[Coco] Maximum CoCo Drives (was: Re: Posted this pic for retro innovations)

[Bruce W. Calkins]

Our CoCo "almost" uses the standard used by IBM in the day.  4 drive 
select lines and one side select line for a total of 5 lines.  However 
the 4th drive select might not even be in our hardware.  I'm sure our 
hardware guys can make a big improvement in how those are used.  The 
CoCo DOS OTOH was hard coded for 4 single sided drives.  Rewriting the 
code to use larger and/or more drives involved moving and rewriting the 
code to allow the space for read/write buffers and drive lookup tables.  
Colon Stearman(sp?) wrote a series called "Cooking with CoCo" in "The 
Rainbow Magazine" on the subject.

Bruce W.


On 1/22/20 2:03 PM, Allen Huffman wrote:
> Heh, we talked about this very thing at an Atlanta CoCoFEST back in the early 90s.
>
> There was a program in Rainbow called SIXDRIVE that did this with six single sided drives of the say, but I never saw someone using the selects to use them as bits.
>
> We should do this and show it off a fest.
>
> --
> Allen Huffman -
>
>> On Jan 22, 2020, at 12:52 PM, RETRO Innovations <go4retro at go4retro.com> wrote:
>>
>> On 1/22/2020 12:41 PM, rietveld rietveld wrote:
>>> Let's turn this back into a coco topic
>>>
>>> is 4 the maximum number of physical drives a coco can have, or has someone ever  patched a DOS  to allow for the use of more drives?
>> Others can chime in, but the FDC has 3 drive select lines.  As such, I would assume one could put a 3-8 decoder on those lines, and, with requisite changes to the DOS, use up to 7 drives (leaving 1 selection for "no drive selected").  I think I saw an advertisement in Rainbow for something that did just this.  Of course, even at 3 drives per interface, in an MPI, isn't that 12 drives?
>>
>> Jim