The 1.2m optical Mercator Telescope (based at the Roque de Los Muchachos Observatory at La Palma) is currently in the commissioning phase of a third permanently installed instrument called MAIA (Mercator Advanced Imager for Asteroseismology), a three-channel frame-transfer imager optimized for rapid photometry. Despite having three cryostats, MAIA is designed as a highly compact and portable instrument by using small Stirling-type cryocoolers, and a single PLC in charge of all temperature control loops, cryocooler interaction, telemetry acquisition and other instrument control related tasks. To accommodate MAIA at the Nasmyth B focal station of the telescope, a new mechanism for the tertiary mirror had to be built since the former mechanism only allowed motor controlled access to the Cassegrain and Nasmyth A focal stations. A second PLC has been installed in order to control the two degrees of freedom of this mirror mechanism by interfacing with its motor controllers, high-precision optical encoders, and limit switches. This PLC is not dedicated to the tertiary mirror control but will serve as a general purpose controller for various tasks related to the telescope and the observatory, as part of a new Telescope Control System primarily based on PLCs and OPC UA communication technology. Due to the central location of the PLC inside the observatory, the position control loops of the mirror mechanism are distributed using EtherCAT as the communication ﬁeldbus. In this paper we present the design and the ﬁrst commissioning results of both the MAIA instrument control and the tertiary mirror control.