[Coco] Re: [Color Computer] Floppy contoler Q

[Mike Pepe]

Phill Harvey-Smith wrote:
> Hi,
> 
> I'm just trying to claify the way the bits at $FF40 work on the RS-DOS 
> cartrage, I have studied the circuit sheets for both the WD1773, and 
> WD1793 based controlers but ams still unsure about bit 6, looking at the 
> circuit sheet this appears to be wired to Pin 32 of the drive cable, 
> which would be side select. This makes sense as the WD1773/WD1793 don't 
> have a side select output, unlike the WD2797 used in DragonDos, which 
> does. However a comment in the mess source says :
> 
> /* An email from John Kowalski informed me that if the DS3 is
> * high, and one of the other drive bits is selected (DS0-DS2), then the
> * second side of DS0, DS1, or DS2 is selected.  If multiple bits are
> * selected in other situations, then both drives are selected, and any
> * read signals get yucky.
> */
> 
> This infers that this signal can also be used to select the 4th drive, 
> however I'm unsure of the way this could be done. It may be possible 
> that the Tandy drives had some off board circuitry that somehowe did 
> this (or a non standard cable), since I have never actually seen one, I 
> do not know.
> 
> In case anyone wonders what I'm up to, I'm trying to port SuperDos (A 
> Dragon dos enhancement), to use the RS-Dos hardware, as I figure this 
> way an RS-Dos cartrage could be used with the Dragon (either the British 
> one or the Tano).
> 
> Cheers.
> 
> Phill.
> 
> 

Phill,

The tandy controllers differ from (what would apparently become) the 
industry standard. The pin that Tandy uses as Drive Select 3 (DS3) to 
the rest of reality is actually Side Select.

As it was pointed out, the practical upshot of this is that you can 
combine "DS3" with DS0, DS1, and DS2 to control up to 3 double sided 
drives.

DS3 in the industry standard spec is pin 6 on the connector. On CoCo 
controllers, pin 6 is not connected to anything. Without rewiring stuff, 
a standard CoCo controller can't physically enable drive 3 anyway.

-Mike


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