Path: utzoo!attcan!uunet!math.fu-berlin.de!tmpmbx!netmbx!hase
From: hase@netmbx.UUCP (Hartmut Semken)
Newsgroups: comp.sys.atari.st
Subject: Re: 1.44mb drives
Message-ID: <2029@netmbx.UUCP>
Date: 2 Mar 91 20:26:02 GMT
References: <1991Feb28.085205.28239@phoenix.pub.uu.oz.au>
Organization: netmbx, Berlin, West Germany
Lines: 48

billy@phoenix.pub.uu.oz.au (Jeff Coleman) writes:
> A few months ago, someone posted a do-it-yourself hardware mod to enable
>the St to use 1.44 meg drives. Could the author kindly repost this or
>anybody else with the knowledge please make it known ?
>(Im aware that pre made kits exists to do this already but Id prefer to do
> it myself)

It is pretty easy.

The trick is to double the clock of the WD1772 floppy controller chip to
get HD bit streams.
The regular clock is 8 MHz and 16 MHz can be get from the shifters 16
MHz output (see the Abacus Internals book for the pinouts; if the Abacus
book is the translation of the german Data Becker book, it should have
all the pinouts).
You could use a simple switch to changs the clock, but I found a
Multiplexer (1/2 74LS157) controlled by the "HD disk in drive" output of
the Teac FD 235 HF more elegant...
For the 5.25 inch Teac FD 55 GF I use a switch...

The problem ist, that the floppy controller will step twice as fast with
the doubled clock; that will be too fast for any drive mechnism.
So You'll have to set the step rate to 6 milliseconds to make the
controller step at 3 milliseconds in HD mode (6 ms in DD mode does not
hurt performance too much, so You can leave the Controller at 6 ms all
the time).
Setting the seek rate is easy: write the appropriate seek rate code (6
for 6 ms, I recall) to the seek rate system variable (is it at 0x4c0 ?
Check it...) and then jump to the "hdv_init" routine (the adress of that
routine is found in the system variables, too.) hdv_init will make new
BIOS parameter blocks for both drives with the new seek rate.
This works with any version of TOS, cause it uses documented system
variables and system calls only, but it will affect the seek rate for
both drives.

Now, all You need is a formatter program that will format 18 (or up to
20) sectors per track (or 15/16 sectors per track HD 5.25 inch).
A simple formatter using the XBIOS format routine is in the Mark
Williams C Compiler docs. I hacked it to use a track buffer twice as
large and it formatted HD disks with 18 sectors...


Thats all!

hase
-- 
Hartmut Semken, Lupsteiner Weg 67, 1000 Berlin 37 hase@netmbx.UUCP
Hi! (Zaphod Beeblebrox)