Re: ROMs replacement

From: Jim Brain <>
Date: Mon, 17 Sep 2012 12:50:40 -0500
Message-ID: <>
On 9/17/2012 5:01 AM, wrote:
> On 2012-09-17, at 06:45, Jim Brain wrote:
>>> 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:
>> Looking at the original schematic, I sketched one up using a '148 and a '257.  With the '257 as Gerrit suggested (the NAND idea would also work) and the 148 handling the 3 signals.  The '148 takes care of OE/CE with it's GS pin (14), and the design requires just 2 ICs and 1 resistor (to hold the CHARROM switch position in high unless shorted through the switch/jumper.
> The NAND version looks like this:
> It has four pull-up resistors. One would of course be better. So you mean you used 148 in place of LS11 and 257 for the second one (interchangeable with 00), right?
Yes, I hooked the BASIC/KERNAL/CHARROM to the 148, and the GS output 
feeds the CE/OE line of the EPROM.

A12 is then handled as Gerrit and you have previously described.

BUt, what does JP301 do in the schematic?  I don't think I accounted for 
it, and I have just glanced at your schematic.

I can draw it up again (the 257/148 option was easy to wire, nothing 
special to lay out), as I just drew enough of it in EAGLE last night to 
prove to myself it was doable.



Jim Brain

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Received on 2012-09-17 18:00:36

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