Re: ROMs replacement

From: Jim Brain <>
Date: Mon, 17 Sep 2012 23:53:01 -0500
Message-ID: <>
On 9/17/2012 2:25 PM, Gerrit Heitsch wrote:
>> Ah, well, the '148 option gives you 8 banks to choose from (I used only
>> 3, with a sub selection for CHARROM being only 4 kB).  Adding more
>> options simply means routing the Select lines to another bank select.
>> So, with a 3 pin jumper, you can add 2 KERNALs to the design (middle is
>> KERNAL select, two ends are two inputs to the '148), but you can add
>> more KERNALs as well or BASICs with just more jumpers.
>  >
> Needs a larger EPROM though. 27C512 for example. Would give you 6 
> KERNALs, 1 BASIC and 2 Char-ROM.

Yes, if you want all 8 banks in use.  BUt, you can just use the lower 4 
inputs for a '256.
> I'm looking forward to the finding out how you make the selections work.

Well, I sent BASIC, CHARROM into the 0,1 lines of the '148.  !GS drives 
!CE and !OE on the EPROM, while the A and B lines go to the A13 and A14 
lines of the EPROM.  A12 is handled wither with the NAND gates or the 
'257, nothing new to report there.

Setting up 2 KERNALS means bringing '148 lines 2 and 3 to the ends of a 
3 pin header.  Middle pin goes to !KERNAL  shorting one half will shunt 
KERNAL to one of the two address lines.  It does require 1 more resistor 
than the '11 solution, as it requires both the 2 and 3 lines of the '148 
to be pulled weakly high, but the wiring is simpler.

>  Gerrit
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Jim Brain

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Received on 2012-09-18 05:00:04

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