exercises

16. dialogue notations and design

EXERCISE 16.5

This exercise is based on the nuclear reactor scenario on the web at: www.hcibook.com/e3/scenario/nuclear/

(a) Looking only at the STN diagrams in Figures 16.21 and 16.22 (that is ignoring for now the meaning of the various actions), identify missing elements from the STNs. Taking into account the meaning of the actions, suggest possible corrections.

(b) Taking into account now the meaning of the various states and actions, explain why you believe the consultant suggested the change from the behaviour in Figure 16.21 to that in Figure 16.22.

STN for alarm state

Figure 16.21 - STN for alarm state

Figure 16.22 - STN for revised alarm state

answer available for tutors only

(a)
Figure 16.21 - no '+' transition from RED state, no '-' transition from GREEN state. Suggest that these should simply do nothing, but perhaps have an audible beep so that you know that the action is being ignored.

Figure 16.22 - no '+' transition from TEMP or RED states. No '-' from GREEN and TEMP state. CONFIRM and CANCEL undefined from nearly all states. Would suggest doing nothing (with beep) for all except '-' from TEMP state. In this state the system looks similar to RED state which suggest that '-' should take the system back to AMBER. This also means that '+' followed by '-' always gets you back to the same state. Students may also notice that the TEMP state and RED state are indistinguishable - perhaps make TEMP state be flashing red light?

(b)
There was a potential for accidentally getting into state RED, which is a 'dangerous state' of the STN. The CONFIRM button reduces the likelihood of an accidental RED state (perhaps there had been too many false alarms before the consultant's visit?).

If you are observant you may also notice that there is a potential for missing the CONFIRM action and thus dangerously failing to get into RED state.

Other exercises in this chapter

ex.16.1 (ans), ex.16.2 (ans), ex.16.3 (ans), ex.16.4 (ans), ex.16.5 (tut), ex.16.6 (tut), ex.16.7 (tut), ex.16.8 (tut), ex.16.9 (tut), ex.16.10 (tut), ex.16.11 (open)

all exercises for this chapter