Does anyone know a good, detailed discussion of the pros/cons of different 74xx Logic families (and especially subfamilies) and the rules for mixing them? I get the difference between TTL and CMOS logic levels, and I kind of get the impression that you can combine families with the same logic level, but I'm not 100% sure. Also, is there something "wrong" with The 74Fxx family? It looks intriguing in its speed, but I don't seem to see anyone using it. Is there some major pitfall with using it that I'm not aware of?
The Art of Electronics (Horowitz and Hill) was the standard text in the 1990s. It covers everything you need to know about every electronics technology that existed up to the point of publication. The third edition was published in 2015, so I imagine it's as up to date as us hobbyists need.
The 74 series come in two major flavours: TTL and CMOS. TTL was the original, with bipolar transistors: original (no letter), H, L, S, LS, AS, ALS, and F (and probably some more I'm forgetting). They're all 5V only, and have the same input thresholds (below 0.8V for '0', above 2V for '1') and output voltages ('0' can be as high as 0.4V, '1' can be as low as 2.4V). The different families are various trade-offs between speed and power. They all draw power even when idle, and generally higher speed means higher power. Outputs are limited in the current they can provide, and inputs draw current, so there are limits to how many inputs one output can drive.
They've been pretty much completely replaced with CMOS, which uses MOSFETs. These are the families with a 'C' in their name: C, HC, AC, HCT, ACT, and some without C like LV. There's a huge range. The T in some families means the input thresholds are compatible with the old TTL chips. If you're mixing technologies, you'd want to use them. If you're using CMOS for everything, it's better to use the non-T version. These have input and output voltages symmetric around half the supply voltage, and generally run on a wider voltage range. If you want to run on 3.3V, you must use CMOS. Again, there's a trade-off between speed and power, but with CMOS power consumption is extremely low when idle.
The problem with mixing TTL and CMOS is that TTL '1' outputs aren't guaranteed to be a high enough voltage to be recognised by CMOS. The 'T' families have tweaked input thresholds to handle the difference. TTL inputs draw current, and want a quite low voltage for a '0'. CMOS families with weaker outputs might have trouble with that. The newer ones are pretty strong though.
TTL is a little friendlier to beginners. If you don't connect an input, it floats high. CMOS has a fit. If you haven't sprinkled enough capacitors on the power supply, TTL will forgive you. CMOS will fail in baffling ways. But CMOS has a nice strong drive for both high and low outputs, and the lower power makes power supply and distribution a lot easier to get right.
There are two reasons you wouldn't just use the fastest family you can find: faster means more power, and faster means the outputs change more quickly. There isn't really any such thing as digital electronics, but if you're running slow enough with slow enough transitions, you can pretend the analogue stuff isn't there. With a family like AC you can't: every wire is a transmission line, and you will get reflections and cross-talk and ground bounce. They need a lot more care in layout. I'm not experienced enough with breadboards to tell whether they're up to the job.
I usually stick with HC unless there's a specific need for something else. My designs aren't aiming for the highest speed, and HC is pretty forgiving of almost everything.
