On Wed, Aug 2, 2017 at 6:47 PM, Mike Stein <mhs.st...@gmail.com> wrote: > Good advice; the most likely culprit (because it's indirectly connected to > the outside world with its static, lightning, ground loops etc.) is M35, a > (1)4584. > > The only other IC would be M24, a 40H032 (which could probably be replaced > with a 74HC(T)32).
40H family is directly replaceable with 74HC family. I wouldn't use 74HCT chips in this circuits, the TTL voltage input compatibility isn't needed (and it doesn't do harm anyway) but switching times and power consumption of the HCT family are slightly different from the original 40H and the almost-identical 74HC. By the way, the UART chip could be the culprit too, just trace the signal back from the DB25 until it disappears. (Or start from the UART pin forward to the external world). HTH Frank
