You can export the list of MODBUS communication objects directly from the controller.
In order to do this, you need to connect to the controller using the appropriate software (or use an offline archive).
LiteEdit: When you are connected to the controller or have its archive open in LiteEdit please click on: Controller -> Generate Cfg Image -> Generate Cfg Image (Modbus registers - used)...
GenConfig: When you are connected to the controller or have its archive open in GenConfig please click on: File -> Generate Cfg Image -> Generate Cfg Image (Modbus registers - used)…
InteliConfig: When you are connected to the controller or have its archive open in InteliConfig please click on: Tools -> Generate Cfg image (MODBUS)
In order to get SNMP MIB table just replace the last part with "Generate SNMP MIB table…"
This way you will get an exact list of registers which is based on the actual controller configuration.
For further reference please look into the Communication Guide which describes all MODBUS related details.