Well, since Nicolas also says that the ROMs need decoding logic, I'm 
almost certainly missing something; since I'm metaphorically from 
Missouri I guess I'll have to cobble something together to see just what
it is.

BTW, what *do* I need to print those colour-on-black-background 
schematics?

Hope I'm not wasting too much of your time,

mike
**********************************************************************************

----------
From: 	Jim Brain[SMTP:brain@jbrain.com]
Sent: 	Wednesday, May 06, 2009 8:47 PM
To: 	cbm-hackers@ling.gu.se
Subject: 	Re: FLASH ROM replacement

M H Stein wrote:
> Jim
> --------
> Well, I can't even figure out how to print your schematic, so I'm probably
> not the one to ask ;-). I kinda thought it'd be just another two rows of pads
> beside the 24 pin pads, without any additional logic or pullups, but I 
> hadn't really thought it through.
>   
You need at least 2 select pins, as you've noted the PET uses multiple 
CS lines.  As well, the layout is markedly different from either the 
2364 or the 27XX pinout,  As well, I consider PHI2 to be another CS 
line, as I believe the ROM is only active on PHI2 high.
> I thought that if one accepted replacing both the upper and lower 2KB 
> with a 2732 or greater (except for the E ROM which could be a 2716) 
> and used the lower socket of each pair, then all you'd need is pin 3 to 
> /CS and jumpers for pin 4 to A11 and /OE to Vss or /CE.
> For the E ROM connect pin 4 to /OE instead (The upper half of E is
> memory-mapped I/O).
>   
If you're willing to string wires and such, yes, that's workable, but I 
dislike making such assumptions on a design like this.  Even with the 
concession, you'd need to deal with PHI2 and a couple select lines, so 
dealing with all of them took no more effort.
> But it probably isn't that simple, and no doubt someone will point out
> the flaw(s). Maybe there isn't really enough demand anyway to make it 
> worth while, especially seeing there's one permanently on eBay...
>   
Well, I'm happy to make a run of the design I have, and I am pretty sure 
I can offer the board sans EPROM for $10-$15, if that's of interest to 
enough folks.  I'd be willing to offer them even less if folks wanted 
them in lots of 5 or 10. (individual packaging is what mandates the 
price points)

or, the PCB design is GPL, so anyone can pick it up and run a batch.  
Powell Electronics offers the 24 pin header, and the rest are easy to 
source from digikey, farnell, mouser, etc.

Jim



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