  Programmable logic controllers (PLCs) are the core element of industrial
plants in todays deployments. They read sensor values, execute control
algorithms, and write output values. Furthermore, industrial plants have
lifetimes of one or more decades. Thus, in a realistic Industry 4.0 scenario,
these devices have to be integrated in novel systems. In order to apply
advanced concepts and technologies, such as computation offloading, which
requires data exchange between PLCs and edge cloud, we investigate open
communication interfaces of two typical PLCs of Siemens S7 series. Hence, each
of the interfaces is analyzed based on plug & play capability, if metadata is
provided, protocol efficiency, and performance. For the latter, the smallest
possible update time for each of the interfaces will be measured.
