---------- Forwarded message ----------
From: Jack Carroll
Historically, original 7400 logic could sink much more current to
ground than it could pull high, and the logic-low value was better-defined.
So if there was any risk of other signals coupling to a clock line, it made
the most sense to change state on the negative-going transition, or to
enable other devices on a logic-low. Also, if you needed to drive power
loads such as early LEDs or small relays without a buffer transistor, there
was a chance you could do it with a current-sinking output, but not a
sourcing output. Most TTL derivatives, such as LS, were like that. These
days, most late CMOS such as HC and AC is symmetrical. Be careful with HCT
or ACT, though; they have unsymmetrical input logic levels, for best noist
immunity when driven by TTL.
--
Timothy Normand Miller
http://www.cse.ohio-state.edu/~millerti
Open Graphics Project
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