Base units for monitoring more than 3 safety functions. Various types are available: PNOZ mm op stand-alone; PNOZ mm 0p-T stand-alone, for an extended temperature range. PNOZ mm 0.1p modular expandable, PNOZ mm 0.2p modular expandable with integrated Multi-Link interface for connecting multiple base units. The expandable base units can be expanded with a maximum of four link and communication modules on the left-hand side and a maximum of one PNOZsigma relay expansion module on the right-hand side.