AFAIK a bunch of TTL logic is used on many expansion boards, and Sergei's turbo XT SBC uses 74LS (or 74HCT) glue logic for just about everything.
For TTL 0-0.8V is low, 2.2-5V is high. Everything in between is undefined.

alexanrs wrote:

AFAIK a bunch of TTL logic is used on many expansion boards, and Sergei's turbo XT SBC uses 74LS (or 74HCT) glue logic for just about everything.
For TTL 0-0.8V is low, 2.2-5V is high. Everything in between is undefined.

I get that.

But would, say, low-low-low-low represent 0x0 (thus "active high") or 0xF ("active low")?

AFAIK for the address lines exposed on the ISA slots are active high. A quick look at Sergei's schematics seems to confim that (ROM chip being enabled when A16-A19 are high)

Yes address lines are non-inverted, i.e. low=logic 0 and high=logic 1.

SquallStrife wrote:

Everything I've found via Google on the topic has been high-level and abstract.

I want to know, if you probe a low signal (<0.8v) on A0...A19, is that a 1 or a 0? I know I can find out myself, but I'm sure someone here will know off the top of their head!!

Only control lines (e.g. IO port read/write or memory read/write) are active low signals. Everything other is active high. On the web there is good book about x86 architecture and ISA bus https://archive.org/details/ISA_System_Architecture