Strategy for up to 40 ADC inputs using alert feature
If you have 40 inputs that are normally at zero and you want to know if one of those signals has gone high you can use logic gates to detect this or if you want, you can use diode orring to give you one signal that tells your one of the inputs has risen high.

The same can be done if the inputs are normally high and one goes to 0V, It's fairly trivial with real voltage signals but a little more care has to be taken when the signals are from an open-collector as per a photo transistor - you've got to make sure the loading of the diodes/logic isn't too great to cause the signal problems.

By the way - using diodes means that the inputs remain independant and still individually readable should you choose to do so.