On 11/24/2012 10:28 AM, firstname.lastname@example.org wrote: > > On 2012-11-24, at 09:29, Gerrit Heitsch wrote: > >>>> No, the next step would be to design one such circuit for a 250469 board, the one where the BASIC and KERNAL are in one ROM and only the char-ROM is extra. >>>> >>>> Should be a bit easier since _KERNAL and _BASIC are already added together by an AND gate on the board. >>> >>> Ironically this seems to do more harm than good for the glue ;-) >>> >>>> That way one doesn't need a 74LS11, a 74LS08 should do. >>> >>> I save one input on the _OE/_CE gate but then have to handle three states instead of two to distinguish between BASIC and KERNAL.. which so far seems to me like I can't make it with the same EPROM content layout and chip count I had before. With shifted content I came up with something like (still have to double-check for correctness): >>> >>> http://dl.dropbox.com/u/58002657/cbm/c64/rom_adapter_narrow0.png >>> >>> Can you have a look? Maybe you have some better ideas, especially ones that would allow to keep the old EPROM content layout? >> >> I'm not quite awake enough yet to solve logic gate problems, > > :-) I think I know what you mean. > >> but here are some hints to think about: >> >> - You have A14, it is HIGH if KERNAL or CHAR are selected and LOW >> if BASIC is selected. > > Yes, I treat it as poor man's _BASIC signal ;-) Ok... Looking at your new circuit, using A14 instead of _BASIC on IC1B should work, it's LOW when BASIC is active and HIGH when KERNAL or CHAR is active. Behaves the same as the old _BASIC signal. The generation for _CS and _OE for the EPROM is also OK. Now the only signal left over to take of is A14 of the EPROM. This one must only go LOW when _KERNAL is LOW and A14 is HIGH. Means you need 2 Inverters. One for _KERNAL and one for the output of the AND gate. Sounds like you need at least 2 Transistors + 4 resistors or you need to add a 74LS04 or you need to rearrange the EPROM contents. Currently, I see no other way to do it. Anyway I try, I need extra logic that I can't build by what the 74LS00 and 74LS08/11 supply. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2012-11-24 12:00:09
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