The claimed interface extension device is based on a network device (DCC) having at least one previously unoccupied digital data output (OUT), to which an interface extension module (IOE) having at least one supply voltage input (S) and having a plurality of digital interfaces is coupled in the claimed manner. In this way, the supply voltage input (S) of the interface extension module (IOE) is interconnected with the digital data output (OUT) of the network device (DCC), so that in event of an error, the interface extension module (IOE) and all the digital interfaces thereof can be switched off. In addition, by measuring the sum current flowing through the outputs of the interface extension module, the load diagnosis capacity of the network configuration can be extended to the interface extension device. The claimed interconnection of the supply voltage input (S) of the interface extension module with the digital data output (OUT) of the network configuration has the advantage of also implementing a fail-safe behavior for conventional interface extension modules.