An electronic data processing device comprises: a processor (1); a serial interface comprising a connection for incoming data (16) and a connection for outgoing data (18); a hardware serial-interface controller (6) for controlling the serial interface; and a reception buffer (22) for receiving incoming data. The processor is arranged automatically to read data written to the reception buffer. The device is arranged so that the processor can indicate to the serial interface controller that it is unable to accept data. The controller is arranged to respond to incoming data by sending a rejection message from the outgoing serial connection and to prevent incoming data from being placed in the reception buffer.