Re: More Zip stuff...missing docs

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

Devin Reade <gdr@eddore.myrias.com> wrote:

> In article <5361ue$m0b@strange.dfwmm.net>,
> Martin Gruen <mgruen@chaos.dfwmm.net> wrote:
> >I still don't know what slots to accelerate, or what options to have on
> >or off, some one help.  Especially since this modem problem is really
> >becoming a pain in the butt!
> 
> Start out with the default settings they show in the manual.  That
> is accellerated for all slots except 2 and 6.  Slot 6 must remain
> unaccellerated if you are using a 5.25" drive.  I suspect the slot 2
> is due to problems with some modems.

There is probably no need to set slot 6 to "slow" mode if you are only
using 5.25" disks, because the IIgs will slow down to 1 MHz anyway, and
the ZIP should obey this.

However, problems will occur with an Apple 3.5 drive (or equivalent)
connected to the IIgs disk port if you have slot 6 set for accelerated
operation.  I had a repeatable problem with a small number of disks that
was corrected by setting slot 6 to "slow" (a long seek was mistimed,
causing the head to recalibrate forever).  I also had another problem,
but I can't remember what it was (it was also fixed by setting slot 6 to
"slow").  Something to do with copy-protected software or a demo, I
think.

A UniDisk 3.5 might be OK, but it depends on what the SmartPort firwmare
and GS/OS driver do to the system speed while accessing the drive.


Stop reading now if you are not interested in the technical explanation.


SW1-5 on the ZIP ("CPS Follow") must be enabled (OFF) for reliable
operation.  This causes the ZIP to run in sync with the computer
whenever the computer is running at "Normal" speed (including temporary
changes to "Normal" speed for timing reasons).  If the computer is
running at "Fast" speed, then the ZIP is able to run in accelerated
mode.

The IIgs automatically slows down to "Normal" speed while the drive
motor is enabled for a 5.25" controller in slots 4-7.  This facility is
not available for slots 1-3, so a 5.25" controller in these slots will
only work if the IIgs is set to run at "Normal" speed in the Control
Panel.  The GS/OS driver lets you use a 5.25" controller in these slots
with the computer set to "Fast" mode in the Control Panel, by
temporarily setting the speed to "Normal".

On the other hand, an Apple 3.5 Drive is accessed while the computer is
in "Fast" mode.  The extra speed is required to get the data through the
controller quickly enough, because the Apple 3.5 Drive has twice the
data transfer rate of a 5.25" drive or a UniDisk 3.5.  This is too fast
for a 1 MHz Apple II to keep up.  (The various 3.5" controllers use a
faster processor or special buffering hardware to get around this
problem.)

Since the computer is operating in "Fast" mode, the ZIP will allow
accelerated operation while the 3.5" drive is being used.  This doesn't
cause a problem most of the time, but there are cases where the firmware
or device driver uses a delay loop to time a long operation (e.g. a
track step).  This runs faster than expected on a ZIP, so the operation
is not timed correctly.  (Reading data from the disk will usually be OK,
because most problems will use a loop to poll for the arrival of the
next byte.  Writing may be less reliable, but it is not as timing
critical as with a 5.25" drive.)

The disk controller hardware lives in slot 6, so if you set slot 6 to
"slow" mode on the ZIP, then every access to the disk controller
hardware causes the ZIP to run at the current speed of the computer
("fast") for about 50 milliseconds.  This is sufficient to keep the 3.5"
firmware or driver in sync with the hardware.


Note that all of this only applies to an Apple 3.5 Drive, SuperDrive or
equivalent connected to the built-in IIgs disk port.  If you have a 3.5"
drive connected to a separate controller card, then it is almost
certainly safe to leave the slot set to "fast" mode on the ZIP, because
the actual access to the disk hardware is handled by a separate
processor.  There might be problems with the interface between the card
and computer which require running the slot in "slow" mode.  I have my
SuperDrive card in slot 6 (set to "slow" because I'm also using the
built-in disk port), so I cannot test this.

> Since I don't normally have the 5.25" drive attached, I keep slot 6
> accellerated as well.

I'd recommend setting it to "slow" if you are using the disk port for an
Apple 3.5 drive.

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