The module FDO2-EXT is an extension module for the optical oxygen sensor FDO2 from PyroScience, providing 4 analog outputs and a Modbus interface. The module has two interfaces, a RS485 interface and a UART interface. The UART interface can be directly connected to the FDO2. The RS485 interface can be connected to a Modbus bus. Further, the FDO2-EXT provides 2 current outputs and 2 voltage outputs, which can be used even if the module is not operated via the Modbus interface.