From OpenSCADAWiki
In the case of different origin of PLC and SCADA, greater unification, than layer of the exchanging protocol, is difficult to achieve. On the other hand, the deep unification of PLC and SCADA data models can be found at the same time by PLC and SCADA manufacturers, which, however, only strengthens their monopoly status and in fact reduces the unification benefits through the overall cost of the solution.