Posted By
dmantione
on 2022-12-07
09:12:02
|  Re: External User Port for the C16: Let's make it!
My thoughts about getting the 74x652 working were as follows:
https://i.postimg.cc/gcMc60V3/afbeelding.png
i.e. the diode correctly emulates open collector in case of a output, but in case of an input, it prevents the external driver to pull the input down. The extra pull-down resistor in front of the diode will fix this. I have decreased the resistor value a bit in the example, because they are now pulling against each other.
Now, 68K is a rather large resistor value to connect to an input. It might be too much for a chip with bipolar logic, because for those you must actually pull a current to keep the input low and with 68K it might be too low. It may work better for chips with (C)MOS logic, because there the voltage on the input is what counts and 68K will get to a low state. However, it may still pull down at a rather slow pace. And the chip must not have an internal pull-up. If it has an internal pull-up, this trick will fail of course.
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