Re: ROMs replacement

On 09/18/2012 06:53 AM, Jim Brain wrote:
> On 9/17/2012 2:25 PM, Gerrit Heitsch wrote:
>
> 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.

There is another problem with the solution with the '148 that is not 
present when using the '11. In order to switch KERNALs, you have to 
route _KERNAL to one or the other input. _KERNAL is the signal used to 
select the ROM, meaning it changes state twice per every cycle the CPU 
access the KERNAL ROM. That makes it high frequency signal, not 
something you want to route through a mechanical switch in a convinient 
position at the end of a long cable.

The solution with the '11 has only a static signal on the header but is 
in the present state limited to be used with a 27C256 and gives you 2 
KERNAL images.

  Gerrit



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