[Coco] Need a FD 502 controller that can accept 28 pin roms

Little John sales at gimechip.com
Wed Oct 13 02:03:52 EDT 2010

Cool. I was going by what I had read in the Extended ADOS-3 Manual which 
If you have the Tandy FD-502 controller, you already have a 28-pin socket: 
however, the controller needs a minor modification in order to be usable 
with a 27128 EPROM.
It then mentions how to get the instructions from SpectroSystems, followed 
The instructions may also be found on Page 76 of the March 1989 Rainbow 
magazine except that there is a typographical error: It is pin 26 not pin 28 
that must be connected to the controller's edge connector.

But are you saying it can handle a 16K EPROM without modifications? or are 
you saying that it can use any 2764-27512, but only as an 8K device? Just 
curious. I use 27256's for everything because I have a ton of them - I just 
burn an 8K image into the 27256 stacked 4 times - so it doesn't matter the 
status of the upper address lines or even if they are floating - it still 

If HDB-DOS is only 8K then it would be fine without mods, but if you tried a 
16K DOS (like CoCoNet) it would need the mod - I think?
-John #1

----- Original Message ----- 
From: "Darren A" <mechacoco at gmail.com>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Wednesday, October 13, 2010 12:24 AM
Subject: Re: [Coco] Need a FD 502 controller that can accept 28 pin roms

> On 10/12/10, Little John wrote:
>> The FD-501 drives will work with the FD-502 controller (or pretty much 
>> any
>> coco controller). I may be mistaken, but I believe that the FD-502, 
>> although
>> having a 28-pin socket, is still only wired for an 8K eprom and the
>> additional address line will still need to be hacked into the socket with 
>> a
>> jumper wire. Can anyone else confirm this?
> The FD-502 will accept anything from a 2764 to 27512 without
> modification.  For anything larger than a 2764, the code must be
> programmed into the upper-most 8K of the device.
> Darren
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