Victron energy Venus GX Manual page 75

Besides double checking the mapping, there are two ways to see where the error lies. One is to check
the last error as shown in the GX Device menus. And secondly you can look at the error code as is
visible on (most but not all) PLCs. See the two sections below for details.
6.2 Looking up the last ModbusTCP Error on the GX Device
To see why a certain register is not available, use the Settings → Services → ModbusTCP menu. It
shows the last error, and reason for that error.
6.3 Error codes
These error codes are returned by our software to the PLC; and most PLC software tools allow seeing
them.
Code Name
0x01 IllegalFunction
0x02 IllegalDataAddress
0x03 IllegalDataValue
0x0A GatewayPathUnavailable
0x0B GatewayTargetDeviceFailedToRespond
7. Support
Description
The requested function is not supported.
The requested Unit-ID is available, but one or more of
the requested register(s) does not exist.
The requested quantity of registers is invalid. See the
modbus specs, for the limits per function code.
The requested Unit-ID is defined in the mapping list,
but there is no device found on the mapped port.
Double check the Unit-ID mapping list in the excel
sheet, and make sure that the device is properly
connected to the CCGX, switched on, and that its data
is available on the CCGX display.
Requested Unit-ID not found in the mapping list.
Double check the Unit-ID mapping list in the excel
sheet.