The protection relay's Modbus data can be relocated to the user-defined Modbus memory area. Therefore it is possible to build up the packed set of measurands and indications. The user-defined register area can also be accessed from the SPA protocol.
The SPA protocol standard defines that the highest possible SPA channel number is 999. This corresponds to Modbus register address 999. Some SPA client applications can however access higher channel numbers than 999. If this is the case, then the SPA server application responds with channel numbers higher than 999. In Modbus data mapping of this relay there are measurand registers that are as default located on higher register addresses than 999. If this causes problems in the SPA communication, the desired measurands can be remapped to a user-defined register area having lower register addresses.
When packing MOM + MCB indication bit pairs into Modbus user-defined registers, it is also possible to map MOM-only bits into the same register. To poll the register using the automatic MOM + MCD bit OR-ing feature, map the MOM-only bit to an even bit location in the register, leaving the corresponding MCD location unmapped. The OR-ing is done for this bit pair, but since the unused MCD bit is always 0, the result is always according to the MOM bit state.
If the Modbus indication point is mapped to the user-defined Modbus area, then the SPA events from this point come from the original Modbus point location. In case the UDR mapped indication point has no original Modbus point location, then the event comes from its UDR point location.