Re: 1551 Repair - 28 pin 6523 Wanted

>If set to input in the DDR they should read as '1'

Only if there is a pull up resistor on the undefined bits, which there may 
or may not be.

It depends on whether undefined means "don't rely on what you currently get 
from one of the chips" or "we know that it will be random because we didn't 
include the pull up resistor". MOS were making their own dies for each batch 
anyway, a cost saving measure of just hacking bits out of the die would not 
surprise me.

However it's only if the undefined values are actually somewhat reliable and 
are required by code that you have a major problem. The rom code can be 
analysed pretty easily without access to real hardware to see whether it's 
sensitive to the undefined inputs, if it's not then it's just accuracy for 
it's own sake (*).

It's very possible that a clone chip is going to be different in some 
behaviour or other, even if it's down to the exact voltage that a 0->1 or 
1->0 will get registered or the time it takes. While it's a digital input, 
the actual input that is coming into it is analogue and is likely to have a 
non vertical slope no matter how steap it is. As long as it meets the 
requirements of the 1551 then nobody will probably care too much.

(*) I'm guilty of that though, so I admire the intention to get it right. I 
guess there is a potential that someone wrote custom code for accessing the 
1551 as well, so just looking at the rom isn't necessarily enough.


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