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Ground-based astronomy requires the implementation of growingly bigger, and thus more complex, instrumentation. A fitting example is represented by the instruments that are being developed for the ESO Extremely Large Telescope, as they pose new challenges under every aspect of their design. These elaborate systems also require a reliable and efficient control and management. For this reason, the current paradigm for the control electronics of astronomical instrumentation is represented by PLC-based architectures, with industrially developed real-time communication protocols. The PLC modular solution allows for an easy characterization of the individual topology, based on the needed functionalities. An interesting additional aspect of the PLC-based solution is the possibility to employ modules that are specifically developed for safety applications, and especially for the implementation of functions designed to protect humans from harm, e.g. during maintenance operations. While this solution is already commonly employed in industrial automation applications, it is still relatively new in the field of astronomical instrumentation. For the previous generations of instruments and telescopes, a fully hardware-based implementation of safety functions has been the norm. With the increasingly common implementation of more automation-oriented components in the design of control electronics, a new baseline for safety applications can be considered. In this paper the safety PLC solution is described and its application to astronomical ground-based instrumentation is analyzed. In particular, a study of the possibilities offered by Beckhoff TwinSAFE modules is presented, and both the advantages and drawbacks a PLC-based safety architecture can bring to the instrument control hardware, and the system as a whole, are taken into account.
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