On 10/19/2012 09:11 AM, silverdr@wfmh.org.pl wrote: > > On 19 paź 2012, at 08:02, Gerrit Heitsch <gerrit@laosinh.s.bawue.de> wrote: > >>> Why? If I want to blend-in the content - say - as if it was loaded in RAM? >> >> Yes, but you will need to tell the PLA about putting ROM where RAM would usually be. > > Why? I put the ROM there, not PLA Yes, but unless the PLA knows about it, it will activate the RAM. >> Otherwise the PLA will generate the _CAS signal for that address space and activate the RAM. That would mean RAM and ROM output drivers fighting it out and the system crashing. > > No, I mean I grab the _CAS signals and select the ROM whenever address decodes properly. If not (or if my ROM is disabled), I pass it through. Not sure about the timing though. Ok, that strikes me as a bit too complicated since you already have all the logic present in the PLA if you use the _EXROM and _GAME lines intended for exactly that purpose and you don't have to worry about routing _CAS through even more logic and incur more delays. >> This 'telling the PLA about it' is done with the _EXROM and _GAME signals. > > Yes, and keep it that way. Let the real cart push the lines so that in such case the real cart can still work and override my ROM. But how will your logic know that there is a real cart present and it should go out of the way? The IMHO easiest way would be to put some extra logic (*) between _EXROM/_GAME from the Expansion port and the PLA. As long as the lines from the expansion port stay HIGH you can use the PLA inputs to activate your internal catridge if you want. As soon as one of them goes LOW, the logic disables the internal cartridge. This has the advantage that you work with signals that are mostly static and you don't have to worry about additional delays for timing critical signals. (*) I see 3 AND gates here... 2 to combine internal and external _EXROM and _GAME and the third one to detect if one of the lines from the expansion port goes LOW to disable the internal cart. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2012-10-19 09:00:10
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