SPA data derives directly from the protection relay's Modbus data. All Modbus data in monitoring direction can be read through Modbus input or holding registers. Modbus registers are theoretically addressed 1…65535, but in practice the highest available address is 9999.
16-bit wide Modbus registers can contain either one analog value or a set of maximum 16 packed indication bits. In some cases two consecutive Modbus registers have been defined to contain one 32 bit analog value.
The SPA channel number corresponds to the Modbus register address in the SPA read messages.
The SPA data type and number define the value response format.