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Integration of Networked Control Systems is always an engineering challenge. Heterogeneous hardware and software environments combined with long lifetime of control systems makes this job particularly troublesome. Modern concepts of integration of automation systems are related to Service Oriented Architecture (SOA). While its suitability is proven in IT systems, SOA has not been adopted yet in commercial Programmable Logic Controllers (PLC), and thus cannot be considered as a solution for integration with already deployed control systems. However, during past years thousands of PLCs with embedded HTTP servers were deployed in the field. These devices, used together with modern PLC that acts as the HTTP client, enable unique opportunity of integration for control systems with soft real-time constraints. In the present study, the performance of PLC-to-PLC communications based on HTTP is evaluated and compared to Modbus TCP.