Well, with bipolar logic, one is driving an NPN emitter. That always biases the input high. What puzzles me, of course, is that the HCT latches, 74HCT573, didn't see the P0 output as being high enough to drive it. They're supposed to accept TTL levels. The HMOS MCU purports to produce sufficient output levels to drive LSTTL. It's still not entirely clear what's going on.

I don't remember whether there were standard TTL '373's. There weren't any 573's, though, and I haven't got any of the F573's, or LS573's, or ALS or AS 573's and probably won't have any. I find it odd, though, that the HCT parts don't see sufficiently high levels to function without the pullups.

In fact, I found it odd that I had so many of these old parts, as I never liked Intel products back in the '80's, and only used them when I had to.

RE