11 Jun
2019
11 Jun
'19
11:40 a.m.
On 2019-06-11 3:17 p.m., Chuck Guzis via cctalk wrote:
On 6/11/19 10:54 AM, Tony Duell via cctalk wrote:
Well in this case I have Mr Duell's schematic to go by to determine what
is input an what is output.? For the 16L8 tristate is an available
output option that you would need to specify in PLD design and I believe
can be selected individually for each output.? In the case of this HAL
it would seem likely that pin 11 tristates all outputs but output 0 at
least when pin 11 is low the output from all the rest is high but I did
not test for tristate as it does not matter in this case since pin 11 is
permanently tied high.
Paul.
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