How to image and create disks for retro computers

It's fun to work with old retro computers, but what's not always fun is trying to make disks for them so you can actually use them. This video is a guide to cover making FM and MFM disks using a PC. Many old computers use this format of floppy disk and if you have one, this video is for you.

This video isn't for you if you are trying to make disks for the Apple II, Commodore PET/VIC-20/64, Amiga or Mac (800k disks.)

0:00 Intro
00:46 Part 0 - The problem
3:20 Part 1 - Floppy disk encoding
8:39 Part 2 - Physical attributes
16:50 Part 3 - The disk drives
20:13 Part 4 - Picking you parts
26:23 Part 5 - Building the machine
34:30 Part 6 - Setting up ImageDisk
41:12 Part 7 - Making disks
53:38 Part 8 - Conclusion

-- Video Links

Michael Haardt's floppy guide with lots of technical information:

For the Apple II and Macintosh, I recommend the FloppyEmu:

For the C64/VIC-20/C128 I recommend a SDIEC: (lots of options here, search around a bit)

For the Amiga, a Gotek with FlashFloppy works well:

ImageDisk:

TexElec Quad Floppy card:
(not out yet)

Recovering a floppy disk using an oscilloscope: (Fascinating read!)

Adrian's Digital Basement Merch store:

Adrian's Digital Basement ][ (Second Channel)

Support the channel on Patreon:

-- Tools

Deoxit D5:

O-Ring Pick Set: (I use these to lift chips off boards)

Elenco Electronics LP-560 Logic Probe:

Hakko FR301 Desoldering Iron:

Rigol DS1054Z Four Channel Oscilloscope:

Head Worn Magnifying Goggles / Dual Lens Flip-In Head Magnifier:

TL866II Plus Chip Tester and EPROM programmer: (The MiniPro)

TS100 Soldering Iron:

EEVBlog 121GW Multimeter:

DSLogic Basic Logic Analyzer:

Magnetic Screw Holder:

Universal ZIP sockets: (clones, used on my ZIF-64 test machine)

RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)

Plato (Clone) Side Cutters: (order five)

Heat Sinks:

Little squeezy bottles: (available elsewhere too)

--- Links

My GitHub repository:

Commodore Computer Club / Vancouver, WA – Portland, OR – PDX Commodore Users Group

--- Instructional videos

My video on damage-free chip removal:

--- Music

Intro music and other tracks by:
Nathan Divino
@itsnathandivino

[adriansdigitalbasement]

FAQ:

1. 5.25" drives come in 48, 96 or 100TPI versions. 3.5" based on the Sony mech are 135TPI.


Things get a bit messy with High Density. I did mention that High Density is just double density MFM encoding used at 500kbps on 5.25" or 3.5" disks. It only applies to 5.25" or 3.5" as 8" disk drives always used 500kbps as their data rate and early controllers used FM and later controllers switched to MFM. The NEC APC wrote at 500kbps MFM double sided, but this was not called high density, it was always called double density because of the MFM encoding.

IBM muddied things up when they made the 5.25" High Density drive with the non standard 360rpm and 500kbit data rate. Worse yet, in order to pack in twice as much data using 500kbps, the drive needs to use a different write strategy for 500kbps and the floppy disks themselves needed to be reformulated. Think of Metal Type IV audio takes versus Normal Type 1. The problem is HD 5.25" disks are physically identical (As far as the drive is concerned) do the 250kbps SD/DD version. So the drive doesn't know what to do. Sony fixed this with the 3.5" High Density disks later, but even those just use MFM at double the data rate for high density. It's still technically a double density disk as far as encoding goes, just double the data is packed into each track.

I know it's confusing! Hope this helps a little!

3. It might be possible to use RLL encoding on floppy drives. This would give another 50% improvement in storage capacity over MFM using the same 250kbps! FM -> 50% better -> MFM -> 50% better -> RLL. This was done on ST-506 hard drives (often called MFM drives) which used MFM encoding primarily, I think at 5mbit. Switching to a RLL controller would give you that 50% capacity and speed boost. (Note that MFM drives almost always spin at 3600rpm, 10x what 8" drives ran at.) The only problem with RLL on floppies is their speed isn't as stable as a heavy spinning hard disk, so that instability might lead to problems decoding the data properly. A big part of how RLL is more efficient is how less clock data is encoded into the data, but that means that instability in the speed of the disk would result in data loss.

4: Hard sector disks. I didn't touch on this either and it's another wrinkle in the floppy drive universe. These disks are the same as normal 5.25" disks, but instead of one index hole, there are usually 16. (Or is it 18?) The index holes create pulses that the drive controller use as a reference to where to write sector data onto the floppy disk. These were needed on old primitive controller cards that did all of the encoding using TLL logic. Once highly integrated LSI controller chips like the WD1791 came out, they no longer needed all of the index holes as the controller only needed one index pulse to synchronize itself. This allowed for the use of variable number of sectors per track, as the controlled IC was handling all of that. Some floppy drive systems like the GCR encoding on the Apple II and Commodore don't use the index pulses at all. This is why you can flip those disks over to use the back side, while this is not possible with systems using these LSI controller chips.

You can use a hard sector disk just fine in a Commodore or Apple II, as the index pulses aren't used, but you cannot use a hard sector disk in a system that is "soft sectored" as the controller will be immediately confused by the large number of pulses. You can use a Kryoflux to make disk images of hard sector floppy disks, using command line switches. It can then write back the images as well. DIsk drives themselves are not hard sector or soft sector. It is strictly the controller that requires one type of disk or another. (With one index hole per track, or many.)

[stathissim]

This could be a highly paid course on computer history instead of YouTube video :) great job as always

[miketech]

You Sir, are a whole gift to the entire retro computing world! I recently used the TRS-80 diagnostic ROM you co-authored to resurrect one of my Model 3s. Your channel motivated me to finally get working on them, and I’ve started a video series on the process!

[computer_toucher]

Fibre optics has had the same kinds of leaps in modulation technology as modems back in the day, by discovering techniques for multiplexing light in different frequencies. I think it's interesting that bandwidth is basically doubled with each leap in all areas. Like DDR RAM - DDR 4 has twice the bandwidth of DDR 3 and DDR 5 is twice that again. Same with PCI Express, etc etc etc. Maths and physics, man, it's incredible.

[tschak]

The 3" drives you mention for the "Amstrads" are Hitachi 3" mechanisms. They run at 250kbps, and just use the standard signalling. They are configured as single sided units, but the disks can explicitly flipped.

[berndlangerich]

Fun fact: The photographer of the floppy drive motor is my colleague. He was pleased to be attributed in your video.

[stevejones]

I used 5.25 and 3.5 disks and drives starting in the late 80s and 90s but I have never really understood the details behind the drives and disks. This was so informative and cool to watch. Thanks Adrian.

[leo.binkowski]

Well this is nice and timely, I was putting off on building a new floppy extractor for my library of NABU diskettes. I've bookmarked this to review when I take another crack at them with IMD next week.

[johanneswack]

Thank you Professor Black. I love your lectures so much, I do learn so many things from you, so i just want to say thank you.

[adambowman]

I've been using GreaseWeazle lately to write disks for my Amiga 500 and have been very happy with the result.

[pepstein]

Wow that was quite a deep dive into the world of FM and MFM floppies! I’d love a part 2 going into GCR floppies and perhaps also the variable speed 3.25” floppies. I recall Commodore uses fewer sectors on the inner tracks to maintain a more uniform data density on the magnetic surface, while Woz used different RPM for some tracks to achieve the same effect. Would love to learn more about this stuff!

[your_utube]

I cannot believe that I watched all of the video, but I am sure that this will become a REFERENCE VIDEO that many retro enthusists will consult as a vital tutorial. WEll laid out and considered. Thankyou for your hard work!

[melony]

This video really does boil down the different drive and disk formats that we retro enthusiasts have to deal with, and all in the one video. Whenever I need to explain this to someone in the future I won't need to fumble around with my own words trying to explain, but rather just point them to your video here. Thanks Adrian! Love your channel, one of my favourites BTW!

[juunidesu]

I remember we always used TEAC drives back in the day in any PC our family set up. I actually had a combo drive in one of my towers, it was both 3.5" and 5.25" in a single half-height bay, I thought it was just amazing. I dont think I've ever seen another one ever since either.

[fredericksonRules]

I'm so happy you're on here, making such an incredible effort to document old tech, and especially highly detailed vids like this. We cannot let all of this important history become lost. Thank you a million times over for what you do, your knowledge, effort, and these videos!

[bobblum]

I'm only about 16 minutes into your video, and you've covered so many details of the various formats that I remember having to fight through! The Tandy 2000 with 720K quad-density 5.25-inch drives, the oddball HP 150. I don't blame you for leaving out the hard-sectored floppies and things like the DEC RX-50 floppies.

Back to watching the rest!

[admonius]

It’s 2023 for me and this is the first video I’m watching for the new year

[Jody_VESAR]

This was a tour-de-force! It might become THE definitive source of knowledge on legacy disk formats.

[simonwilliams]

Adrian's explanation regarding Amstrad 3" drives at 19:28 is absolutely correct: 3.5" drives can, for example, be plugged into the back of an Amstrad CPC6128 with a slightly modified floppy ribbon cable with the 5.25" / shugart edge connector.

[JasonSobell]

The 'width' aspect of writing to 40 vs 80 track drives was only true for a few very early PC drives. In general both types of drives used the same heads, and many of the non-IBM early 5.25 drives were all 80 track devices with BIOS coded double-stepping to reduce manufacturing costs, meaning they could fleece us as consumers by charging more for 80 track devices at no extra cost to themselves :)

When I was a BBC micro games developer I used the Watford Electronics DFS ROM which had commands *40 and *80 to switch modes and do double-stepping on an 80 track drive, but I created a modified EPROM version that read the track count from the outermost track when the directory loaded and auto-switched, so I could insert any disk and have it auto track-select. Fun days!



These videos are always an interesting trip down memory lane, but they really make me feel old!