Posted By
dmantione
on 2022-12-05
19:00:05
|  Re: External User Port for the C16: Let's make it!
Hello,
I have read up on the discussion between 652 and 654 and I think the point against the 652 is valid. I have indeed designed my replacement to be compatible with both 652 and 654, tested a bit with both, but then never worked with the 652 again. I have sufficient 654 chips, so there wasn't much point until now to look into the 652 topic again.
I'm puzzling a bit with the idea wether a pulldown resistor between diode and input can fix the issue, but then you have a pulldown before the diode and pullup after the diode, it will likely become a mess...
I am not sure I agree wether I agree with the bootup problem:
1 - The 6529 has no reset pin. This means that after a reset, the state of the 6529 should be considered unknown.
2 - The data sheet of the 6529 explicitely recommends that it is interfaced with open collector output devices, likely because of these issues
3- Even in case a push-pull interface is used and a conflict happens, generally no terrible things happen because of the resistors the chips have in the output drivers, and the conflict will be there for a very short time, as the KERNAL resets to input very quickly after reset.
Therefore, in lack of sufficient 654, a two chip solution should work. My first thoughts would be 377+541, but the schematic with 273+573 looks good as well. Indeed you can also use a GAL, but it looks overkill to me.
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