Scott, you've got to get rid of your blindfold to your problem. Slapping bandaids on it is not going to create a good outcome. As I've mentioned before, running you limit switch wires with the motor wires is bad - this is the root cause. Doing this is going to give you lots of spikes when the motor starts/stops - you can measure this so you know exactly what you have to guard against - its not like you're trying to protect against some unknown transient that might occur randomly. I would suggest you get a storage 'scope and make some measurements, this will allow you to ENGINEER a solution. Once you have a picture of the spike, you'll probably find that it has risetimes in the 10's of microsecond range. A RC filter knocks the top off these spikes - again you can measure this. Your assertion that MOVs are slow comes up frequently - have you looked at the datasheet for these items? You'll find they respond in the low nS range - so speed is really not an issue. Some comparisons between tvs diodes and MOVs:

MOVs:
high capacitance - good for low frequency circuits, bad for high frequency circuits.

good overload capability - movs can take a good kick, however they tend to explode if you're too nasty to them.

wide voltage tolerance - they're good for gross overloads.


TVS diodes (tranzorbs(c)):

low capacitance. good for data comms/higher frequency circuits.

average overload capability. give them a good overload and they'll go short circuit.

reasonably tight voltage tolerance.

So , for protecting a HC TTL input, one would most likely use a TVS diode.


So, do some measurements to quantify the problem, then fiddle around with some R & C and see the effects. I'm sure you'll be able to come up with a robust solution that will eliminate the requirement of replacing ics and you'll be able to sleep at night knowing that your circuit will survive.


I have a similar problem in my car - there are a couple of idler wheels that have sealed bearings in them. These are mounted to the engine block. The problem is that the engine block puts its heat into the sealed bearings and the bearings only last a couple of years instead of lasting near indefinitely. The fix is to replace the bearings regularly - its not a difficult job but annoying. Had the designers mounted the idlers on a separate bracket to isolate them from the heat, then there would be no problem.