Re: ROMs replacement for narrow board

From: Gerrit Heitsch <>
Date: Sat, 24 Nov 2012 09:29:24 +0100
Message-ID: <>
On 11/24/2012 08:37 AM, wrote:
> On 2012-10-18, at 18:52, Gerrit Heitsch wrote:
>>> Why stop there? Replace the TTL with micro-sized versions, mount them inbetween the pin header on the bottom, and make the board the same size as the eprom!
>>> Just kidding!  ;-)
>> 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, 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.

- It might be necessary to invert A14 to get back to the original
   design. A full 74LS04 would be Overkill. Use a simple inverter with
   a BC547 or BS170 and 2 resistors.

- Now you have A14 and _A14. That should allow you to easily
   discriminate between KERNAL and CHAR by combining it with the
   _CHAR signal.

- A14 (and) CHAR = KERNAL  (high)

- _A14 (and) KERNAL = BASIC (high)


       Message was sent through the cbm-hackers mailing list
Received on 2012-11-24 09:00:14

Archive generated by hypermail 2.2.0.