11 Jun
2019
11 Jun
'19
11:17 a.m.
On 6/11/19 10:54 AM, Tony Duell via cctalk wrote:
I have just remembered something that I guess you
realised some time
back...
The 16L8 allows you to tri-state outputs under logic control (I think there's
one AND term for the output control of each output). I suspect this is used
in ths PAL, certainly for things like DMAR0 and DMARdy which go onto the
backplane connector (and thus could also be driven by another board).
I mentioned in my blog entry that I had a way of detecting this by
driving the outputs through a series resistor and sensing the logic
level of the pin.
In other words, you've already labeled the pin as an output by
exhaustively running through input combinations. What's needed is to
differentiate a "driven" high level from a tristated one. By driving
the outputs low through a series resistance you can determine if an
output is high-impedance or not.
I explained that badly, but it does work.
--Chuck