Re: Apple II FAQs Floppy Disk Drives

[dempson@actrix.gen.nz (David Empson)]

Pim Blokland <pblokland@planet.nl> wrote:

> In the section about the IIgs Finder showing two 5.25 drive if there is
> only one, you give a solution to this problem: trick the driver into
> thinking it can support only one drive, and therefore never displaying
> more than one.
> While this trick does seem to work, it nevertheless isn't the solution to
> the problem, namely the computer thinking there are two devices connected
> instead of one.

The problem isn't that the computer thinks there _are_ two drives
connected.

The problem is that it cannot tell whether or not the second drive is
connected, so it errs on the side of caution and claims to have two.

Apple's own drives can usually be detected, but third-party drives are
indistinguishable from "no drive connected", so Apple decided to play it
safe.

The only way to identify the presence of a 5.25" drive is to look for
changing data while reading from the drive.  The only way to do this
reliably is to insist on a formatted disk being in the drive.  Since it
requires starting the drive motor, another technique is to try reading
from the drive without starting the motor.  With most Apple drives, this
generates a random stream of data, and the changing data implies a drive
is connected.

A problem occurs with many third-party drives, which latch the read data
output into a fixed state while the motor is not spinning, so the
computer doesn't detect a drive.

The situation is different with 3.5" drives, because there is a positive
feedback mechanism which allows the drive to be detected, particularly
with UniDisk 3.5 and other SmartPort devices, since they use a
communication protocol with error detection.  I'm not sure what method
is used to detect an Apple 3.5 Drive, but it is obviously reliable.
(Probably some kind of feedback arrangement through the one bit control
and data registers.)

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand