| 摘要 |
<p>The protection switch system includes a number of push-button switches which must be operated in the correct order to give access to a particular service. Some of the switches (BP2, 4, 5, 8) are able to provide the required output, while others (BP1, 3, 6, 7, 9) are not able to, and their operation triggers off an alarm. The switches are connected to a shift counter (3) which counts the number of operations of the switches and provides an output when more than a given number have been operated. The correct buttons are connected into a first logic circuit in order to detect which particular combination of buttons has been operated. If a correct combination has been operated then an authorisation signal is produced, but an incorrect combination leads to the production of an alarm signal.</p> |
