The ones in the disc interface have to be plain TTL rather than LS for the sake of the higher drive current - 7438 rated at 45mA, 74LS38 rated at 24mA, driving into 150R termination on the floppy drive (so 33mA actual current if they manage to drive it all the way to zero).So my next job is to remove IC27 and test it. I've got a handy replacement in the disc interface for easy swapping. It is marked as 7438 so basic TTL, before the Schottky stuff became standard.
IC27 I think could be LS - I'm guessing 7438 is used just for BoM commonality with the ones used in the disc interface. Two of the gates are used to give open-collector drive for the NMI (3K3 pull-up), one is used in the printer port (originally driving the printer externally so maybe a reason for robust drive, but there was a cockup with the polarity and it's now just used as an inverter with a transistor (Q11) driving the actual output. The third gate is just used as a spare NAND-gate in the video scrolling logic and the open-collector isn't wanted (and it's constant-state signals so speed isn't important either). In fact, 74LS03 or 74HCT03 would probably do the job for IC27.
Statistics: Posted by arg — Tue Feb 13, 2024 9:01 am