1. A microcontroller device comprising: a host processing module; a debug module; and a data bus network interface module for enabling reception and transmission of application messages to/from a remote network node via a data bus network, the data bus network interface module being arranged to: receive a data bus message from the remote network node via the data bus network, read a non-data field of the data bus message, and make data content of the data bus message available to the debug module if the non-data field comprises a value defined for debug use; and receive data from the debug module for transmission over the data bus network and, upon receipt of said data from the debug module, to generate a second data bus message containing the received data and comprising a second non-data field value defined for debug use.