However, several additions have been made to the design based on the feedback received from the evaluation group.
The primary addition is the availability of a timeout feature, which was not present in the initial design. The timeout should be of a reasonable length -- possibly to be determined by enduser testing -- currently set at about ten minutes. Potentially, users may be speaking to friends or acquaintances, and will not want the machine to timeout within the few minutes that they are talking. However, the machine should not be inactive with a previous transaction for a long time, since other users may not realize that the system in stuck in a previous transaction. Audible warnings will be given for the last minute of the timeout cycle, and warning messages will be displayed onscreen: "The TURBOMail system has been inactive for ten minutes. Please complete your transaction, or the current transaction will halt." If the system times out with money or a credit card in the system, the cash will be returned to the user in the change slot, while the credit card will be retained by the system (this mirrors the behavior of ATM systems; it may also be possible to retain the user's money, although this may not be the desired behavior -- the user may realize his mistake and return for his cash. It is undesirable, however, for the the user's credit card to be returned, available to any other passerby to take).
The second addition is the global availability of navigation options which allow the user to back up or terminate a transaction. A quit button is present on all screens (if appropriate) and the cancel button allows the user to back up to the previous screen in all cases, except when a transaction has been completed but no new transaction has begun (i.e. the screens which inform the user to collect change, stamps, etc.).
In the case where the machine is unable to perform its functions (because internal diagnostics have registered problems with some part of the system, such as being out of stamps, or a problem detected with the scale, etc.), the system is disabled, and the welcome screen is replaced by a screen stating "The TURBOMail system is out of order. We apologize for the inconvenience." All buttons on the interface are disabled and inactive.
Finally, status message updates are provided by the system for processing states (in particular, as the system is dispensing stamps or handling the credit card verification and accounting). These messages are displayed on screen, and typically provide information as to the current state of the machine (such as "Dispensing stamps, please wait..." or "Sending credit card accounting information, please wait..."). Typically, most screens will not need such messages, since new screens will be displayed as soon as the user selects an option, providing immediate feedback. These status messages are used only in those portions of the interface where the screen may not accurately reflect the current system state.