Re: ROMs replacement.. solved!

From: silverdr_at_wfmh.org.pl
Date: Fri, 19 Oct 2012 20:30:35 +0200
Message-Id: <3ECF7931-595E-49AC-8542-BD029CAB9202@wfmh.org.pl>
-- 
Sent from mobile phone (so please have understanding)

On 19 paź 2012, at 10:34, Gerrit Heitsch <gerrit@laosinh.s.bawue.de> wrote:

>> 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?

Because (correct me if I am wrong) in such case PLA will push _ROML/_ROMH LOW _instead_ of _CAS.

> 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.

At first I thought it wouldn't work but I think I got your idea and it might work quite well for the standard addresses. This would mean that I monitor the external lines but supply my own to the PLA and take the _ROML/_ROMH from PLA for my needs. Once I find /external/ _EXROM/_GAME being pushed LO, I still supply PLA with my equivalent but stop using _ROML/_ROMH.. If that's what you meant then indeed it seems like easier to do at acceptable cost.

> 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.

Right. And I don't need [to repeat] any decoding logic. The only disadvantage against what I was thinking about is that the addresses are then stoned inside PLA. With what I proposed, I could use theoretically bigger software as I could select-in ROM at any address.

> (*) 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.

I shall have to draw it but yes, it seems like we are on the same page now.

-- 
SD!
       Message was sent through the cbm-hackers mailing list
Received on 2012-10-19 19:00:12

Archive generated by hypermail 2.2.0.