[Coco] eprom programmer

[Chad H]

I just took apart my FD-502 controller casing again to determine the best
place to modify for this and I noticed on the underside that pins 26,27 and
28 are all adjoined by factory trace.  Pin 28 has a trace leading out on the
top side  I suppose I could interrupt the trace between 26 & 27 on the
underside to work with the 26 pin-only modification.  That would be quickest
and simplest.  Then I just solder taps in place to a switch...you wouldn't
even notice from looking at the top.

-----Original Message-----
From: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com] On
Behalf Of Darren A
Sent: Saturday, March 24, 2012 12:40 PM
To: CoCoList for Color Computer Enthusiasts
Subject: Re: [Coco] eprom programmer

On Sat, Mar 24, 2012 at 10:49 AM, Chad H wrote:
> Hmm...so let me see if my thinking is correct here...
>
> I currently have a 8K 2764 EPROM in my 502 Controller... If I replaced 
> this with a 27128 16K chip I could combine 2 .ROM files, write them, 
> but I would be stuck on either the low or high bank.
>
> Now Darren said the high address line was tied to Vcc on the controller.
> What if I removed that and ran the Vcc to a switch along with ground 
> and used the switch to flip Vcc/Ground on the high address pin...  
> Would this achieve the bank switching effect?
>

Yes.  You would need to bend out pin 26 (A13) on the EPROM or perhaps cut
the trace on the board.  Then connect a switch to that pin which toggles
between Vcc/Gnd and you have a switchable, banked EPROM.

A more elaborate modification would be to do the same with pins 1 and 27,
then you can use a 27512 EPROM with eight banks of 8K.

Or..connect pin 26 to the CPU address line A13 and only put switches on pins
1 and 27, giving you four banks of 16K.

Darren

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