The first monostable
multivibrator must be retriggerable
The output pulse length of the
multivibrator must be longer than the period of the keep-alive signal. The pulse
length of a multivibrator is T=kRC. The recommended configuration is a 100-kΩ
resistor and a 0.1-uF capacitor, for a pulse length of around 10 ms.
Either falling-edge or
rising-edge trigger configurations can be used for this application. See the
datasheet of your selected multivibrator for details.
A pull-up or pull-down resistor
is required to hold the input at a valid state during reset. The recommended
value for this resistor is 10 kΩ.