saya plc software
A SCADA system is connected to the PLC using "tags" that are basically memory location found in the PLC and are cyclic read/write. The MODBUS or TCP/IP are only protocols to do this read/write access. The interfaces could be serial (rs485, RS232), ethernet, wireless etc. This memory location contains varibles that could be digital input/output, analog variables, flags, alarms, strings, recipes etc. The main problem is not what interface/protocol you use (rs485/rs 232/modbus tcp/ip or modbus/profibus/tcp/ip). The problem is that the PLC software have to match the existing tags already declared on the SCADA software. That means that you need to know the address location of the tags in the SCADA runtime and you have to design the PLC software using the same address. Not very likely to happen.
The solution is to rebuild the SCADA runtime based on the new tags that you declared on the Siemens PLC. But this suppose that you have the sources and design software for your existing SCADA.