Re: ROMs replacement for narrow board

From: Gerrit Heitsch <>
Date: Sat, 24 Nov 2012 15:46:48 +0100
Message-ID: <>
On 11/24/2012 02:40 PM, wrote:
> On 2012-11-24, at 13:09, wrote:
>>> Anyway I try, I need extra logic that I can't build by what the 74LS00 and 74LS08/11 supply.
>> Yup - same here. I am now trying with swapped parts:
> With limited success as well.
> A12 is handled the old way, A 14 is simple: goes LO whenever _CHAROM or _BASIC (A14B) goes low - one '08 gate. But A13 doesn't work. It goes LO whenever _BASIC (A14B) goes LO. So far so good but then it has to be switched via jumper when and /only/ when KERNAL (A14) is HI..

A14 on the ROM doesn't work that way. With the gate like this, it means 
that BASIC and CHAR would have to be in the lower part of the EPROM. The 
other circuit assumes the KERNAL there.

I really see no other way than adding 2 inverters, either discrete or in 
the form of a 74LS04.

Also, there is another problem. Your KERNAL-Switch uses the _KERNAL 
signal which we don't have.

We really have to regenerate _KERNAL from  !(!A14 & !CS-ROM) which is

          A14 ----------| \     |\
                |\      |  )----| )O--  _KERNAL
     _K/B ------| )O----| /     |/
                |/      |/

Once we have that, the old circuit and ROM layout can be used again. 
Since you use a 74LS08, we have enough AND gates, the only thing missing 
is 2 Inverters:

                  | |  4.7K?
                  | |
                   *----------------- out
   In    ___     |/

If you use a BS170 (cheap FET), you should be able to get by without the 
input resistor.

Still not pretty, but the simplest way to fix things IMHO.


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Received on 2012-11-24 15:00:09

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