HP9816 PAL16L8

[Paul Berger]

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:
>
>> 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
>
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.