Index signal for flip side of the disk (flippy).

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e5frog
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Index signal for flip side of the disk (flippy).

Post by e5frog » Mon Jun 16, 2014 3:57 pm

So I thought I should try the reed version shown in the Catweasel menu on my Mitsumi D509v3. Someone told me it wasn't reliable but why not check it out. I don't have tons of disks so the work of flipping the disk will hardly be more than modifying a disk drive to read both sides at once or try and earn enough to buy one already modded...

Found a magnet in a good size (CD-ROM laser focus assembly) but no reed anywhere, just in my demagnetizing cassette, but I didn't want to grab that so I had to order it from somewhere.

As I'm impatient with these DIY things I thought I might be able to solve it some other way, with an IR led and a photo transistor in the right spot, I had seen that somewhere - but that would require a lot of work as there are things in the way in that particular spot. So maybe a light barrier fork instead and a piece of plastic attached to the flywheel (?), the rotating part of the motor... that circular magnet covered in metal...
http://www.conrad-electronic.co.uk/medi ... PS_250.jpg

Didn't have one loose but there should be two in an old mouse. Checked my scrap drawer and started dissecting mice...
Only found versions with two loose components, IR led and a three legged component which I later found out was a double photo transistor (to sense scroll direction I guess). Googled some, found a schematic someone else had used using the same type of parts. Could work perhaps, I cut a piece off from some PS/2 track ball that I bought NOS really cheap and that never really worked unless you'd use rubber gloves or something to get that ball rolling.

http://s2.postimg.org/tsjrwzl2h/IMG_4614.jpg

Soldered needed resistors and wires to hook up measuring clamps and power. 18kOhm on the photo transistor output to GND and 1kOhm in line with the IR led.

http://s2.postimg.org/9nq7rir8p/IMG_4621.jpg

Then I needed to decide the width of the plastic piece rotating between them.
An index hole in a 5.25" disk is about 2.5mm in diameter, placed about 25.75mm from the (imaginary) center of the disk, converted to a triangle that gives about 5.56 degrees, so a circle sector with a 5.56 degree angle placed on the back should do it. I calculated further that such a shape is about 4mm wide at the position my index sensor could be placed.

With a 2.5mm hole rotating at 360 rpm it flies past the sensor at 0.971m/s, that mean it takes about 1.03ms/mm, so something around 2.57ms could be sensible, something in that area is to be expected, not that it will digitally turn on and off, there's probably some rise and fall time as there's light scatter and the sensor is at a distance.

Before doing anything else I hooked up the drive to an old Pentium III computer and (finally as it took quite a while before I realized B: won't work by it's own here, I have to make a twisted cable) set it to some chkdsk activity while measuring the output of the original photo transistor:

http://s2.postimg.org/yyux80zu1/index_s ... iginal.jpg
Well, not that shabby calculating, in the middle it's about 2.5ms (0.5ms per square).
Rather slow rise and fall, I wonder what parts of this pulse is important, I imagine it's an edge triggered signal perhaps.

After just doing a test with a paper checking that the signal levels (when on/off in the optical sensor ripped from a mouse) were OK I decided to try and mount it. With the distance of about 85 mm from the center circa 4mm of plastic needed to fly by. I cut a strip with 5mm black plastic and taped it at the opposite edge of the magnet I glued there two days ago in hope of finding one of those reeds.

http://s2.postimg.org/iesn81l61/IMG_4620.jpg

Tacked the sensor with some sticky tack or whatever it's called in "outlandish" at a position that suited best for measuring.

http://s2.postimg.org/rlaxvbqeh/IMG_4617.jpg

Looked fine, so back in the computer again, make it spin with the disk in... I could maybe have figured out some way to do that but I didn't want to mess about too much.

And here's my homemade index pulse:
http://s2.postimg.org/t6ut0vq09/index_s ... cement.jpg

Funny that 5mm turned into about 5ms pulse width, I can just put a thinner slice there to make it smaller, if I adjust the distance between sensor and plastic piece covering it then it might look more like the original, perhaps rounding it a bit will help. I'll fine adjust if needed when sensor is permanently mounted.
I thought I'd just glue it along the PCB edge like so:
http://s2.postimg.org/ur04z9yeh/IMG_4624.jpg


So, the question is, for Kryoflux purpose will this be a good solution?
Does anyone know if the pulse width is important or maybe the way the pulse looks (like it's slope) or can I expect it to be edge triggered and I'm fine the way it currently is?
What's the main reason for a magnetic reed not being reliable? I'd imagine it could bounce and perhaps change elasticity over time...?


For final assembly I'm planning to desolder the wires from the original sensor on top and use these for 5V and signal and also solder a wire to ground.
Problem solved?


BTW, it sucks that I can't display postimage uploaded images here, it just complains about "It was not possible to determine the dimension of the image."

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mr.vince
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Re: Index signal for flip side of the disk (flippy).

Post by mr.vince » Mon Jun 16, 2014 4:43 pm

Images work for me/most but I am very sure there is a way to break it, by using some flavour of JPEG, like progressive 10, whatever. Maybe they were just too big in total.

Apart from this... Congratulations, nice mod. The issue will be like with any other fake index: it's fake. You lose the coupling of a disk replicated in one pass, but at least no issue for personal use. I'd object for preservational use, especially for systems that can read the index, as side 1 will always be shifted from the real one, so systems that depend on it would see the difference.

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e5frog
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Re: Index signal for flip side of the disk (flippy).

Post by e5frog » Mon Jun 16, 2014 5:58 pm

So it's not a perfect option, perhaps not better than the reed even? I wanted to place double IR:s there but it looked like it would be difficult to fit a second IR setup in that position without having to rebuild a lot. If you think it's worth the trouble I might look into other ways to fit an indicator in the reversed position.
A semi good solution would be to use the original for the normal side and fake one only for when reversed... ehh? ;-)

I don't quite understand what you mean with coupling.


I'm just going to use it for C64 disks, AFAIK. Would it be safe to use this false index with that?

Do you mean that data is placed on the disk relative to the index hole and that if I use a fake index signal I will write the data in the wrong place unless the hole of the disk is lined up exactly with my index indicator - which is not likely to happen even if I try. So that means it will guaranteed be off synch with the index next time I put the disk in or use it in another drive?

So for C64 use, good enough with a fake index like this (proper signal but not in the right position) or is double indicators/double index holes a better (and the only recommended) option?

Thanks for your feedback, I might upload the images as attachments instead but it would be better if the forum added compatibility with the good service that postimage provide - as in a lot of other "forums". http://postimage.org/mod.php
First image is not from postimage, it had the same problem though not recognizing the size.

spags
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Re: Index signal for flip side of the disk (flippy).

Post by spags » Mon Jun 16, 2014 11:29 pm

I've often thought that an RPR220 would be a good component for this sort of operation (to read the actual index hole). It is a single sided compact optical switch (basically a one-sided version of what you find in a mouse), which is probably handy given that for many drives it is difficult to easily access both sides of the drive at the flipped index position (usually one side is covered by one or more of PCB, motor or chassis).

Another single-sided option I've also thought about involves separate emitter, detector and fibre optic (shrounded in heatshrink to each component) all pointed at just the right position. Those luck enough to have 3D printers could even create a compact guide to encase the fibres at the end points, allowing the emitter and detector to be position at any appropriate location (thanks to the flexible fibre).

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e5frog
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Re: Index signal for flip side of the disk (flippy).

Post by e5frog » Tue Jun 17, 2014 1:00 am

I figured out how to fit the two components in the proper position, I noticed I had well over 5mm under the disk to the metal plate below it and the holder for the spindle (plastic piece that locks in the disk from above) could easily be mounted in an other way so a hole could be made on top...
I was almost about to but then I though about it and the original index hole photodiode is mounted on a PCB and is adjustable in the head moving axis so to speak and locked in with red lacquer (that transparent nail polish looking stuff that doesn't seem to be available anywhere anymore). So if I add another one detector it'd better end up in the right place or be adjustable. I guess to set it in the right position a disk with some kind of magnetic marking at the spot of the index hole is needed and then you hook a scope to signal from head and the other channel to the index hole indicator and make sure the signal peaks at the same time - or however they are supposed to be synched... I think.

I don't know much about it but if I'm going to add this I want to be sure I get it right, don't know where I find that information though or what type of disk is needed, there needs to be some kind of signal that can be measured and is in synch with that hole...

Don't know if a signal in the wrong position is better than a fake index signal?

My current components also have the reversed function, but I might as well buy a new photodiode if I go through with it, don't know what frequency the IR diode is peaking at so perhaps it safest to get a matching pair.

Fiber optics sounds neat, if you use a reflective detector then perhaps a mirror could be placed on one side for good reflection when hole passes. Still need some kind of calibration though.

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IFW
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Re: Index signal for flip side of the disk (flippy).

Post by IFW » Tue Jun 17, 2014 1:14 pm

It's not good for preservation of commercial software as it would be always out of alignment apart from your drive.
Once your drive stops working it will be out of alignment for all drives... ;)

However, as long as it for reading scene, work etc. disks and you are happy flipping the disks, then it's fine.

ErikFromCalifornia
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Re: Index signal for flip side of the disk (flippy).

Post by ErikFromCalifornia » Sun Aug 30, 2015 5:52 am

@mr.vice: You wrote, "The issue will be like with any other fake index: it's fake. You lose the coupling of a disk replicated in one pass, but at least no issue for personal use. I'd object for preservational use, especially for systems that can read the index, as side 1 will always be shifted from the real one, so systems that depend on it would see the difference."

It's unclear to me if the index signal timing factors into how a disk reads the data on side 2 of a flippy disk. My main question is does the drive actually read data based on the timing of the index hole on a flippy disk OR does the drive simply care that a pulse is generated to indicated the disk is spinning?

@anyone: Can someone clarify for me if flippy disks have two holes in the disk itself? If so, then wouldn't BOTH sides of the disk when read by the index sensor be DOUBLE? Or, is there two wholes in the disk jacket, but one index hole in the disk itself?

e5frog raised a question I'm also trying to understand: "Do you mean that data is placed on the disk relative to the index hole and that if I use a fake index signal I will write the data in the wrong place unless the hole of the disk is lined up exactly with my index indicator". There are two things: reading and writing with the index hole of a flippy disk. I'm more intrested in the READING of the data. If a fake index sensor is used, again, does this affect how the drive reads the data? Or does second index hole of a flippy disk just used to tell the drive the disk is spinning? I'd like to think that the second index hole of a flippy disk is just used to tell the drive the disk is spinning and that the drive can then determine where on the disk it should start reading? Or, is the reading and the second index hole on a flippy disk coupled together?

brightcaster
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Re: Index signal for flip side of the disk (flippy).

Post by brightcaster » Sun Aug 30, 2015 8:20 am

For question #1 I can say that I used a jacket with two holes wich I used to insert a few flippy disks (to avoid the modification procedure). The original disk which I took of this jacket had only one hole!

Regards. David

spags
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Location: Australia

Re: Index signal for flip side of the disk (flippy).

Post by spags » Sun Aug 30, 2015 2:05 pm

Genuine certified double-sided soft-sector flippy disks use to have two holes through the jacket and one hole through the actual media (and of course if they were blank media, the two write protect notches cut out).

ErikFromCalifornia
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Re: Index signal for flip side of the disk (flippy).

Post by ErikFromCalifornia » Mon Aug 31, 2015 7:01 am

@brightcaster and @spags, thank you both for clarifying as that does certainly help! I'm therefore of the understanding that you can only have one hole in the media then.

Apparently, the index holes are also used for data alignment and timing. I suppose a 1571 may have two photo diodes? Or I wonder how the 1571 reads flippy disks when the disk is actually flipped? I'll keep looking into this. It is still a mystery.

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