Last revision of this page: March 2, 2025

• Introduction
• Frequency
• Trick
• Power Supply
• Adapter
• TG43 Signal
• Result
• Links & Information

I have had or have the following 8" floppy disk drives since 2018:

1. Full-High

   1x Shugart SA800/801 ? (115V, 50Hz)
   1x Shugart SA850/851 N (230V, 50Hz)
   1x Shugart SA850/851 R (230V, 50Hz)
   3
2. Half-High

   2x Mitsubishi M2896-63 (best one's)
   3x NEC FD1165-FQ (one is missing the PCB)
   2x Tandon TM848-02
   1x Y-E DATA YD-180-1601 (→ QumeTrak 242)
   8
3. Sold

   1x Tandon TM848-02
   2x Shugart SA860
   1x Shugart SA850/851
   4

The most important thing in advance. Yes, it is possible to operate 8" drives within MS or PC-DOS. Looking back, it was not that difficult to do this. From concept to implementation, it took me a few months. Most of the time took the search for a good working drive (Shugart 860) on eBay and shipping from the US to Germany. The actual implementation was done on a weekend. But first things first.

Whether the drive is actually good is unfortunately often a matter of luck. It can also go wrong. For a new, modern, i.e. half-height drive, a price of 400 EUR is not unusual.

As stated first, it is possible to run 8" drives within MS-DOS, but ... Basically, I have not found a single hint in any old textbook of a true operation of these drives in DOS PCs. I have several issues of Scott Mueller "Upgrading and Repairing PCs ", also the first two editions from 1988 and 1992. Mark Minasi: "The Complete PC Upgrade and Maintanace GUIDE, 1991". Corey Sandler: "Fix your own PC, 1997." Result: Nothing! See [Literature]

IBM PC, XT, AT and the other later compatible PCs were equipped only with 5.25" floppy disks (single-sided and double-sided) and later with 3.5" floppy disks. But not with 8" drives!

Although Microsoft's support page (Q75131) and "The MS-DOS Encyclopedia" explicitly mention 8" drives (Version 1.0 - 2.0) the direct file access with IBM compatible PC's is not possible. The mere fact that the 8” recording format is labeled single density in the (support page) table means that it could have been never used. IBM-compatible DOS PCs or the BIOS have never ever supported the FM format.

Where does the origin lie? Here are some information to this subject: Tim Patterson, SCP and 86-DOS (supported disk formats).

American computer programmer Timothy Paterson, a developer for Seattle Computer Products [SCP], wrote the original operating system for the Intel Corporation’s 8086 microprocessor in 1980, initially calling it QDOS (Quick and Dirty Operating System), which was soon renamed 86-DOS. A year later, fledgling company Microsoft purchased exclusive rights to sell the system, renamed MS-DOS, to IBM for their newly developed IBM-PC. IBM-compatible versions were marketed as PC-DOS. Version 1.0 was released in 1981; ... [3]

Seattle Computer Products adapted DOS to an array of floppy disk systems. We used North Star, Cromemco, and Tarbell floppy disk controllers, and later designed a floppy disk controller of our own (the “Disk Master”).
Floppy disks came in two sizes, which we called 5” and 8” (but the 5” was really 5.25”). The 8” disk came first, introduced by IBM with 77 tracks of 26 sectors, each storing 128 bytes of data, for a total capacity of 256,256 bytes. A single small hole was punched into the disk near the hub as the “index”, or starting point of each revolution. This means the disk was “soft-sectored”: there were no physical indications of sector boundaries. The sector start could only be determined by reading from the disk. [5]

Have a look at the 86-DOS Programmers's Manual. Exactly here is the origin for the support of 8" floppy disk drives; 26 sectors per track, 128 bytes per sector, FM recording. If you look at the source code of MICROSOFT DOS V1.1 (IO.ASM), the relationship becomes even clearer. The various floppy formats are defined at the very end. This 86-DOS was able to handle 5.25" & 8" formats. Have a look for: LSDRIVE, LDDRIVE, SSDRIVE, SDDRIVE, OLDLSDRIVE, OLDLDDRIVE.

This origin can be traced very well on the page: Timeline of DOS Operating Systems.

• 08/1980: Paterson's operating system, which he calls QDOS 0.10 ("Quick and Dirty Operating System"), ships. It's crammed into 6 KB of code. Seattle Computer Products runs an ad in Byte marketing it as 86-DOS for $95. Seattle Computer contacts Microsoft about adapting Microsoft BASIC for the new operating system, proposing a cross-licensing arrangement.
• 07/1981: Microsoft buys all rights to 86-DOS from Seattle Computer Products, initially for a further $50,000 and favorable licenses back from Microsoft. After settling a 1986 SCP lawsuit, the total cost to Microsoft was $1 million.
• 08/1981: Microsoft delivers its adapted 86-DOS 1.14 to IBM. ... This product [86-DOS 1.14] was later called MS-DOS 1.0 by Microsoft.
• IBM announces the IBM Personal Computer (PC), model number 5150, featuring: ... the IBM Personal Computer DOS 1.0 (PC DOS).
• [7]

Now you know why DOS supports 8’ drives. According to Dave Dunfield, the NEC APC is one of the few computers that runs with 8-inch drives under MS-DOS. The NEC APC is of course not an IBM PC compatible computer. In Wikipedia the operation of a 8" drive under MS-DOS 2.0 is mentioned. But this is the Xerox 8/16, a dual CPU computer. These both are not IBM PC compatible computers, too!

Retro computing, i.e. the employment and repair of old computers, is currently experiencing a rebirth. Many people are particularly taken with the old 8" floppy disk drives. "I want one of those too!" some say to themselves. And why not?

Getting a drive to work is a problem I will describe later, but first you need to buy a good 8" drive. And this is where you can make the first big mistake.

One can divide the 8" drives into two basic categories with regard to the power supply (for rotating the floppy platter).

1. Alternating current

   Power grid Europe: 220/240 V, 50 Hz
   Power grid USA: 110/120 V, 60 Hz
   AC
2. Direct current

   Europe, USA: 24 V
   DC

Category 2 includes the modern flat 8" drives such as the Shugart 860, Mitsubishi M2896-63, NEC FD1165 or the YE-DATA YD180-1601. These drives can be used without any problems, no matter in which power grid you are located (USA or Europe). These drives are exclusively operated with direct current.

Category 1 drives on the other hand can cause problems when you buy a disk drive in the USA and want to use it in Europe or vice versa. Why?

The diskette drive motor uses AC to rotate the floppy (at 360 rpm) through a belt-driven system. The rotation speed is controlled by the frequency of the AC power grid. The belt and drive pulley have a precisely defined length and diameter.

The voltage (110V, 115V, 220V or 230V) is secondary because this can be adapted with a voltage converter (for little money).

Mains time deviation
Mains time is a time measurement which in Europe is based on the standard mains frequency of 50 Hz. 50 oscillations of the alternating current correspond to one second of mains time. Frequency fluctuations cause deviations in the grid time. If the frequency is lower than 50 Hz, the 50 oscillations take slightly longer. If, on the other hand, the frequency is higher than 50 Hz, the 50 oscillations take less time. As one second of mains time is always exactly 50 oscillations, the mains seconds therefore last slightly shorter or longer depending on the frequency.

Netzzeitabweichung
Die Netzzeit ist eine Zeitmessung, die in Europa auf der Standardnetzfrequenz von 50 Hz basiert. 50 Schwingungen des Wechselstroms entsprechen einer Sekunde der Netzzeit. Durch Frequenzschwankungen kommt es zu Abweichungen der Netzzeit. Ist die Frequenz tiefer als 50 Hz, so dauern die 50 Schwingungen etwas länger. Ist hingegen die Frequenz höher als 50 Hz, so dauern die 50 Schwingungen kürzer. Da eine Sekunde der Netzzeit immer genau 50 Schwingungen sind, dauern daher die Netzsekunden je nach Frequenz etwas kürzer oder länger. [8]

In the past, this was not a problem because you could simply buy another belt and another motor pulley from your local computer dealer. But this is not possible nowadays, there are almost no spare parts on the second-hand market like ebay.

This problem also existed back than with so-called synchronous clocks (in German: Synchronuhren). In the seventies and eighties, these clocks were in great numbers in the cities and at the railway stations. If the frequency in the electricity grid decreases, the clock runs too slowly; if, on the other hand, the frequency rises, the clock is running fast.

In January 2023 I bought a Shugart SA800/801 (new old stock) with 115V and 50 Hz (I am located in Germany) on ebay US. A stroke of luck because you can not use it in the USA. Although the AC capacitor for the drive has lost a few drops of oil over the last 30-40 years, but otherwise it runs just wonderful; really like new. Excactly with 360 revolutions per minute.

The boom in the solar industry (maybe) has solved the frequency problem. With the help of a solar inverter, you can generate alternating current (AC: 115V, 230V) at 50 or 60 Hz (pure sine wave) from direct current (DC: 12V, 24V, 48V).

The simple trick is to fool DOS and output an 8" drive as a 5.25" drive (1.2 MByte, DS/HD). That's it. Of course, this is only possible because the characteristics of both drives are almost identical.

The only difference is the number of tracks. A 5.25" high-density (HD) drive has 80 tracks, a 8" DS/DD disk has only 77.

8" → 360 rpm 500 kbit/sec 77 tracks/side

HD → 360 rpm 500 kbit/sec 80 tracks/side

... Two years later IBM introduced the Personal Computer AT, based on the Intel 286 microprocessor. With it came the high-capacity 5” floppy disk. It basically gave the 5” disk the same specifications as the older 8” disks – doubling the data rate and spinning the disk at the faster 360 RPM. Capacity increased to 1.2 MB, the same as the 8” disks. [9]

This trick is not new. Back then with the advent of the first 1.2 MB HD drives, the clunky 8" drives were replaced by these. The M4854 was intended as a direct replacement for an 8" drive. This drive was even marketed with 77 tracks!

The newer (half-high or flat) 8" floppy drives only need +24V for the drive motor and +5V for the electronics. The standard PC power supply unit only supports +12V and +5V. You therefore always need a separate +24V DC power supply or DC-DC converter.

Who owns a laboratory power supply owns luck, appropriate socket or table power supplies are available to buy. Only get the right plug and a few adapters and problem no. 1 is solved.

On eBay Germany, jojobondjs88 (Joel Schmidt) offers both, a power supply unit and an adapter for 8" drives.

This solution with this Meanwell PSUs is easy peasy and works flawless with all my drives! I now use a 5V PSU with 5A and a splitter for five connections. I really like using the hollow plug connections. By the way. I have converted my old 12V fans, which were too loud, and can use them for a wide variety of purposes. Operated with only 8V, an old and loud 12V fan is really quiet again.

In the next picture you can see my universal PSU. Also easy peasy to assemble, with really all voltages you may need for your retro tinkering.

You can also try the DC to DC converter (+24V, +5V, -5V) by dbit (Digby's Bitpile, Inc).

What about -5V?

Briefly mentioned by the way, some old full-high 8" drives like the Shugart 800/801 also still need -5V DC, but ...

... with the Shugart 800/801 the -5V are not needed if you have the LSI PCB version. This is the case with mine for example. But I only found this out by accident, because I shone a flashlight through the PCB. And lo and behold the -5V pin (4) is not connected; it is a dead end.

For other drives you have to look into the manual! Note: The FDDC converter by dbit (see above) supports -5V.

The 8" adapter was a problem in 2018 but no more today. Background: 8" drives have a 50 pin (50p) connector (Shugart-50), but today's floppy controllers (PC-34) use a 34 pin cable (34p).

You have several options to choose from:

1. This solution is only for people who have some experience with the soldering iron; fiddly work but it is well priced. See: Daves Old Computers.
2. Adapter by Thomas Brase. This one requires only minimal crafting skills. I worked it out. See next figure: two terminal block breakout connectors.
3. Adapter by John Wilson (D bit): FDADAP floppy disk adapter.
4. Adapter by Texelec: 34 to 50 pin 8" floppy adapter.
5. Adapter by Thomas Brase: Interface PC-34 to Shugart-50.
6. Adapter by Joel Schmidt
7. Adapter by Gary Kaufman

The next picture (above option 2) shows how it all began in 2018. I didn't know how to solder at that time. Since 2020 I use options 3 to 7.

Admittedly, this adapter is very bulky and doesn't look very nice, but it works flawlessly and you can plug the connection cables as you like. This may be necessary, although very rarely once.

However, this adapter still has its justification. I needed it in March 2024 because I connected a 5.25" floppy drive to an Altair FDC+ controller on my IMSAI 8080. This pair of adapters can be used universally.

With these two jumper options (PCB V2.1), you are very flexible with your floppy cable. Version 2.1 is very flexible. You can make the 34p connection both before and after the cable twister. According to the jumpering, the 8" drive has the designation A: or B:, just as it fits your system. The default* is variant B: with both jumpers on (2-3).

With the signal routing shown in the following figure, you should be able to operate almost any 8" drive on a standard PC floppy controller (with drive A and drive B). A special feature is signal no. 2, TG43 or WRITE CURRENT LOW, see below.

With these signal connections I have so far been able to connect and operate five different 8" drives (Shugart 850/851, Shugart 860, Mitsubishi M2896-63, NEC FD1165-FQ, Y-E DATA YD-180-1601) to a PC controller (WD1006V-MM2 with WD37C65BJM).

And here is a condensed summary by Dave Dunfield. This corresponds exactly to my variant B:. A small mistake has slipped in here. The DISK CHANGE signal is on 34/34p and 12/50p and not READY.

To add a little more confusion, I have copied the following figure from the CompatiCard I or IV manual.

Yes, this method also works. You just have to make sure that YOUR personal combination of cable (twist or not), interface and DRIVE SELECT on the 8" drive is logical and coherent. You will recognise the correct setting immediately during the boot process, during the so-called floppy seek. If this is missing or drive A: and B: have simultaneously the „floppy seek“, then at least the DRIVE SELECT (DS) jumper on the 8" drive is set incorrectly.

In addition to the obligatory 34-pin ribbon cable, you also need a 50-pin (SCSI) cable and a card edge connector with 50 contacts as well. The corresponding terminal blocks and connectors can be obtained at Farnell or TMU. But they are not cheap!

My solution with the „breakout blocks“ has the advantage that you can change the wire bridges at any time, because, as it should be otherwise, the Shugart 50 interface is sometimes slightly different in the 8" drives.

In addition to the well-known BUS systems such as the Shugart-50 and the PC-34, there is also the Shugart-34, quasi the predecessor of the PC-34, and then a real old exotic one, the so-called Philips BUS system for 5.25" but 50 pins.

Dave Dunfield (see link above) described this in detail and presented also possible solutions. Briefly. The signal TG43 (track greater 43) reduces the write current from track 43 to 77 (the inner tracks). However, this signal never occurs with the PC interface! Either HD disks are written with a high write current (coercivity 600) or DD disks with a low write current (coercivity 300). It never follows a change on a floppy disk depending on the tracks. There may be problems with the operation of 8" drives in PCs, but it does not have to!

The TG43 signal is expected at the Shugart interface on line # 2. Since the PC controller does not know this signal, there can be problems during the writing process, but reading is no problem. The Shugart 860-1 is a newer drive and can control this signal internally via its own logic. There is an extra jumper for this.

Other drives with internal TG43 jumpering (write current low) are the Tandon 848-1E and 2E (1983) - please do not confuse with the 848-1 or 2 (1982) - and the Mitsubishi M2896-63-02M. I also have two M2896-63-02M in operation. Working flawlessly with my terminal block solution (with no external TG43 signalling).

I only listed the TG34 problem at the very end, because it is not clear if this is really a problem. All the authors I've researched are also not sure. Can be or can not be! In case of doubt only trying out helps.

So, if you're seriously considering running an 8" drive, then study the manual carefully before you buy it! You can easily find the appropriate manuals on the Internet. The FDADAP adapter from John Wilson (D bit) supports the TG43 or LOW CURRENT signal!

And so it looks like in the finished state (in 2018).

Update 01.04.2021: Of course, this „result“ is already hopelessly outdated, but that's how it started for me. Today I use either the FDADAP or my own adapters (Interface PC-34 to Shugart-50).

Please do not disturb the imprint of the 34 pin floppy cable. This is originally from a tapedrive and was used the other way around. Only red line 1 on pin 1 is important. You can also see the attachment plug here. Here a second floppy cable can be connected. So two floppy drives can be used!

• Vintage Computer Federation - Forum
  • Need some help with 8" floppy drive ... / This is a very interesting thread with a lot of information. Read it!
    • And here the whole thread as PDF (page 1) (page 2) (page 3)
  • DOS File Level access to Secondary (370H) FDC - Driver / A very interesting article, which also deals with the history of 8" drives.
• boginjr.com
  • Using 8-inch diskette drives with a PC
  • 8FORMAT - A formatting utility.
• Howard's Seattle Computer Products SCP 86-DOS Resource Website

1. (↑) SA800/801 Diskette Storage Drive, Service Manual, 1982
2. (↑) dbit, http://dbit.com/fddc.html
3. (↑) Britannica, https://www.britannica.com/technology/MS-DOS#ref1278700
4. (↑) Curt Mayer, Facebook, Vintage S100 Computer Enthusiasts
5. (↑) Tim Patterson, http://dosmandrivel.blogspot.com/2011/04/all-those-floppy-disk-formats.html
6. (↑) Seattle Computer Products, Programmer's Manual, Version 0.3, 86-DOS
7. (↑) https://en.wikipedia.org/wiki/Timeline_of_DOS_operating_systems
8. (↑) https://www.swissgrid.ch/de/home/operation/regulation/frequency.html#netzzeitabweichung
9. (↑) Tim Patterson: http://dosmandrivel.blogspot.com/2011/04/all-those-floppy-disk-formats.html

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