Abstract: A communications unit includes a processor (11), a data bus (12), an address bus (13), memory means (14,15,16), a data interface (36) between the data bus and a host computer (35), and a monitoring interface (30) with the host computer. The monitoring interface (30) includes monitoring address generating means fed from the host computer and including a bank register (31) loadable by the host computer as its high-order portion and a direct path (32) from the host computer as its low order portion. The monitoring interface also includes address coupling means (18) for passing the contents of the monitoring address generating means to the address bus (13) to address the memory means (14,15,16) of the communications unit directly. The host computer can also write to the unit via the monitoring interface.