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Several types of industrial Real-Time Ethernet (RTE) networks could be present in the same plant. This work deals with the clock synchronization problems that arise when different RTE network infrastructures are interconnected. Specifically, this paper is focused on the exploitation of PROFINET IO Conformance Class C infrastructure for the interconnection of other industrial communication devices or measurement instruments that use IEEE1588 for clock synchronization. Actually, such devices (e.g., LXI instruments, EtherNet/IP devices, etc.) cannot be satisfactorily synchronized if directly connected to the PROFINET infrastructure, because of the large time errors (up to 100 μs). The solution proposed in this paper is an intelligent clock synchronization converter that has fewer limitations if compared with other systems, like boundary clocks. The basic idea is the creation of a “black-box” (a sort of remote bridging device) for the interconnection of IEEE1588 nodes through a PROFINET IO host plant, with zero-configuration on both systems. The proposed approach differs from boundary clock since its goal is to keep the PROFINET IO and the IEEE1588 synchronization domains separated, exploiting the possibility to tunnel the IEEE1588 time information through the PROFINET IO infrastructure with sufficient precision. In order to verify the practical feasibility of the proposed solution, LXI Class B instruments have been connected to the infrastructure of a real PROFINET IO network. The results show that the standard deviation of the synchronization accuracy is only 10 ns higher than the one measured in the case of a dedicated (separated) network for the LXI instruments.