This was a 14 mark question and should not be considered to be difficult.

The Human Interface Model: (5 marks)

Human -à Interface -à Machine à World with feedback arrows from world to interface, machine to interface and interface to human

Redone for the story (5 marks)

Mary Beth -(mary beth's command) à Interface--(control signal) à Therac --(rays) à Patient

Feedback goes Patientà Interface (patient state)

Therac à Interface (Therac mode)

Interface à Mary Beth (interface information)

Mary Beth needed to know the control signal, the Therac mode, that the Therac had sent out rays, and patient state. (4 marks)