A communications interface for interfacing between a host system and a state machine, the communications interface comprising: an event slot, the event slot comprising a plurality of registers including: a write register for writing by the host system, and a read register for reading by the host system, wherein the event slot is addressed from the host system by a single address location permitting the host system to write data to the write register and/or read data from the read register; and wherein the write register and the read register are individually addressable by the state machine.