Hitachi SJ-P1 Series User Manual page 63

Chapter 7
*2) The exception code indicates the factor of the
exception response.
■Exception codes
Code
01h The function code received is not an allowable action for this device.
02h The data address received is not an allowable address for this device.
03h A value contained is not an allowable value for this device.
21h When writing the data to the holding register, it is out of the range in the inverter.
It is the situation that the function is not allowed by the inverter:

To be configuring the register that is prohibited to change while driving.
 To be writing to the register which is soft locked.
22h  To be executing the enter command while driving.
 To be executing the enter command in under-voltage.

To be writing to the register while operating auto-tuning.
etc.
23h Configuring the function of writing to read-only parameter.
26h Writing when a prior data writing or data initialization execution.
27h Accessing only the high order parameter of 2 register length.
About the Modbus TCP
Description
7-18