In this application, two 2-input NAND gates are used to create an active-low SR latch as shown in Figure 9-1. The two additional gates can be used for a second SR latch, or the inputs can be grounded and both channels left unused.

The SN74HCS00 is used to drive the tamper indicator LED and provide one bit of data to the system controller. When the tamper switch outputs LOW, the output Q becomes HIGH. This output remains HIGH until the system controller addresses the event and sends a LOW signal to the R input which returns the Q output back to LOW.

The inputs of this active-low SR latch can often be driven by open-drain outputs which can produce slow input transition rates when they transition from LOW to Hi-Z. This makes the SN74HCS00 ideal for the application because it has Schmitt-trigger inputs that do not have input transition rate requirements.