Re: ROMs replacement for narrow board

From: Gerrit Heitsch <>
Date: Sat, 24 Nov 2012 12:49:10 +0100
Message-ID: <>
On 11/24/2012 10:28 AM, 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):
>>> 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.


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Received on 2012-11-24 12:00:09

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