Re: ROMs replacement

From: Gerrit Heitsch <>
Date: Sun, 16 Sep 2012 15:04:18 +0200
Message-ID: <>
On 09/16/2012 01:31 PM, wrote:
> On 2012-09-16, at 12:57, Gerrit Heitsch wrote:
>> The simplest way to achieve that is a 74LS257.
> Yes, or 157.. as we don't need the cutoff of the output anyway.

Yes, but the advantage of the 74LS257 is that if you do hardware work on 
the C64, C16, +4... you will have some 74LS257 at home.

The LS157 doesn't have tristate outputs, but still has an enable input, 
so no matter how you slice it, it's a signal you need to take care of.

> Yup, levels are very much my worry here but I wanted to avoid adding another IC, especially one with most pins unused.

After thinking about it for a bit... You can also use a 74LS00 (or 
74HCT00) which is fully used up in the process and has 2 pins less:

How to set up:

Input1 is A12, it gets fed into one input of one NAND.

Input2 is your switch. It gets fed into one input of another NAND. 
Remember the pullup here.

The select signal (_Char-ROM) is directly fed into the other input of 
the NAND with A12. It is also inverted by using one NAND (second input 
tied HIGH) and then fed into the second input of the NAND that takes 
your switch input.

The outputs of the 2 NANDs are then fed into the last available NAND and 
its output is A12 for the EPROM.

Should look like this:


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Received on 2012-09-16 14:00:06

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