Interconnections between process control networks and enterprise networks has resulted in the proliferation of standard communication protocols in industrial control systems which exposes instrumentation, control systems, and the critical infrastructure components they operate to a variety of cyber attacks. Various standards and technologies have been proposed to protect industrial control systems against cyber attacks and to provide them with confidentiality, integrity, and availability. Among these technologies, data diodes provide protection of critical systems by the means of physically enforcing traffic direction on the network. In order to deploy data diodes effectively, it is imperative to understand the protection they provide, the protection they do not provide, their limitations, and their place in the larger security infrastructure. In this work, we briefly review the security challenges in an industrial control system, study data diodes, their functionalities and limitations, and propose a scheme for their effective deployment in trusted process control networks (TPCNs.)