c3dbfd83_imply_Modbus___Exception_Codes__Meaning
[Exception Code] 03 (03 hex) [Name] Illegal Data Value [Meaning] 
A value contained in the query data field is not an allowable value for the slave. This indicates a fault in the structure of 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 significance of any particular value of any particular register.
Simply Modbus - Exception Codes
Meaning