Hello and welcome back.
If you follow the channel, you know that we recently battled with an early 8” floppy
diskette drive, but eventually managed to connect it to a PC.
In the process, our buddy Antoine developed an adapter for it, available for all on Tindie.
We discovered there were no less than 11 major differences between 5-1/4” floppies and
8” floppies.
But, curiously, signaling wasn’t one of them, so our PC was able to read them after
we figured out all the details.
We could even boot from our 8” floppy drive and play a DOS game.
We showed 4 different types of 8” floppies: the single sided, the double sided, the single
density and the double density, in all combinations.
But commenters quickly pointed out that I had omitted the hard sectored 8” floppy.
Ah, yes, right, the hard sectored floppies, I don’t have any.
This is even more ancient.
But Eric is about to straighten us out.
And have you ever heard of an IBM demi diskette ? I havn’t either.
So stay tuned for Eric’s story on that, since his grandfather worked on it!
In the same vein, some viewers asked us to look at really odd ones that they had not
managed to read: a Minnow disk from an IBM 370, *the* one and original read-only floppy,
and a weird Intel one.
Both contained quite important pieces of historical software.
So let’s complete our old and bizarre floppy education and see if we can rise to the challenge
of reading the really weird ones.
[Marc] Eric's brought a diskette, some diskettes that I got flack for ignoring in my 8-inch
diskette video.
Take it out!
And that's the hard sectored one?
[Eric] Yeah.
So it's got the index and sectoring holes at the edge here, instead of the middle of
the disk.
And there are 32 holes on here, because this is a 32 sector diskette.
[Marc] Right.
And then they appear here.
They're measured over there I guess.
And you were telling me, this?
[Eric] Yeah.
The right protect tab is in a different spot.
[Marc] And it's it's a hard tab!
Can you can you flip it over?
It's a hard piece of plastic.
All right.
So here it is.
I've never used one, so that's the first one I'm seeing.
And you and you have other ones?
[Eric] I do!
[Marc] And that, I had never seen either.
[Eric] This is a 4-inch floppy disk.
And this is the prototype drive for it.
[Marc] And this was a product that never went commercial?
Or it went commercial and something happened to it?
[Eric] Well, sort of.
It eventually was announced as the IBM Demi Diskette.
And they changed the disk: instead of being a low-cost welded disc, like this, where you
could see they welded the plastic case shut, they turned it into a molded plastic case.
A lot like the Sony floppy.
[Marc] Oh, okay.
Before they saw the Sony, or they had the same idea?
[Eric] Right around the same time.
So, unfortunately, it didn't do very well against the Sony and the other micro floppy
drives.
Because it only stored, I think, it was 90 kilobytes.
And it was more expensive than the competition.
And so the OEMs just didn't use it.
My grandfather worked on this.
So this is the prototype mechanicals of the drive, and the prototype electronics.
But you could see this is something that someone has wired up by hand.
So there's a lot of wiring in the back here.
[Marc] Sweet!
Since last time, we got one more disc...
[Eric] That's right!
[Marc] ...that we added to the collection.
Yet another format, that's quite important actually.
It got sent by a viewer.
[Eric] And it's this format here, which probably the earliest floppy format, right?
[Marc] It's the original!
[Eric] I think it was the original, yeah.
[Marc] It's the original.
This one has the hole on the side.
It has plenty of holes.
[Eric] Yes.
[Marc] Hard sectored?
[Eric] Hard sectored.
I think it's got eight sectors.
[Marc] Right.
And it's different from your other hard sectored Memorex disc, that you had shown just before.
[Eric] Yeah, yeah.
So this...
[Marc] It's different, but very similar.
[Eric] Very similar.
So the index hole is in the same spot, but it has a notch on the edge for right protect.
[Marc] And it's upside down.
[Eric] Yeah.
The label, they put on the other direction.
So, it goes into the drive the same way, it's just the label faces down.
[Marc] Oh, really?
So, it's clearly inspired.
And this one was sent by a viewer, because it's the original, it's IBM 360 for the original
usage of the disk, which is firmware loading.
[Eric] Right, exactly.
And that would have been a read-only drive.
The drive was called the Minnow, I think.
It was the 23 FD.
I think that was the format.
[Marc] Yeah, the original Minnow.
[Eric] And, unfortunately, we can't just pop this in a regular drive, because there are
certain critical parameters that are different.
So the disk spins at a different speed.
So this one's designed to spin at 90 RPM instead of 360.
[Marc] All right!
[Eric] And that's not even the most difficult change!
This actually uses 32 tracks per inch, whereas every other 8-inch format uses 48 tracks per
inch.
So, that means that you need a different stepper motor or a different drive mechanism, or you
need to do a micro step.
And so, I figured out that I could probably do like a half step on the stepper motor.
And if I move one and a half steps, then I should be able to look at the tracks, even
though there are 32 tracks per inch.
[Marc] And it's also different encoding?
The sectors won't be recognized by a PC chipset, right?
[Eric] Yeah, the encoding is different.
It's still FM encoded, but the data rate is also slower.
So it's at 33.3 kilobits per second.
[Marc] And the sector header is different!
[Eric] Yes!
[Marc] But fortunately, you inherited the IBM disk drive, which it was Carl's, and then
became mine, and now it's yours!
That’s the proprietary IBM drive, that I showed in my last video.
It’s not an original 23FD, but a later model, a 31SD.
It had been bouncing around in the team, and has an IBM proprietary interface that’s
unsuitable for connecting to a PC.
But it looks like we’ve finally found the right owner and the right project for it.
You managed to make it work, on its weird interface and totally undocumented stuff?
[Eric] Yeah.
I reverse engineered the control board, so I can understand how everything is supposed
to work.
And so, now I know the pin out, and I know all the signals.
And so, I can control it.
[Marc] Make it work, make it work!
[Eric] Okay.
So it's spinning, and we already have a disk in the drive.
And so, the first thing we have to do, is actually load the heads.
And so, I've put a little switch in here for that.
And so, now the heads are loaded, and we can actually see data coming off the drive now.
[Marc] And the control for the stepper motor is different too.
Instead of being a pulse and direction, it's a phase thingy?
[Eric] Exactly.
So there are two wires, but instead of being step and direction, like you're saying, it's
the raw control over the phases.
And so, if I want to make a step, I have to manually put in a gray code.
[Marc] First button and second button.
And depending which one you press, first...
[Eric] Yeah, one, two and then I release the first one and release the second one.
So I'm just manually putting in a gray code.
[Marc] Oh yeah, that works!
But, still, this is a later drive.
So it still doesn't have the index hole at the right position?
[Eric] It has it at the new position!
So I have to figure out how to retrofit the drive with an index sensor at the position
that the Minnow disk uses.
[Marc] Right!
So nothing that the Drillenium Falcon couldn't do.
We used first an end mill and then a drill to and add a little hole, so we can put a
detector on either side of the disk.
Actually, can you put the disc in there?
See if we didn't mess up, if we centered it correctly.
There we go!
And that should hold up a photo-transistor on one side, and the new IR LED on the other
side.
[Eric] And, I forgot to mention, the track zero is in a different place too!
You need room for the sector holes on the edge of the disc, and so everything is offset!
And I don't know what that offset is going to be!
(One week later...)
[Marc] It's another day, and Eric has managed to fit the sensor in there.
And made a controller for his half step.
And we're going to test it.
[Eric] In fact, we should be able to test the index sensor right now.
So let me hook up one of the scope channels to do that.
I should say these are our sector pulses.
And if you look carefully, you can see the index pulse appearing.
[Marc] Oh yeah!
[Eric] So it appears right in between two sector pulses.
[Marc] So, there's a double pulse?
[Eric] On this particular disc, there is.
So what I've done is, I built this half step driver.
And normal full steps on this drive, are 48 tracks per inch.
And so, if we do a half step, that's the equivalent of 96 tracks per inch.
And so, that means that if we step 3 half steps, then that's 96 over 3, or 32 tracks
per inch.
Which is what we want for the IBM disk.
[Marc] So, step and a half.
[Eric] So this is a half step.
So you can see the head moves very little.
[Marc] Hard to see.
[Eric] So, if we go ahead and load the heads, then we should be able to see data.
And then, we can look at track alignment.
[Marc] All right, go for it!
[Eric] Okay.
So, these appear to be data pulses.
[Marc] Oh yeah, you can tell the signal to noise is much better.
Do a half step?
This is much better.
{Eric] Yeah.
[Marc] Do a half step?
This is worse.
[Eric] Yeah.
So, this is going to be our zero point on this particular disk.
It turns out it's not the same on the Minnow disk: its track zero starts in a different
spot.
[Marc] All right.
[Eric] All right, there's one more thing I wanted to try real quick, just to make sure
my logic analyzer setup was working.
[Marc] Oh, you capture data with a logic analyzer?
[Eric] Yeah, so this is what a typical capture looks like.
There's the index pulse, and then, there should be 32 sector pulses
[Marc] That's your Salaele, laelae, lele..
Selae, Salee?
[Eric] Saleae, I think.
And then we can zoom in here, and we can see all of our flux transitions.
[Marc] This is the actual Minnow disk.
[Eric] All right.
So our first task will be to find track zero.
There it is.
[Marc] There you go.
[Eric] So this is our track zero.
So I'm going to zero it out.
And then, if I do fine steps, we can look at the amplitude.
Oh, this is larger.
See that?
[Marc] Yeah, much better.
So you're on the edge?
[Eric] Okay.
So this is actually track zero.
So let's mark that.
Let's go ahead and acquire this.
And there it is!
Let me go ahead and unload the heads, so I don't put any extra wear and tear on that
disc.
So let's see what this looks like.So here we have our sector pulses.
There's no index pulse in this particular design.
And, let's take a look at the sector data.
And here we are.
So it's about 7.75 microseconds.
These are all zeros, because every sector is separated by a huge long string of zeros.
[Marc] Okay.
[Eric] And then, if you want to figure out where the sector data actually starts, you
have to look for a one bit.
Oh, there it is.
So there's a one.
[Marc] Okay.
[Eric] And because this is FM, a one is just a pulse in between the two clock pulses.
[Marc] Yep, and it's easy to decode.
[Eric] So we have a one, and then the next nine bits here are the sector byte.
[Marc] So, you're going to just do it sector by sector, and then write a program that tries
to recover the sector data.
Eric was able to recover all the bits and separate sector info from data, but only after
he figured out that the Minnow disk rotates in the opposite direction of a standard disk.
The disk is the all important IPL disk for an IBM 370 model 135.
It contains the firmware IPL microcode, without which you cannot boot the machine, or IPL
it, as IBM would say.
Unfortunately the microcode on the IBM 370/135 is undocumented, and made up of commands rather
than instructions.
So we have the bits safely archived (linked in the doodly-doo) but don’t know what they
mean.
Please have a hack at it if you feel like it.
The next challenge came in the form of Intel diskettes from an Intel Intellec MDS-80, an
early 8080 microprocessor development system.
Although way more modern, using MFM encoding and regular 8-inch diskettes that would fit
in my drive, they were totally unreadable by a PC.
This is because the sector structure is non-standard, hence the floppy disk controller in the PC
could not recognize valid sectors.
Fortunately someone has already engineered a floppy disk controller, called Applesauce,
that supports that sector encoding, so it’s not worth our time to redo it all over again.
However, we checked if the diskettes had good data, and it turns out only one of them had.
We will let the user procure an Applesauce system to read it.
And that’s it for this episode’s adventures with early and esoteric 8-inch floppy formats.
But it’s not the end of our adventures with 8-inch floppy drives, because I need to go
through a heap of them for my HP computers.
And guess what, they’ll all fail in a different way.
This will challenge our analog debugging expertise, elicit our greatest McGyvering, teach us a
thing or two about magnetic heads and test our alignment skills.
But you know us, we’ll get them working.
See you then!