:>reference the 74LS145 Datsheet ... diode with ... 17K
:An open-collector output with 17K pull-up diode? What is that? I checked a bunch of datasheets, but couldn't find any mentioning such a bizarre thing. What datasheet was saying that???
:If you measure the Com pin, at BASIC prompt: you'll see it's always LOW, with some short +1V edges. If you attach a pull-up, the signal looks inverse: Always HIGH(+5V), with some short 0V edges. As I said, the 74LS240 in your circuit might maybe work as pull-up, the 74LS145 in the CPC doesn't do so.
:>pull-up resistor to "sharpen the edges" of the signal isn't required.
:Makes sense for low frequencies. Though the inputs are read at falling edge, aren't they? Well, okay, you are right, the falling edge should be stable, only the raising one would be improved by pull-ups.