Update your old 720k to a 1.44 drive etc.
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Post by exxos » Mon Aug 21, 2017 1:58 pm

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The HD6.x board holds a simple but clever design which changes the ST's disk controller instructions to make it function in 1.44 and 720k modes. Most older kits require you to modify TOS or install software patches for TOS to work with high density disks. These patches often change the default seek rate to 6ms. This is required since when running in highdensity mode the seek rate halves. If this modification wasn't done the seek rate would go down to as little as 1.5ms and most floppy drives would not function at this speed.

The HD6 module keeps a watch on the commands which the ST sends to the floppy controller and when it sees a seek rate command the board jumps into action and changes the commands "on the fly". This gives you the freedom of a fully automatic board which requires no special software to be installed. Its a simple "set and forget" type solution. The module supports both 1.44 & 720K modes internal/external drives in any combination.

Also I noticed a lot of other designs when switching to 16mhz mode, stayed in 16mhz mode even when the floppy drive was not in use. This just unnecessarily stressed the WD1772 causing overheating and failure over time. My design only switches to 16mhz when its accessing the floppy drive. So overheating and stress is kept to a minimum. With this method, almost zero 1772s failed in 16mhz mode even after consecutively formatting several 1.44 floppies.

The HD kit was Developed in the very early 90s when 1.44 kits were all the rage. Having fitted various kits to my own ST's over the years I decided to build my own. The documents online were, as always, full of errors or incomplete making study difficult. They all needed track cutting, wires all over the place soldered onto other IC's, and more annoying needed software to work. I decided I wanted a software free version as other kits I used I kept on loosing the driver floppy! So I designed the HD4 module which is a software free fully automatic hardware solution. HD5 was produced in 2012 with a updated PCB design and now supports 6ms/3ms on the fly switching.

HD6 sees the move from dedicated logic chips to a single GAL IC. Years ago, circa 1990, Logic IC's were easily obtainable and in recent years are increasingly difficult to find. Because of this the choice was made to move over to a GAL which makes the whole design a lot smaller and gives greater flexibility of the design. While GAL chips are obsolete in recent years, they still seem obtainable reasonably well currently.

One thing that has been in question for some years is the media change issue. I looked into this with TOS104. I found if I booted from a blank floppy then placed in a floppy with programs on it, and opened up drive A:, the GEM window would show blank contents. Even if the window was closed and reopened several times, the new contents would not show. Pressing ESC with the window open refreshed the window and the contents showed. So this clearly shows a issue with TOS in not recognizing the floppy change. As the design now uses a GAL, It allows more features to be added without having to add more logic chips.

From my searching, the correct method for media change is to set WP (write protect) line LO when not accessing the drive. TOS polls the floppy drive while not in use and checks the WP signal. If it sees the signal is LO, then TOS assumes the media might have changed. So upon disk access, TOS re-reads the floppy FAT and then displays the correct contents. So with the jumper pad selected, this enables the WP line to go low when the floppy drive is not in use. So repeating my tests, when booting from a blank floppy, and then opening up the floppy after changing it, the correct contents are now shown without pressing ESC.

I wrote a small program in STOS (compiled as as TOS executable) to print out the media change status. It can be downloaded below. If run on a unmodded ST the message will say "Disc has definitely not been changed" constantly. On a modded ST. After loading and the floppy drive light turns off, the GAL will set WP LO and the message will say "Disc may have changed!". The message shows up regardless of if you actually change the floppy or not. The Media Change Fix is enabled by soldering over the MCF pad on the PCB.

While Beta testing on my STE, I found that I had a patched TOS which sets the step rate to 6ms by default. So it would work with 720K or 1.44 floppies without having to worry about the step rate times. Though a lot of people (including myself) do not like the 720K floppies running at 6ms, it often makes the floppy drive sound really loud as its running slower. This was a annoyance of my V4 modules. V5 saw the "step rate fix" so 720k floppies ran at the proper 3ms speeds. So it seems at some point TOS206 had the same "bug".

However since I have now run into issues with TOS forcing 6ms, I adapted V6 to force 3ms back on 720K floppies. 1.44 floppies are forced to 6ms (which then becomes 3ms at 16mhz speeds). So now V6 series sees correct step rate times regardless if it has been forced in TOS or not.

Over all, this is likely as advanced as a floppy upgrade kit can go. Once set, copying between 720k, 1.44 floppies is all done automatically and everything is taken care off.



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