Crawly Crypt Collection 1/crawlyvol1.bin/program/hard_prj/flop14st/hdflop.2

Text File | 1993-07-01 | 6KB | 102 lines

Introduction and Notes for adding High density disk drives to the Atari ST

The first thing you must do, is to verify that your existing HDD controller IC
is a suitable one for the modifications. The controller on the Atari ST is a
Western Digital WD1772 and carries a suffix (usually) of 00-02 or 02-02. As it
is explained in the following texts, the original chip is designed to run at
10MHz and only the 02-02 chip will handle the higher clock speed of 16MHz, so
ensure that you have a compatible HD controller first.

There are no major problems with running two disk drives either one of each or
two high density drives but a little jiggery pokery is required to get it all
rolling properly. The type of HD drive you select will depend upon you but some
are easier to work with than others - it is essential that you get a drive that
will handle the full Shugart specs and not one of the (increasingly common) PC
type HD drives that has limited functions. The HD drive MUST have the ability
to send out the HD select signal on it's interface connector. The PC does not
use this normally as the HD select is controlled by the PC host card and tells
the drive what kind of disk is inserted (this is the reason that the PC can
mess up a HD disk by formatting the media with the wrong data rate).

For full compatibility you'll need to gate the HD select signal with the drive
select lines so that you can mix both high density and normal density drives
on the same bus.

Some of the later series of HD drives have a pull-up on the HD select line that
will stop you accessing the normal 720k drive as a second unit. This can be
overcome by swapping jumpers (assuming you get a drive that can be configured
properly) or by changing the pull-up to a higher value and arranging gates so
that it is driven and held low for non high density disk access.

You may also experiance timing problems with some obscure makes of drive, so
be careful to make sure you know what the drive unit you are buying can/can't
do.

Finally, if you boot from the A: drive and you make this your high density one
then be aware that you will need a small "autoboot" program to set the step
rate BEFORE you can boot from a high density disk. This program MUST reside on
the first track of the disk or the boot will fail for resons explained below

                              HOW IT WORKS

Basically the 720k and 1.44 meg disks drives are compatible in as much as that
they both have 2 sides and 80 tracks. Now the important difference is the speed
at which they write to the media. With 720k disks, the data is written out by
the drive at 250k bits/second - this is known as "double density" and uses MFM
encoding. A high density drive uses exactly twice this speed at 500kb/s and
hence, writes twice as much data to the media. This is layed down by the format
program as 18 sectors (instead of 9 on a 720k) per track of 512 bytes per
sector. This faster rate is still MFM encoded and is known as "high density".
There is yet a faster rate again at 1Mb/s that is used on the IBM machines and
creates disks of 2.88 meg capacity but a new controller IC would be needed for
this (anyway, the disks are too expensive yet) and it's known as "quad density"
and there is an older standard known as "single density" which was half the
data rate of the original 720k we discussed at 125Kb/s (remember the old CPM
machines and things).

OK, you've swallowed all that (what you haven't....) now consider that since
the physical interface is the same and only the data rate changes (a high
density drive unit switches it's data rate up to 500kb/s when you plug in a
disk with an extra hole) between 720k and 1.44 meg disks, it becomes easy to
see that if we send the data at the same speed as the high density standard,
then we can talk to high density media. How do we do this - easy, double the
clock rate to the disk contoller chip. It's normally run at 8MHz, so run it
now at 16 MHz and presto... you have your high density interface. You can pick
up a 16MHz clock in the Atari from the video shifter.

One unfortunate side effect of doing this, to which I referred earlier was that
since the data rate has changed and the speed doubled, then it now takes the
HD controller chip half the time to carry out any operation than it did before.
Now this means that effectively, the STEP rate of the disk drive (a function of
the controller chip) is now halved from 3ms to 1.5ms (and I haven't yet found
a drive that'll go this fast). So the problem when booting from a HD disk, is
that you must load a little utility that will change the STEP rate  (programmed
by the operating system) to 6ms (which when clock doubled becomes 3ms).

Now, this utility has to be small enough to fit on track 0 otherwise the disk
drive will try and step out to the second track at 1.5ms (which it won't do)
and the drive goes out of sync. Quite a number of programs incorporate the
ability to change step rates and a small program can be written to do the job
(I have written such a small utility if anyone wants it).


                             COMPATIBILITY

There are one or two commercial 720k disks that complain and refuse to load if
tried in a converted machine with a 1.44 meg as drive A: and if you're a games
freak then it is probably better that you make the HD unit, your B: drive to
maintain compatibilty. Once written and formatted, TOS takes care of how many
sectors per track there are and I have found no (serious) problems at all with
two HD drives fitted to my Atari Mega 3. One last point to bear in mind is that
a lot of (both commercial and PD) utilities were written to allow the use of
9, 10 or 11 sector formats, so these will not work with an 18 sector disk and
any cleanup/repair/disk doctors that can't handle HD media, will most probabyly
foul things up for you and make an even worse mess - you have been warned.

Oh yes, forgot to mention you can handle 5.25" HD drives as well but you need
to fit an external switch (the 5.25" HD doesn't have an HD select line OUT)

Happy hacking.....

Martin GW6HVA @ GB7OSP
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