values return an exception response.
5.10.2
User Defined Response
The addressed slave responds with its address and function code followed by the information field. The information field
comprises of the control byte, one byte defining the quantity of data bytes in the response information string and the data string.
Example
:
