Posted By
MikeZ
on 2015-12-20
08:36:07
 |  Re: unused I/O location for debug purposes
The reason for the mirror images is that the pla doesn't decode the 4 LSBs of the address buss. In sends a chip select to the ACIA when it reads FD0-. To use the addresses FD04 to FD1F you must decode the 4 LSBs on the address buss when the ACIA is active. I did this in 1987 by using a 16 to 4 decoder chip. It got its chip secect from the PLA's FD0 chip select. The 4 pin side was connected to the 4 LSBs of the address buss and 12 of the 16 outout pins corresponding to FD04 - FD1F were each connected to an additional 6529 I/O port. That was my application and it worked.
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