Much modern laboratory equipment including many computers and spectrometers contains extensive TTL logic. Problems can arise when one is trying to replace circuit boards in such equipment if the equipment is more than ten years old. "New" circuit boards are not necessarily the same as "old" circuit boards even though they are specified to be the same.
Yau-Man Chan and Herbert L. Strauss, "Not All TTL Circuit Boards are Compatible," Appl. Spectrosc. 40, 563-563 (1986)