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

Arthur Flexser flexser at fiu.edu
Wed Oct 13 02:15:27 EDT 2010

That modification is only necessary because Extended ADOS-3 requires 16K of
EPROM space;  for an 8K DOS, the FD502 works fine as is, even if the DOS is
in a 27128 or 27512, so long as the code is in the correct 8K of the chip.
The relevant address line is grounded on the FD502, and the modification
requires cutting a trace to un-ground it along with running a jumper wire to
the edge connector.

On Wed, Oct 13, 2010 at 2:03 AM, Little John <sales at gimechip.com> wrote:

> Cool. I was going by what I had read in the Extended ADOS-3 Manual which
> states:
> 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
> by:
> 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
> works.
> 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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