Noted that Ken had an issue when a 74S TTL part was replaced with a 74LS because the characteristics are different.<p>Back in the day (mid 1980s), when I was going through an electronic engineering apprenticeship with a flight simulator company and many boards were end-to-end TTL, it was generally understood that power-hungry 74S series TTL were used for a reason (generally speed) and should not be substituted lest 'funny things' happen.<p>Mind you, there was one time when debugging a glitch led to one chip being replaced with the same type from a different family - something like a 74LS replacing a 74F - and the propagation delay difference was enough to fix an edge-case timing issue. Due to project constraints, the root cause wasn't tracked down and so the board bill of materials made it clear that the 'odd one out' was correct and should not be changed.<p>PS: Ken - I think I still have a quantity of 74S TTL 'pulls' from the day, so if all else fails sourcing a part, look me up!