[Coco] Four floppy drives?

[Mark J. Blair]

Please forgive me if this has been addressed before. Did any of the floppy disk systems made for the Color Computer support four floppy drives in their standard configuration?

The hardware documentation refers to four drive select lines, but the fourth line (variously called drive select 3 or drive select 4, depending on whether an author used 0-based or 1-based numbering) of the Radio Shack controllers was connected to drive pin 32, which is the side select line for standard Shugart interface double-sided drives. Pin 6, which is the fourth drive select line on standard (non-IBM style) Shugart interfaces, is left open. The first three drive select lines are connected to pins 10, 12 and 14, which is normal for Shugart interface drives. The fourth drive select bit of the control register is not adjacent to the first three, almost as if it was an afterthought.

This makes it easy to connect up to three double-sided drives to a standard Radio Shack controller and then use double-sided disks with patched ROMs or different operating systems. But did the original Radio Shack single-sided, 35-track systems support 4 drives? And if so, did they use nonstandard drives which used pin 32 as the fourth drive select line instead of pin 6?

Using drives with the standard Shugart pinout, only three single or double sided drives could be used with a straight-through ribbon cable. A fourth drive could be used if all drives are single-sided and the cable is rewired to move pin 32 on the controller to pin 6 on the drives. Four double-sided drives wouldn't work with a standard Radio Shack controller, because there aren't enough control lines available.

Did any of the aftermarket controllers map the fourth drive select to pin 6, add an additional control line to allow four double-sided drives, or have any other interesting variations compared to the Radio Shack ones?

Last, but not least, are any of y'all aware of Radio Shack's reasoning for mapping the fourth drive select signal to pin 32 instead of pin 6?


-- 
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/