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Old 26th Feb 2021, 9:54 pm   #698
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 676
Default Re: Non-working Commodore PET 3016

Originally Posted by SiriusHardware View Post
Mark, are you thinking that PROM _CS is held low for the entire time that the address is within the address range of the PROM? It does look that way - 4 inputs from BA12 to BA15 to the 74154 4-line-to-16 line decoder UD2. No other signal involved in the generation of the chip select. If so this is somewhat reminiscent of the situation with the MK14, you could intentionally damage something by deliberately writing to the PROM address area.
That does look like a possible problem, but when the PET is running NOPs it shouldn’t be a risk as there are no write operations.

I have heard it was possible to damage PETs by poking to the wrong memory addresses, maybe the PROM decoding was responsible for that.

It would also mean that PETs could be damaged if the software crashes, seems like a very poor design, especially with all those unused chip select pins on the proms.
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