9.1 Application Information

In this application, two 3-input NOR gates are used to create an SR latch as shown in Figure 9. The additional gate can be used for other applications, or the inputs can be grounded and the channel left unused.

The SN74HCS27 is used to drive the tamper indicator LED and provide one bit of data to the system controller. When either tamper switch outputs HIGH, the output Q becomes HIGH. This output remains HIGH until the system controller addresses the event and sends a HIGH signal to any of the R inputs which returns the Q output back to LOW.

The user can add a small RC to the feedback path of the NOR gates to default the output to a certain state, which can create slow transition rates. This fact makes the SN74HCS27 ideal for the application because it has Schmitt-trigger inputs that do not have input transition rate requirements.